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.