Tuesday, September 7, 2010

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.

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