Insulation

Here we have a nice shot of the damage caused to the terminal when my meter's probe slipped off the top of the nut and shorted against the base of the terminal and that aluminum angle piece to the right.


Clearly, I couldn't leave it like that. Being as clumsy as I am, it's bound to happen again as long as that aluminum perches so dangerously close to those terminals. I can't move the angle aluminum because it's what's holding the batteries in the car. I figured the best thing to do was to put some sort of rubber insulation on the aluminum to keep any accidental shorts from occurring. So, I went down to the local hardware store to look for a sheet of rubber. What I ended up finding was a sheet of PVC used for lining the floor under a shower pan. Waterproof, flexible, and more importantly, electrically non-conductive. It's sold by the linear foot, and one foot of the stuff cost me less than $6.

I cut a few carefully measured strips from the sheet and painted some contact adhesive on one side.


I masked off the angle aluminum as best I could and painted those with the adhesive as well. After a couple hours of drying time I carefully stuck the strips to the aluminum.

It stuck on very well and should do the job. I had cut the strips so that I could wrap them around the bend, hoping that the sheet would stick to the inside edge's 1/8" surface of the aluminum. But the material simply isn't flexible enough to make that bend. If I could find a sheet of real rubber, and if it were thin enough, it would make that bend, but I think this will work fine. Take a look, there's no way to short the terminals against those aluminum pieces by accident.

Chargers Away

One of the tough things about building this car was the extremely tight spaces I had to work with and in. Getting my arms back there to turn the screws and bolts that held the charger in was so difficult. It took me the better part of an hour, but I finally liberated the charger from it's home.


Here's the charger, which looks totally fine.


I packaged it up very carefully and handed it to UPS this afternoon. In an email exchange with Rich from Manzanita, he said that assuming nothing other than the input AC rectifier is damaged, they should be able to fix it up in a day or two for around $200. How great is that?!

As I've mentioned before, I need to raise the height of the front of the car. I found a place that will build new springs for me, but they want one of the springs and some measurements off of the front suspension. Well, I can't take a spring off the car and mail it away, I'm using them at the moment. While looking around on eBay for a spare spring I can send off to Kansas, I came across an item that I had always felt sure must have existed, but I'd never seen. A quick search in Google and I found these items.



These are spacers that you can slip between the coils of a spring to give it up to a 1" lift. The springs on the Z3 were already riding low when I bought the car. The car was riding about 3/4" lower than stock. With the extra weight of the batteries and motor, it dropped another 3/4".

I took a look at the springs and the bottom 2 coils were very close to each other. I jacked the car up one side at a time and slipped them in, wedging them as far down in the coil as I could. I sat the car back down, drove it around a bit and then measured the ride height. It gained 2cm, or .79". That's more than I lost from doing the conversion. It looks much better too.

Of course that means I've lost a bit of spring travel, but not 2 cm since the gap I squeezed the rubber piece into was more like 1 cm high when the car was on the ground. The real question is will that loss of 1 cm of spring travel cause me problems. I don't think it will, I already drive it very carefully and slowly over bumps. But it's the bumps you don't see that get you. The good thing is that if the spring should collapse all the way, that big grommet is rubber and will flex. Hopefully enough to protect the pillar.

I think it's a good band-aid solution, but I'm going to continue my efforts to get the car sprung properly.

Something Stupid

Just when everything is going well...

I've been making slow progress zeroing in on getting the charger set to bring the batteries up to the optimal charge point before it turns off. It's tough because the charger uses a little screw potentiometer to adjust it up and down. It's exceedingly difficult to get it right where you want it. Plus I've found that if you change the current you're pushing to the pack, the high voltage point at which the charger believes it's done changes.

When I have the charger turned up all the way so that it's pushing 28 amps, it would cut off at 164.5 volts. But if I ramp the current down to 10 amps, it trips off at 165.8 volts. It would be so much nicer if the charger had a digital interface for setting the cut off voltage. I realize that would add to the cost of the charger, but I have to tell you, I'd pay for it.

This morning I was charging the car at 10 amps The charger hit it's limit threshold at 164.7, which is about where I want it. I started taking measurements on the cells to monitor how they were doing. There are a couple that come up to 3.6 volts faster than the others. While measuring one of the cells I slipped and touched the probe to the chassis while it was on the positive terminal for that cell. Well, there was a loud pop, and a nice bright flash. When I looked down, the point on the end of my probe had been melted to a nice rounded blob, the terminal had a big black spot around it, and the charger had flipped off.

I checked everything and couldn't find any obvious problems to any of the systems. I tried to turn the charger back on, but there was no power to the charger; the breaker for the outlet had popped too. I reset it and tried to turn the charger on again. Loud pop, but no flash anywhere, and the charger tripped it's breaker and the breaker on the house tripped too. *Sigh*

So it would seem that through clumsiness or carelessness, I've damaged my charger. I've sent an email off to Rich Rudman at Manzanita Micro to get his advice. But I'm pretty sure that the chargers coming out of the car soon and making a trip to Washington. Man I hate it when I do stupid stuff. And just as the weather is really getting beautiful for top down driving! *Sigh*

UPDATE:

I heard back from Rich at Manzanita. Apparently I've blown the input AC rectifier. So, I'll be taking the charger out of the car ASAP and sending it back to them for repair.

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.