EVCCON 2012, Charging, and a Watchdog

There's no denying it, this is a long over due update.  Since I last wrote about the charger blowing up, there have been a few events worth noting, and a change in the charging system worth mentioning.  Let's get to it.

EVCCON 2012

You may remember that my father and I took the Z3 out to Missouri last year for the inaugural EVCCON.  It was a fantastic event and before it even ended, I decided that Dad and I would return to EVCCON 2012.  There was a big difference this year in that we didn't take the car along with us.  Having the car out there last year was terrific, but trailer-ing it out there was stressful, expensive, and it took three days each way.  So this year the car waited back in the garage while we went to Missouri.

There has been plenty written of the convention on some very well written blogs, particularly on Mike Brown's blog about his Porsche 914 conversion, ( day 1, day 2 and day 3), and of course on EVTV.  What I was most taken back by was the quality of the builds this year.  I've been told by a number of people that they think I did a great job on the Z3.  Compared to most conversions I've seen, it is great.  Virtually all of the people that brought cars to this year's convention attended last year's and had a chance to see all of the cars that were brought at that time.  There were a handful of car's at last years convention that made the Z3 look like a kindergarten project.  It was evident by everyone that saw them, that they were clearly a different animal.  In fact, it's clear that the community has decided those cars are the new standard.  If you were going to convert a car, they were the benchmark that you need to aim for.  A challenge that all took seriously.

The quality of the cars this year was simply outstanding.  Everyone had all the components laid out and organized better than you'd expect an OEM to do.  All of the wiring was routed perfectly and protected in looms.  The connectors all neat and orderly, everything labeled.  Looking in to these cars was like looking in jewelry boxes.  Just astounding.  Here's a sampling of a few.  I wish I'd grabbed more photos.

Fred Behning and his MGTD

Jeff Greeson's 914.  Take away that blue cord and it's looks like a show piece.

John Allen's Celica.  A beautiful job in every respect.

Kevin Heath and his RX8.  He has every reason to be proud of this build.

Jason Horack's Daytona.  Well laid out, neat and tidy.

Dale Friedhoff's Ranger.  It won best wiring/layout award.

It's inevitable that a fair number of people are going to look at any conversion.  The better they look, the more professional they appear, the more likely people are to take them seriously and not view them as a science project, or worse a rolling death trap that is bound to electrocute someone.  I personally think that producing fine cars like this pushes the cause of EVs forward a bit faster.

If I'm not mistaken, last year Dad and I were the only father/son team at the convention.  Well apparently that inspired a few others.  Several decided they wanted to share the experience with their fathers as well.  I was thrilled to see that list grow to 5 teams.  We took a moment to pose in front of Jack Rickard's original Speedster, along with Jack.

Left to Right, Me, my dad Bill, John Allen and his father, Nabil Hanke , John Hanke, Jack Rickard, Fred Behning and his dad Fred,  Brandon Hollinger and his father on the end.  

After a public car show in the park, the EVs went on parade through town and back to a local hotel were many of the attendees were staying.  They rounded up all the EVs that were in the parade for a photo op.

I think there are 33 cars in the photo, and I know of 5 others that didn't make it.  So that brings the total to 38 EVs at the convention by my count.  Not a bad turnout.  If you have the means to attend, I encourage you not to miss EVCCON 2013.  They are only going to get better from here on out.

Charger News

You may remember from my last post that my charger up and exploded on me.  It went back to Manzanita and they repaired it in a couple days and had it back to me right quick.  I think the charger was out of the car for less than a week.  All seemed well until a week after the install I noticed the charger behaving very erratically.  Though I'd dialed in 20 amps of current, I saw the meter bounce all around from 18 to 2 then 10 then 3 then 5 amps.  It was clear that it wasn't healthy. 

I contacted Manzanita again and explained what it was doing.  They asked me to take notes for a while to see if we could see a pattern to help them determine a cause.  If there was a pattern somewhere buried in the data I gave them, it escaped me.  After a couple weeks, we decided they needed it back to fix it.  I took it out and mailed it the day before Dad and I left for EVCCON.  

As it happens, Rich Rudman owner Manzanita Micro was one of the featured speakers at EVCCON, so I got to talk to him about the charger.  I offered to let him pick my brain about it's behavior hoping that he might have some insight into why it was doing what it was doing.  Instead it turned into a very short conversation.  Rich asked me if it was behaving erratically, and listed off a few of the key characteristics of the behavior.  I said "Yes! Exactly!"  He said "Yeah... we don't know what's causing that."  Apparently they've seen this a few times.  They suspect a specific chip on the power board is causing it.  When they come across this, they replace a few key components and that resolves the problem.

I got it back last Wednesday, they had it for about 2 weeks, and so far, I'd say they nailed it.  It has been rock solid reliable.  Boy, do I like that.  They really are a first rate shop.  

A Watchdog Circuit

It's happened twice now, which isn't a lot, but enough to make me a bit nervous.  The charger which usually cuts off when the pack reaches 164.5 volts has error-ed during the constant voltage portion of the charge cycle.  Instead of cutting off the charge in 10 minutes I set it for, it was well past 15 minutes, and I turned it off manually because the battery was going too high.  I was present on both occasions to catch it and prevent it from over charging the batteries.  But that begs the question, how many times did I not catch it because I wasn't there to watch? 

The truth is, the charger is pretty reliable, but it uses electronic components to process that logic and is subject to the same faults any electronic component is.  How many times does you computer do something unexpected?  The world is an imperfect place.  To that end, I've always thought that it would be worth while to have a separate circuit watching the charge cycle, one that had the ability to cut off the charge if things got out of hand.  Fortunately a couple simple components allow anyone with a Manzanita Micro charger to do just that.  I believe that other chargers have this capability as well, but I don't own any of them, so I can't say.

The Manzanita chargers were built incorporating what Manzanita calls the REG Bus.  I hear it does many things when coupled with other hardware that they sell.  But the truth is, I only need to use one of those functions, and that's the one that allows me to stop the charger.  

Pin 1 on the REG Bus supplies 5 volts DC.  Pin 2 has no voltage on it; however, if pin 2 sees 5 volts, the charger interprets this as an over charge condition and shuts the charging cycle off.  The charger remains on, with the fans cooling it, but the charging cycle terminates.  The bad side is, if you remove that 5 volts from pin 2, the charger immediately starts charging again.  So the trick is to get pin 1 and 2 shorted under the right conditions and leave them connected until the over charge condition goes away.  Enter the JLD 5740 volt meter.  At $37.50, it's a bargain.

The 5740 will measure anything from 0 - 500 Volts, DC or AC should you need it.  It has 2 relays that you can set independently.  You can set a relay to latch closed at one voltage, and open at a different voltage.  In this way, you can build in any logic for on/off you want.  Here's how I set mine up.

When I turn the meter on, the relay is open.  As the charge cycle runs, the voltage rises to the expected 164.5 volts.  If for some reason, the charger misses it's target, the meter is set to close the relay at 165.2 volts.  When the relay closes, the wire from pin 1 is shorted to the wire from pin 2 and the charger stops dead.  That voltage is low enough that no cell in the system will exceed 3.6 volts.  That relay then is latched closed until the voltage should drop below 158 volts.  Since the resting voltage of the system, after a charge is 160, then the meter will never (without intervention) open that relay, and the charger will never restart.  

When I built the car, I positioned a small switch under the charging door.  When that door is open the switch sends 12 volts to the charger, and it's presumed that I'm charging and the car is plugged in.  I run that 12 volt signal to the Zilla, which sees it and disables the car from moving.  I've simply spliced into that line and use that signal to power a separate relay, that then powers the meter.  I've incorporated the whole thing in a project box.  

Notice the neat glowing switch on top of the box.

The switch is depressed and the meter is on.

I've run the power from the switch in the charging port to a second switch in the project box.  If I want to use the meter to watch the charger, I simply press that button and the meter comes on.  When I press the switch again, or if I close the charging port door, the meter is turned off.  I don't need for it to be on when I'm not charging, so there's no reason to have that parasitic load running.  

The only thing I haven't mentioned is that the 12V from the car that powers the meter first runs through a small 3 Watt DC to DC converter that isolates the high voltage on the meter from the car's 12V system.  We don't want the high voltage system leaking back to the chassis.  Pretty slick, and the whole thing cost less than $50 and it works great.  

Heat: Not So Good for Chargers Either

A couple weeks ago, on a Monday morning, we were in off peak hours and I figured it would be a good time to top off the batteries before I ran the day's errands.  You may remember that I recently destroyed my eXpert Pro meter by re-applying the pack voltage to it and not removing 12V power first.  By the way, DON'T DO THAT!  Any way, I'd received a new one, but I hadn't yet installed it.  Without it, I have no idea how much current I'm pushing to the batteries, so I need to keep an eye on them to be sure I'm not over charging them.  The fact is you learn things when you watch what the batteries during charging.  Over time, I've learned quite a bit, and by watching them closely I can pretty much tell what's going on and predict when the charger will terminate.

At any rate, I know which cell will start to rise above 3.45V first, so I put a meter on it and started the charger.  For me, 3.45V is the top of the charge.  There simply isn't enough energy put into the cells above that point to justify the potential risk and damage to the cells by getting them to the manufacturer's 3.6V.  Ten minutes later, I walked out to see the voltage on the cell had climbed from it's initial reading of 3.26V  up to 3.380V.  I know by experience that means I'm about 35 minutes away from 3.45V when the charger should terminate it's charging sequence.  So you can imagine my surprise when I walked out 25 minutes later and found the battery at 3.364V, and the charger still running.  The voltage had dropped, yet the charger was still running.  I stood there for a moment, completely confused, it just didn't compute in my head.  Kind of like walking into your favorite BBQ place and seeing that it's full of vegans.  Not that there's anything wrong with it, but it just doesn't make sense.

Figuring that the charger must have just gotten confused (after all it does have a simple logic board in it), I figured I'd just reboot it.  I turned it off, waited 4 or 5 seconds and turned it back on.  The moment I turned it on, there was a succession of 3 pops, with the last one being quite loud and producing a bright orange flash and rush of hot gas shooting out the charger's vent port.  This was followed by me yelling "Oh shoot!", or something similar but perhaps more colorful.  I turned the charger back on... nothing.  Wasn't too surprised by that.  I checked the breaker on the house and had not tripped, so whatever failed in the charger failed in such a way that it simply consumed all the power coming into it and blew up, rather than shorting.  Or at least that's my take on it.

In either case, there was no doubt that I was going to need to remove the charger and send it back to Manzanita Micro for repair.  This was the second time I've had to do this.  The first was entirely self inflicted when I mistakenly shorted one of the batteries to the chassis with my multi-meter probe, while the car was charging.  That blew up the AC rectifier and melted the end of my probe.  I had no idea what happened here, but I boxed it up, got an RMA number and away it went to Washington.

It arrived in their shop on Friday and they sent me a note Monday afternoon that it was repaired, tested and ready for shipment.  Remarkably fast turn-around.  I called and spoke with Clarice, their office manager, to ask if they knew what caused the failure.  Clarice has seen enough repairs that she can recognize the likely cause of the failure just by looking at the parts that were replaced.  In this circumstance, both IGBTs failed as well as a capacitor.  She said she'd honestly never seen anything like it, but she'd ask Rich when he got it.  Rich thinks that one of the IGBTs failed and when it did, it took the other with it.  He thinks heat may have been a contributing factor.  This is where things get interesting and where there may be a lesson that all of you can learn at my expense.

As regular readers of this blog are aware, I live in Phoenix Arizona, which is slightly north of, and roughly rock throwing distance from Hell.  It's not uncommon for the inside of my garage to be 105°F.  If my wife pulls her car in after getting home from work, as it ticks itself cool, all the heat that was happily stored in all of that steel, gradually works it's way into the garage raising the temperature well over 110°F.  It's lovely.

Here's the thing.  Two years ago the car lived through it's first summer as an EV.  A significant portion of that summer saw the car dis-assembled because I had sent the motor back to Netgain because the balancing putty had fallen off.  Plus I took the opportunity to redo the battery layout.  So when it was back on the road in late July and I was charging it on a regular basis, I would come out to find the charger's yellow light flashing at me.  I went back to read the manual to find out what that meant, and I couldn't find a reference to what that was all about anywhere.  I figured it was just an oddity with this charger.  What it really was, was the charger warning me it was over heating.  I blissfully ignored this warning for the remaining portion of the summer.

As it happens, the car was taken apart for a good portion of the following summer to again fix a balancing putty issue on the motor, and to add air conditioning to the car.  In fact, I didn't get it back on the road until the first week of September, just in time to tow it out to EVCCON 2011.  But even then I would see the flashing yellow when charging, but by then I'd figured out what it was so I would dial the current back until the light stopped flashing.

That's been my modus operandi this summer.  I'd turn the charger on and dial in between 21 and 22 amps.  I'd poke my head back into the garage in 20 minutes or so and if the charger was over heating, I'd turn it down.  About 90% of the time, it was over heating.  I'd turn it down to 16 amps or so.  In retrospect, I think this behavior, and the initial instances of ignoring the warning, damaged the charger over time.  Heat is the enemy of all circuitry and when the charger was overheating, it felt quite literally like a blow drier firing out of that little vent port.  So my advice to you is don't to this.  I realize that most of you don't live in such hot climates, but for the few that do, pay attention to your charger.  Heat kills.

At this point, I intend to charge at 16 amps or so during the hotter months to protect the charger.  It's remarkable how much cooler the unit runs at that current level.  The problem is the charger was configured at a higher current level.  That means if I charge at a lower current level, there's a risk that the charger will overshoot the voltage I've set and consequently over charge the batteries.  Re-tuning the charger for a lower level is possible, but such a huge pain in the butt that I don't want to do it for the 1 month it's necessary before cooler weather arrives.  But I would like to be able to charge at any level I like and not risk over charging the cells.  I do have a solution in the works which I will be implementing and writing about soon, so stay tuned for that.  In the mean time, I'm going to have to watch the end of the charging curve very carefully to protect the batteries.

What I've Learned This Year

You may have noticed that I haven't posted anything in a while.  That's largely due to the fact that nothing interesting has happened in a while.  I'm pleased to report the Z3 continues to zip along trouble free and in relative quiet (still can't find that pesky rattle).  People occasionally ask me about the car, but I haven't had any interactions that are worth mentioning lately.  I've had a number of people tell me I should drive it down to the BMW dealership nearby and show it to them.  I'm kind of torn on this, I can see it going one of two ways.  One, they are interested and a few people come out to look at it.  Or two, they don't give a damn and are irritated that I'm taking up their time and not there to buy a new car from them.  I'm just not sure it's worth my time.

The batteries have been a complete non-issue.  Meaning that I charge them, I discharge them, and they do nothing out of the ordinary or unexpected.  I still need to bottom balance the pack, but I simply haven't had the time, what with the holidays and all.  However, I have had time to think about it and draw my own conclusions on the topic.

As I see it, bottom balancing means I'm not going to run the risk of ruining one or more cells by driving them to reversal if I draw the pack down too far.  So that's one argument for the case.  I may not be able to charge all the cells all the way up if they have slightly different capacity, but that difference is so minimal, it has almost no impact on range; less than a mile, probably 1/2 a mile at worst.  I may have to tinker with the charger to make sure it cuts off at the 3.45 volts I've been using, but so be it.  Having to set the charger for the proper cut-off is necessary regardless where you balance the pack, top or bottom, or even if it's not balanced at all.  So that is not a factor.  If we tally that up, we have one positive and two non-issues.

If I top balance the pack, I can be sure all the batteries are fully charged at the end of a charging cycle, which gets me nothing really.  I may be able to squeeze out that extra 1/2 mile I mentioned before, but there is always the ever present danger of reversing a cell or two if I discharge too far because some hit bottom first.  Setting the charger for the proper cutoff still has to be done, so that's not any different from bottom balancing.  So, that's one strike against top balancing, and two non-issues.  If you weigh it up, you see where the balance falls (no pun intended).

I learned something about Ohm's law in September.  I had no idea why turning down the current I was charging at would push the batteries to too high a voltage.  To be clear, I had no idea it would happen, and after it did, no idea why!  I tinkered around with the charger settings for a month before I understood what was happening, but still had no idea why.  Jack Rickard was kind enough to explain it to me and mentioned that even he'd fallen victim to it.  I suggested to him that if we both had been bitten, others might as well, and that he may want to mention it on one of his weekly shows.  Boy did he!  Fully half of one of his shows was dedicated to showing how I messed up and explaining why it happened.  To me, this may be one of the best shows he's published.  If we can point out the land mines involved with these batteries, hopefully people won't step on them.  They really are the best solution for enthusiast, home built EVs at this time.  Fortunately, the batteries are robust enough to withstand even my clumsiness. Dr. Jay Whitacre pointed out that as long as the cells are not charged above 4.3 volts, they are more or less unaffected.  Mine never came close to that kind of voltage.

From the time the car rolled out of the garage until roughly two months ago, I watched the Link-10 meter like a hawk, and range anxiety was a real issue for me.  I suppose that watching the meter over that time was necessary for learning purposes.  But eventually what I learned is that there really isn't much reason to watch it at all.  I know how far the car will go, I know how far my trips are, so range anxiety is really not part of my reality anymore.  This year I've come across only a handful of instances where I needed to use my ICE car instead of the Z3. Most of those times it was because I needed to carry more than one person.  One of those was because I needed to carry some 2x4's home from the hardware store (tough to do in a Z3).  I can recall only one occasion where I needed the ICE because I needed to drive across town and range was an issue.

Summer was tough.  It was too hot to drive the car because there was no AC.  The Zilla kept flashing at me warning that it was getting too hot.  I'd back off the throttle, or coast,a bit and the Zilla would stop complaining.  I can fix the AC problem, but it will involve removing the batteries up front and hiring someone to come out and fabricate hoses for me.  I could throw a bigger radiator at the Zilla to dissipate heat better, but I'm not too sure that would be very effective when the air is 120 °F or hotter on the street.  I may try, but it may be a mute point as I'm about to explain.

My real concern is the batteries.  They typically warm up to between 15° to 25° over the ambient air temperature, and that's the temperature taken at the terminals, internally it's bound to be warmer. I have no idea how much warmer, but it's probably on a few degrees.  They all have plenty of airflow around them, but there is only so much cooling 120° air can do.  The problem is the electrolyte in the cells starts to degrade between 135° and 140 °F.  It's easy to see how that temperature can be achieved in the Arizona summer if the batteries run 25° hotter than the ambient air.  Keep in mind that when you hear that it's 110° in Phoenix, that's at the airport.  If you actually measure the temperature of the air over any surface street in the valley, it's going to be North of 118 °F.  Heat is the second biggest threat to these batteries, and unfortunately summertime in Phoenix has that in spades.  I may very well end up using the car only 9 months out of the year, which is too bad.

There you have it; the good and the bad, the ups and the downs.  Nothing really earth shattering on either side of the spectrum, and I suppose that's good.  I was shooting for a car that I could just charge and drive, relatively worry free, and that's pretty much what I got, now that everything has settled down from the motor problems in May and the charging problems in September.  I've put over 3150 miles on the car.  It has become my daily driver.  It's rare that I have to drive my ICE car.  I've said this many, many times before and I think that manufacturers need to use this logic when selling EVs, If you have more than one car, the chances are very good that one of them could be an EV and you wouldn't notice a difference in your life except that it would save you money in the long run.  If you have more than 2 cars, then it's an almost certainty that one of them could be an EV and you'd never feel any inconvenience apart from a heavier wallet.

When we get to March 2nd, the anniversary of the Z3's road debut, I'll publish every sordid detail and piece of information I can find on the car.  I've been keeping tabs on quite a lot of data, and I'll share it all then.  In the mean time have a wonderful new year, and thanks for reading!

Charging Mystery Solved

Well at least I think it's solved.  The mystery I'm referring to is why did some of the batteries suddenly start accepting more charge than they should have?  My first assumption was that they'd moved in their state of charge relative to the others.  But really I had no reason to believe that other than I couldn't think of any other reason.

It was then suggested to me that perhaps those particular cells simply have a lower capacity than the others so they reached their full state of charge first.  I think that's likely the case, but truthfully it could be a combination of both reasons for all I know.  But that really doesn't answer the fundamental question as to why, when they'd been behaving in a very predictable way for 5 months, did they suddenly start showing the tendency to run away? 

Well I think I figured it out today, and I'll explain, but I need to give you some background first.  I'd mentioned before that the Manzanita Micro charger has a little potentiometer screw that you use to set the cut off voltage for charging.  When the pack reaches that voltage, it switches from constant current to constant voltage and starts ramping down the current according to a timer you set.  I'd also mentioned that it can be a finicky thing to set. 

Some time at the beginning of August, I was charging the car and I happened to stay in the garage for a while.  Normally I charge it at night during off peak hours and I just let it finish on it's own.  Well this time I hung around, cleaning up a bit and I saw the charger start flashing, warning that it was overheating.  When I set the charger up last February, it was nice and cool in the garage.  Not so much in August.  To make a long story short, I realized I didn't want to shorten the life of the charger by overheating it every time I charged up, so I decided to turn down the current a bit.  Turned up all the way and it sends 28 amps to the batteries.  I decided I'd turn it down to 15.  Problem solved.

Well, little did I know that I'd just introduced different problem.  Apparently the point at which the charger changes from constant current to constant voltage is also dependent on the current out.  I now have it set to start it's ramp down when the charger reaches 164.7 volts.  But that was set when I was pushing 15 amps.  Today was a beautiful cool day, so I decided to give it the full 28 amps and low and behold, the charger switched to constant voltage when the pack hit 163.4 volts, leaving the pack about 3.2 kWh's short of it's full capacity.  I stood there for a second thinking "what the hell?"

I started experimenting with different charging schemes and trips around the neighborhood to draw some power from the pack.  I left the potentiometer screw alone and just tried different charging currents and I found a curve.  Send the batteries less current and the cutoff voltage is higher.  Send it higher current and the cutoff voltage dropped.  Here's what it looked like:

15 amps = 164.7 cutoff
20 amps = 164.4 cutoff
25 amps = 163.9 cutoff
28 amps = 163.4 cutoff

So back in August I simply dropped the current I was pushing through the charger to keep it from over heating, and the result was that the cut off voltage I had so carefully tuned for 28 amps went up.  Suddenly it was too high and ultimately ended up pushing more current to the pack than it should have.  I don't know the exact date this happened, so I don't know exactly how many times it happened.  I think it's more than 3 but less than 7 times.

But there's a lesson to be learned here.  Don't change any thing, be it a setting or a process, and expect everything else to remain the same. Always check and double check when making a change to a procedure or setting.  Also, for any of you that have a Manzanita charger, or are thinking of getting one, remember this potential issue.  Don't get caught by it.  I don't consider this to be a major flaw in the charger, it's minor at worst.  But forewarned is forearmed. 

On Battery Balancing and Charging

As I've mentioned in the past, I'm not an expert.  On anything.  But in the time I've been operating the car and charging it, I've noticed a few things about these LiFePo4 batteries that I think is worth sharing.  In addition, think you can draw some simple conclusions from these experiences.

You may remember that just over a month ago I noticed that a few of the batteries were hitting their max voltage before the others.  The question became, did those cells move in their state of charge relative to the others, or do the simply have a slightly lower capacity.  My assumption right away was that they'd drifted.  Quite frankly, it never occurred to me that they might just have a lower capacity, which meant I immediately thought they'd moved relative to the other cells.  Of course the first thing I thought to do to fix that was balance them to the others.  I drew energy out of the all cells until every cell matched, down to 1/100 of a volt.  I drove the car a few miles and then charged up.

That proved to be a complete waste of time.  The very same cells that I so carefully matched to the rest exhibited the same behavior the very next time I charged them.  This time though, I was watching them. But I'd say this is a clear indication that those cells simply reach their fully charged state before the others. They had not moved relative to the other cells, they just have less capacity.  We're talking about a fraction of an amp/hour.

Early on, and for the first month I carefully watched the batteries every time I charged them.  I saw very small differences in the finished voltages between the cells; one hundredth of a volt here, 2 hundredths there.  And so it went until I had to pull the motor.  After the motor was back in, they did the same thing, but I was checking them less often.  I guess you could say I got complacent. I went from checking once a week, to letting it go nearly a month.  When I did that last check, I found those cells I mentioned running away. There's no question they'd done it before, but there's really no way for me to determine how many times.  I know it was less than 20, more than once.  I also think there's no question those cells sustained some damaged by it.  How much remains to be seen, but I don't think it's substantial.

I think it's likely that those cells were a slightly lower capacity than the others to start with, so they filled up first.  In that 20 or so charges, they'd reach their 3.6V maximum and be forced a little higher. Each time, just a little further.  In this way they accrued a little more damage each time.  Had I watched them more carefully, I would have caught it.  But, it is what it is, and lesson learned.  Since I rarely pull more than 50% capacity from these batteries, I don't think I'll ever really feel the impact of that.  Perhaps I'll get only 2900 cycles out of those cells instead of 3000.

But all of this got me thinking about battery management systems. I don't mean to imply I never thought of them before.  I have, a lot. But, the ones on the market shunt off power in the form of heat when a cell reaches capacity.  I can't stress enough how much I don't want that anywhere near my car.  Maybe that extra heat is OK when you're in Minnesota.  It's not in Arizona.  No extra heat is acceptable as far as I'm concerned.  I'd rather ruin a set of batteries than burn my house down.

I believe what would be ideal is a charging/battery monitoring system that has the capability of charging cells individually.  A monitor on each cell would watch the cell, and when it reached it's determined state of charge, the monitor would simply shut the charging for that cell off.  Not shunt the power away, but rather throw a relay that would simply stop power flowing to that cell.  Of course it would need to send signal back to the charger so the charger would know that one less battery was accepting a charge.  The charger would then ramp down the current it's putting out by the corresponding amount that the deactivated cell had been drawing.  Otherwise, you'd have a cascade failure as more cells dropped off line, the current traveling to the remaining cells would climb until you started burning wires.

This strikes me as spectacularly complex.  Doable, but due to the complexity, likely very expensive.  Not only that, but complicated which means an increase in the likelihood of failure. You could certainly design the system so that failures resulted in a system shutdown, not a runaway charger.  But could such a system be designed?  I think so.  The net result is batteries charged to the same voltage every time.  This of course would be a top balanced pack.  Jack Rickard has done quite a bit of work and demonstrated the dangers of top balancing.  So the only way this is practical is if you simply hold the bottom 5% of the batteries state of charge as off limits.  Meaning you could never run the batteries close to depleted for fear of having a cell reverse it's self. 

The next best option would be similar but much more simple.  A similar system with a monitor on each cell.  It watches the voltage for one cell.  When any cell reaches the target charge state, it sends a message to a main board that in turn trips a relay that cuts power to the charger.  Sure the batteries might be at slightly different states of charge, but who cares.  No battery is ever endangered and your pack is charged.  The advantage to this is that you could, if you chose, bottom balance you're pack with this set up.  Once balanced the charger would cut off as soon as the first battery reached it's limit and there would be no effort to move the others any further, keeping the batteries in the same positions relative to each other.

It seems to me that this would be a far simpler solution to charging and protecting your batteries and far safer than creating heat in a confined space but shunting off lots of power.  I would consider putting that in my car.  Heck I'd design it and build it myself if I were talented enough.

When it all comes down to it, I think my problem was one of charger set up.  Setting the charger and thinking once it was tuned in nothing would change was clearly a mistake.  Like I said, lesson learned.

One More Thing

Today I met with a gentleman named Alex who has been following the EV Z3 blog and the progress with the car since just before it hit the road.  Alex is traveling at the moment and had a stop over here in Arizona, so we decided to take the opportunity to meet up.  We had a great time.  We took the car through down town Chandler, (which is unfortunately under construction) and out on the freeway.  People often ask whether it can reach freeway speeds, so I thought it would be a good idea to show Alex just how well it does. 

He's been considering doing a conversion of his own for a while, although his ideal car to convert is a Porsche 911.  How great would that be?!  Hopefully the Z3 helped to inspire him to tackle the project some day.  Keep in touch Alex, and if you decide to tackle that Porsche, let me know so that I can follow the blog on your build.  You will keep a blog, right?  :) Thanks again!

More Battery Info

I've spent the last week and a half working with the batteries and charger, trying to fine tune and develop a charging plan. The more I've worked on this, the more I'm starting to think the problems I've encountered up to this point were simply a result of setting charger's cutoff voltage too high.

Initially I'd set it at 168 volts, because that's the target voltage I wanted (3.5 x 48). But of course, that was too high because the charger simply begins it's ramp down of current at that level. That lasted one charge, then I turned it down to 166.5. That worked out well as the voltage on the pack was at 168 when the charger finally cut off.

But clearly as the cells broke in, or started showing their individuality, some started reaching their target voltage before others. In terms of actual current pushed to the cell, the differences are so small. In the last post, I'd figured that the difference between the extremes was about 7.3 Watt hours. That may actually be a bit high. It may be closer to 5 Watt hours.

I lowered the chargers cutoff to 165 volts, and that seemed to be working quite well. That put the finishing voltage at around 166.8 volts. The batteries prone to runaway were hitting about 3.58 volts, by the time the charger kicked off. The rest of the cells were only just behind. The difference in the entire pack's charge state reduced by only 150 Watt hours or so. But on two occasions, I saw one of the cells climb to 3.60 volts when there was around a minute left on the charging cycle. Had I let it go, they would have climbed to somewhere close to 3.64 Clearly, I needed to back it off just a bit more.

This morning, I charged the pack with the charger starting it's ramp down at 164.5 volts. It finished at 166.3 volts, with the highest cell at 3.52 volts and the lowest at 3.43. That, I think, will be just fine if we get something like that each time. I'll continue monitoring over the next few weeks to see how things progress, but it's looking good at the moment. As best I can figure, that cost me another 100 Watt hours. So I'm down my a grand total of 250 Watt hours in the pack. Less than one mile.

Springs

I had a local shop look at the springs on the car Tuesday. Since the car was riding 3/4" lower than stock when I got it, I had always wondered if it had received aftermarket springs to lower it, or if the original springs were just tired. They invited me down to take a look. Turns out they are the originals, which means I have two options. I could get coil overs for around $1500. Or I could have some springs made. A place out of Kansas City will fabricate new springs for me which will take the proper weight, if I send them one of my springs, some geometry measurements off the suspension, the new weight of the car and $285. Sounds like a good deal to me, but I'm not in a position to take a spring off my car. I need to find one at a local salvage yard. Over all the car is riding 1.5" lower than stock, but just on the front end. The back end is perfect. I look like I'm going down hill all the time.

Plates

The good people at ADOT have seen fit to grant my request for a custom license plate. Check it out!


How sweet is that?!

First Charge After Balancing

Yesterday I drew off about 5 kWhs from the battery pack while driving the car around. This was after I had balanced it the day before. So I was all set to charge the pack to find out how the cells behave.

Before I get to that though, I had been in contact with Jack Rickard letting him know of the imbalance in the pack. Jack is the champion of the "No BMS necessary" movement. He warned me of dangers involved in "top balancing" the pack. For those of you unaware, the concept here is that there is far more danger involved in having the state of the cells differ while down at the bottom of the charge curve, than while at the top. The danger being that if one cell drops below the 2.0 voltage limit, the others will drive it into death very quickly.

I believe this to be a real threat, Jack's tests have demonstrated this. But for the moment, I'm not worried about cell behavior at the bottom of the curve as I'm not taking them there. I am worried about what they're doing at the top because that's where I'm seeing the discrepancy. But it looks like that concern may be misplaced.

When I charged the pack last night, I watched as all the batteries marched up the voltage curve in nice even step. They all arrived at 3.38 volts together. But then a few of them hit 3.39 while others lagged behind. There was one cell in particular that started climbing faster than the rest. When that cell hit 3.50 volts, the others were around 3.44, plus or minus .02 volts. I watched carefully until that one cell hit 3.60 and I killed the charger. By that time the others were around 3.49 volts.

So now I think I know what's happening. I have my charger set to cut off when the pack reaches 165 volts. Or rather, it starts ramping down the current when it gets there. The idea being that it stops charging when the pack is at 168 volts. That would put each cell at 3.5 volts. Early on I had made the assumption that the cells that were coming up too fast had shifted in the pack, that their state of charge had moved relative to the others. I don't think this was the case.

Jack had pointed out that another possibility is these cells have developed a diminished capacity. Well, I think that is the likely explanation as sad as that is. Think of the batteries as if they were a jar you poured fluid in. If one jar in the mix suddenly has a little less capacity (you've dropped a rock in it) when you're adding water to them all evenly, one will start to overflow before the others. What is clear is that it wasn't a result of cell drift within the pack because balancing them would have resolved the difference. The question at this point is how much has this rogue cell lost?

Keeping in mind the characteristics of these cells to runaway very quickly at the top and bottom of the charge curve, what I need to do is figure out how much more time would be required to fill the rest of the batteries after this one was charged completely. Based on what I was seeing during that last charge, I'd place that at about 5 minutes. I know that each cell has 408 Watt hours (120 Ah x 3.4 volts). During charging, each cell gets 1.46 Watts per minute. So if this cell is full 5 minutes before the others, that means it's capacity is 7.3 Watt hours below the others, or 1.8%. It also means that if I cut off the charger when that cell has reached it's peak, the rest are missing out on that 5 minutes of charging, which translates into 350 Watt hours, or one mile of travel (at this time).

To the best of my knowledge, all this is correct; however, I'm not above making boneheaded mistakes, as anyone who reads this blog can attest to. But all of this raises a few questions. The most obvious is would a BMS help the situation. I think it would, but there are trade offs I'm not comfortable with. The first being the cost. The second being the fact that the extra power to the cells which reach their capacity first is simply shunted off as heat. No thanks, it's already 110 °F in my garage. There's no way I'm risking setting my car on fire. To me, that negates all the benefits that come with the slight reduction in worry to the health of the pack.

Besides, at this point, what's it going to gain me? It would seem one mile. If I wanted to be extra sure I'm not endangering the pack, I could dial the charger back a bit more, reduce my range by say 5 miles and really ensure there's no risk to the pack! I've got 60 miles to play with, what's 5? Nope, still not seeing a compelling reason to add a BMS.

The other question is would it be worth going through the process of bottom balancing the pack? I'm not entirely sure what it would gain me at this point. It would make longer trips safer for the batteries as they'd all meet their discharge point at the same time. But I never really drive the car more than 40 miles at a time, usually only 25. That's something I'll reserve for some future time.

For now, I need to fine tune my charger to be sure and protect that cell. The others that weren't far behind it in voltage during last Friday's charge are presumably in the same boat, but not as bad. It stands to reason, if I charge the pack being sure to protect that cell from over charge, the rest are safe. Still, I'm going to have to watch them all for any further deviation.

Battery Balance

I'd hoped this day wouldn't come, and that this problem wouldn't occur, but it has. I was charging the batteries last Friday and I found a couple of them heading North of the target 3.5 Volts. CALB (the manufacturer of these cells) recommends that you charge them to no higher than 3.6 volts. You can imagine my concern when I found 4 of them higher than 3.8! Well, that just won't do, and if that continues, I'll shorten the life expectancy of these rather expensive batteries.

Now, I know what some of you are saying. "Of course you idiot, you're not using a BMS. You must use a BMS with these cells, what did you expect!?" To you folks I say you're right. Sort of.

There is no question that these, and in fact all batteries need battery management of some sort. The conventional wisdom regarding LiFePo4 cells is that you must use an automated BMS to protect them from overcharging. But, keep in mind that even lead acid batteries need BMS. The primary difference being that people tend to manage lead acid batteries manually, watering them and equalizing them on occasion. As I see it, the real question is whether you need an expensive automated BMS system for these LiFePo4 cells, or can you manage them yourself like you would lead acid batteries?

So far these batteries have been really boring. They've all charged up to 3.5 volts reliably and stayed in balance nicely. Keep in mind that after you remove the charger current and let the cells settle, they end up around 3.4 Volts. Their stability had lulled me into complacency and I hadn't checked the individual cells while charging for a month or so. In that time, a few of them drifted. I don't know why; the heat in the garage, bad connections, differences within the cells themselves? Chances are I'll never know, and in all likelihood they'll do it again.

Right now I'm equalizing them, getting each battery down to 3.350 Volts. This is a long, boring procedure, but not particularly difficult. I've simply hooked a 12 volt brake light to some alligator clamps. I clamp the meter to the posts and then the bulb. The bulb glows and burns off the extra electricity. It's just a question of stepping through each cell. About half are at the right voltage already. The rest are within 0.05 Volts, except the 4 naughty ones that were at 3.6 after resting, or .25 Volts too high. Like I said, not difficult, just boring. You must pay close attention.

Now the question becomes how often am I going to have to do this. Lets assume that it's in their very nature for the batteries to do this. Then how often will they drift? What's the rate of drift? Can I expect them to fall out of balance on the very first charge, or will it take 6 months? Will it be a gradual change, or will it happen within one charging session? The answer to these questions really determines the risk that the batteries face and whether I'm going to need to give in and get a BMS.

If the past 7 months can be used as a baseline, I'm thinking that I may be able to go 4 months before I need to worry about balancing them. Remember there was about 2 months of down time when I had to remove the motor. If I really can wait 4 months before I need to take action, then I'd say a BMS is not necessary. However, if I charge them up tomorrow and see some of the cells immediately going higher than 3.5 volts, that likely means that the only way to prevent that is a BMS system. If the mean time to imbalance (mti, for all us geeks who live in a world of three letter acronyms) is somewhere in between those two figures, then it just becomes a question of weighing the inconvenience involved in manually balancing them, and how often I have to do it. Not to mention whether I'm willing to endure the pain of having to regularly measure the cells close to the end of a charging cycle.

One thing is certain, and that is I'm going to have to monitor them very closely over the next several months to get a clear idea of what's happening and when. In either case, I intend to protect these cells from that kind of event again. Along the way, I'll record what I'm seeing here. Those of you who care to follow along may learn a thing or two about the cells and how they behave. I may find a BMS is essential. Or I may find that with a little care, you can do just fine without one. Apart from the fact that I'd rather not spend somewhere around $3,000 for a BMS, I have no passion for one position over the other.

A Few Things Worth Mentioning

Today, my dad and I finished up the electric run from the service panel to the garage. I'm waiting on one part (the outlet) so that I can wire it to the box and then call the city for inspection.

Long ago I had chosen the form factor for the power plugs I would be using. Unfortunately, it was before I really understood what kind of power I would be drawing. I ended up choosing components that simply aren't up to the current I'll be sending through them.

I found out what I really need is something that can handle 240 Volts and up to 30 Amps. The best fittings for the job turn out to be NEMA L6-30. Since almost none of it is installed, it's no big deal, I just have to get the right parts and install them. Unfortunately, there is one bit that is installed, and that's the receptacle in the car. So, I'll be needing to take that apart and re-do it. The truth is, I'm kind of glad, because the new configuration will be sturdier and it will look way cooler. I'll include pictures once I'm done.

However, the best news for the day just came from FedEx. They called to arrange delivery for the batteries. Tuesday the 19th, between 2 PM and 6 PM, I will be losing some floor space in my garage to a palate of batteries. Joy!

Good News Everyone!

I've received word that the batteries have arrived in the country, Seattle to be specific. The company I ordered them from is going through the shipping container and portioning out everyone's order. I should have the batteries here within a couple weeks.

While I'm talking about batteries, it's occurred to me that I haven't really gone into too much detail about them, the range they should provide, and other such pieces of data. If you're interested in the nitty gritty numbers read on. Other wise... you've been warned.

I'm getting 48 of the Sky Energy SE120AHA batteries. They have a nominal voltage of 3.4 volts and provide 120 Amp/Hours. The batteries will be wired in series. This is just like the batteries in a normal TV remote are set up (positive to negative, positive to negative, etc). What that means is that all of the batteries become one big battery, and the voltage of that one big battery is the total of each added together. So, the math looks like this:

3.4 x 48 = 163.2 Volts

So that whole pack will have a nominal voltage of about 163 volts. Each battery offers 120 Amp/Hours of current, so to figure out the total number of Watts provided, you multiply the Volts by the amps:

163.2 x 120 = 19584 Watt/Hours Or roughly 19.6 kilowatt/hours.

Now, I don't want to draw all the current out of the batteries, because that will shorten the life. If I use only 80% of the power, or less, I'll extend the life of the batteries to upwards of 3000 cycles. So, if I want to leave at least 20% of the power in the battery, that means I'll get to use this much power:

19.6 kWh x .80 = 15.68 kWh

The car will likely use between 225 and 250 Watts/mile. Erring on the conservative side, lets say it will be 250. So then we figure out the range of the car, using only 80% of the current:

15680 / 250 = 62.7

So conservatively, I should have a range of 62 miles. Of course if I'm flooring it when I leave every stop light, that number goes down. If I'm on a freeway going 65 miles an hour, that number will go down. If I travel conservatively on a surface street, traveling no faster than 45, I should be able to extend that a bit. Or in the event I need to tap into that remaining 20% reserve energy for some reason, I could get as much as 16 more miles out the car.

You can see why people that drive EV's cringe at the question "How far can you go on a charge?" (Would you really want to explain that every time?) Or why the manufacturers that are just getting into the market of promoting EV's that they hope to build at some undetermined time in the future are reluctant to give out those numbers. Well that's not true of all manufacturers. Some promise 100 miles when practical experience or simple math like I've done above shows that the range would be more like 50.

To charge the car is simply a matter of how much electricity I can get a hold of and how fast I can dump it into the battery. The charger I have will run off of anything from 110 to 240. It has a dial that will allow me to limit the current (amps) it draws from the socket so that if I'm at a friend's house and they let me charge the car, I don't draw more than the outlet's breaker is rated for. The charger can only draw 20 amps off the service line which is probably the biggest limiting factor I face. But if the battery pack voltage is less than the outlet's, the charger modifies the power by decreasing the voltage to the battery pack, while at the same time increasing the amps it puts out to as much as 30. Such will be the case with the service line I'm running to the garage now.

For the sake of simplicity, lets assume I've drawn the battery pack down by 15 kWh's and the charger opeates at 100% efficiency (more on that in a moment). The charger will essentially be putting out 172.8 volts (the charge voltage for the battery pack) at 30 amps. Here's how it pans out:

172.8 volts x 30 amps = 5184 watts

15000 watt hours / 5184 watts = 2.9 hours

A 120 circuit is a different story. A 120 volt circuit putting out 20 amps equates to 2400 watts no matter how you cut it. The charger will modify that by raising the voltage to 172.8, but that means the amperage output will drop correspondingly to 13.8.

172.8 volts x 13.8 amps = 2400 watts

15000 watt hours / 2400 watts = 6.25 hours

As you can see a 120 volt outlet will take a while longer, but 6.25 hours is still quite acceptable. Now there is some loss when the charger is modifying the power from the wall and pushing it to the battery pack. It won't affect the time on the higher voltage charge, but the 120 volt charge time will likely go up by an hour or so.

So, there you go. Way more data than you ever wanted to know. When ever anyone asks me about range I'll simply refer them to this page. :-)