TickTock wrote: GregH wrote:
TickTock wrote:On page EVB-26 of the April, 2011 rev of the SM, it states that the highest cellpairs are discharged until they reach the target level. I see random cellpairs getting selected - sometimes even the low ones. Under no condition should the lowest cellpair get shunted. Also, others have shown captures with lots of CPs highlighted. I really doubt more than a half dozen or so are ever shunted since this would reduce the available pack voltage.
If the BMS is not very intelligent and only tries top side balancing without considering the SOH at low SOCs, then it is entirely possible at a low SOC to see a low cell have balancing prescribed hours earlier when it may have been much higher.
That's another thing - it makes no sense to me for these to be active while driving. It's been a while since I had the conversation, but I am pretty sure the Nissan engineer told me they only do top-side balancing (which is consistent with the description in the SM). There is no reason to shunt the cells while driving - it should only be done while connected to the charger so the remaining cells can, then, be charged more.
When we did the first lithium PHEV Prius conversions I would do exactly this. The iron phosphate batteries really pop up at the end of charge so it was really easy to identify the fullest cells and bring on balancing... shut off the (1kW.. 5A, 200V) charger and balance for 10-20 minutes (@40mA) then turn off balancing, turn on the charger.. repeat.. This of course required being plugged in for hours after a full charge.. not optimal.
If you know which cells you'd like to remove a few mAh from then it really doesn't matter much what the voltage is at the time the balancing is done. You just need to keep the shunt on for X minutes or hours.
If my Leaf is balancing, it always seems to be off when I turn on the car then I can see the shunts turn on over a few seconds from left to right (lowest cell to highest cell). I can see the same pattern come back on over a few seconds every time I turn the car back on. This indicates to me the car does NOT balance when off.. but it certainly does when on or when charging. As a matter of fact it could be balancing in the middle of an L2 charge based on an assessment made the day before at the end of a previous L2 charge. Note this would also mean the car does NO BALANCING more than 15 minutes after a full charge.. ie when the 3 blue LEDs turn off and the car goes to sleep, the balancing turns off.
I'm not sure why they don't balance with the car off, perhaps to make sure no shunts get stuck on.. more likely the LBC shuts down when the VCM goes to sleep seconds after key-off and doesn't have the ability to leave shunts stuck on while sleeping. For better or worse, I designed my battery management systems so that I could leave balancing on when the processor powered down. I had LEDs on each shunt visible from outside the pack so you could see at a glance which shunts were on.