Saturday, October 30, 2010

Back to a Routine

With the last couple month's posts dedicated to various issues surrounding charging and the batteries, I'm happy to report things have become boring again.

I've already written about the two mistakes I made through this whole thing, hoping that others can learn from my mistakes.  First, if you have a charger with the capacity to adjust the current it pushes to the batteries, don't expect that adjusting it up or down will have no influence on the total amount of energy that is ultimately pushed to the pack.  Second, a misguided attempt to balance the batteries has done nothing good.  In fact it's forced me to sacrifice a small portion of the top of the pack and likely made the bottom more perilous (but I don't intend to find that out).

My goal since all of this began was to find the correct cutoff voltage for the charger.  It was a long process, trying different voltages, watching the batteries closely each time.  To be honest, it sucked.  But during the whole endeavor, I never allowed any of the cells to pass the 3.6 volt cell limit.  Ultimately what I found was setting the chargers cutoff voltage for 164.4 volts, at a current of 20 amps seems to do the trick nicely.

The goal has been to charge the pack so that no cell exceeds 3.45 volts.  You may recall that originally I was charging to 3.5 volts, but when attempting that now, I found that some cells would occasionally try to head North of 3.6 with a few minutes left on the charger's timer.  So, 3.45 seems to be the order of the day.

I've charged the pack some 35 times now and in every case it has gone perfectly.  The highest I've seen any cell go was 3.46, and that happened once. Close enough.  The difference in capacity seems to be a whopping 1.3 amp/hours, which equates to 3/5's of a mile.  I'll just trim off two or three quick launches from stop lights and call it even.

I'll continue monitoring them closely for some time, just to be sure nothing strange happens, and I'll report it if it does.  For now though, we're back to a boring, predictable routine of  charge, discharge.  And I'm quite happy with that.

Thursday, October 21, 2010

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. 

Wednesday, October 13, 2010

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!

Thursday, October 7, 2010

The Charger Has Returned Home

The charger arrived back from Manzanita Micro today, safe, sound and in perfect working order. They received it last Thursday, and had it fixed and on it's way back to me by Monday. I found out that it was the AC rectifier bridge that I'd damaged. They repaired that and then fully tested the unit to make sure it was healthy in all respects. The grand total in charges... $125, and $50 of that was shipping. How great is that!? I seriously hope I never have to send it back, but if I do, I know it's in great hands.

I got it back and began re-assembling the car. I hadn't done too much to it, so it only took me an hour or so. It was with some trepidation that I threw the switch to start charging the car. But it turned out my concerns were not needed. It fired up and started working like a dream.

I would still like to install the inrush limiters, or thermistors on the DC to DC converters. I bought a little project box, meaning to do it during this week's down time. But I found out more about they way they work which forced me to change my plans. When cold, they offer high resistance to the current, which keeps the arcing to a minimum, but they quickly heat up as current flows through them and the resistance drops. It's not unusual for them to reach 200° F. Well the project box I bought was plastic, so that wouldn't do. But worse, the only place I have to mount them is against the plastic box in the trunk that holds the charger and DC to DC converters.

So, I need to re-think how I'm going to do that, and where I'm going to mount them. I'm kind of at a loss right now, but I'll figure something out eventually. I could disassemble the DC to DC converters and actually put them inside the housing, but I really don't want to do that. Regardless, tomorrow I'm back on the road with a nice big EV grin!


A friend of mine named Fred and I have launched a new blog. It is called F1-Geeks, and as you might have guessed, it's devoted to Formula 1. But that's not all! We actually intend to talk about several topics that we and others like us find particularly interesting; cars, gaming, tech gadgets, home theater stuff and EV stuff too. Check it out, and leave us a comment or two if you feel like it. Thanks!