I was thinking of a constant current then constant voltage charger. Your charging algorithm is a good idea and takes care of that issue to a degree. Though a few minutes at 1%C still only allows for a 0.1% difference in cell balance (@6 minutes). So that either means far more than "a few minutes" or there is so little imbalance what's the point?weber wrote:I'm not sure what you're missing here, so I'll try explaining the whole thing in more detail.Faz wrote:Waiting several days to charge the battery is exactly my point "why bother"?!?!?!
Commissioning (initial balancing) only has to be done once. From then on, a full recharge from flat can be done in around 8 hours (in our case) because our charger can then run at its full 5.5 A for most of that time. The balancing phase at the end, where the current tapers down to 0.4 A, only takes a few minutes. But it's exactly the same charger and exactly the same BMS-controlled charging algorithm in both cases.
Our BMS shunt capability is exactly the 1%C or C/100h that you mentioned as typical (0.4 A for a 40 Ah pack). Of this, you wrote:
So I've explained that even with only 1%C shunt capability you do not need to have a charger that's only capable of a ridiculously low current, and you do not need to commission your pack cell by cell.Which is great if the pack was already commissioned cell by cell or your charger is of a ridiculously low current. Otherwise what is the point of a full blown bms?
So we are in agreement that an initial pack balance is important.weber wrote:If I had commissioned my 228 cell pack cell-by-cell it would have taken far longer. But even if you only have say 45 cells and so it would take less elapsed time to commission them cell by cell, that requires a different charger and a lot of your time spent moving it along from cell to cell. Whereas my BMS-controlled initial charge did not require any attention from me and so let me work on the many other jobs required to get an EV conversion on the road. So from my point of view it took zero time.
You didn't previously say anything about it being "corrupt". You wrote:Um destroyed cell? Well it is clearly already stuffed if a properly commissioned pack has a corrupt cell.
I presumed it was clear that if a cell was so far out of balance that it is corrupt.I have the same thought on "lvc" if a cell voltage is dropping while driving, well either you stop or drive till the pack makes you stop. The end result is similar, but one way gets you closer to home.
True and a good point. This is still only the case if some cells are very far out of balance. I presume that most people don't drive to 100% DoD. This scenario happening at 90% DoD means a cell with 10% less (or worse) capacity than its neighbours. There will also be clear indications of problems before this occurs, ie: drop in vehicle performance and then clear indications before battery ignition ie: burning smell as the battery heats.weber wrote: If you "drive till the pack makes you stop" you will by then have completely flattened that cell and begun to "charge" it in reverse. This will lead to it going high resistance (where "high resistance" here may still be less than one ohm) and thereby ending up with a large fraction of the battery voltage across it in reverse, while the remaining cells continue to force current through it.
Both jonescg and I have packs with around 200 cells. Imagine one of those cells with say 100 volts across it (in reverse) and 100 amps being pushed through it. That's 10 kilowatts being dissipated in one cell! How long do you think it will last?
Then I apologise for not making myself clear. I believe in monitoring, not automatic cutout/off. I don't believe a balancing shunt needs to be perma-mounted to the cell and I believe that if it is worth having it is worth making of a suitable capacity.webber wrote:You refer to "our bms" I wasn't referring to indi built bms's.
For the purpose of this argument, I don't see how it matters who builds them. You're talking about two of the most basic and necessary functions of a BMS for Lithium-ion batteries. You seem to be saying that an undervoltage alarm is not worth having, and that a balancing shunt is not worth having unless it can bypass far more than 1% of C/1h. I'm trying to explain why you might be mistaken about those.
Again I don't believe this needs to be perma-mounted to the cell.weber wrote:There is a third basic and necessary function of a BMS for Lithium-ion batteries, and that is an overvoltage alarm (or a single combined over/undervoltage alarm).