Thats exactly what i was trying to say with "current can flow always in one direction". It seems that it wasn't well expressed.ElectricGod wrote: ↑Mar 21 2018 2:18pmIE: C- mosfets have their diodes reverse biased when charging from C-. From P-, current flow into the BMS if the mosfets are off is always possible since the back diodes are forward biased. However current flow out of the BMS if the mosfets are off is stopped since the back diodes are reverse biased. Effectively what this means is from P- you can't stop charging current only discharging current flow.
The C- FET's cannot switch off a load (or doing low voltage cut off), and the P- FET's cannot disconnect a charger (or doing high voltage cut off).
If the current is flowing through the body diode there will be a larger voltage drop (around 1V instead of the usual few mV), but current continuous to flow if it gets turned off (or with 0V on the Gate).
Glad to hear that there was no fire^^So then, lets talk about drastically BAD cell voltages for a minute. I'm sure NOT perfect and sometimes I do stupid stuff and kick myself really hard for it. This was at 12S and 48 volts. I was a noob to EV's. I had built 4 12S LIPO packs and was using a crappy charger that topped out at 52 volts. That's 2 volts over 4.2 per cell or 50.4 volts. This was not ideal for my LIPO packs and I had to watch that the voltage at the EV never went above 50.4 volts...or at least not by much. Charging over night was out of the question as all of the current from the charger would get spent in lots of balance circuits among 4 BMS in parallel. Fortunately the charger was capable of 2 amps so this wasn't a huge concern if I was watching the charge process periodically. I had purchased 3 BMS and they were in active use, but my 4th pack was waiting for it's BMS to arrive. It was common that I would plug in the charger into the EV and a couple of cell monitors into the unprotected pack. When the loud beeping started, it was time to pull the unprotected pack and finish charging via my RC charger. For whatever dumb reason, I flat forgot to do this. As you can imagine, that 12S pack was sitting at 52 volts or 4.33 volts per cell when I realized my mistake. I was thinking to myself why wasn't I hearing the cell monitors? I immediately realized it was becasue I had never plugged them in. I ran to the scooter and sure enough, every cell in that pack was at 4.3 volts and the pack was quite hot. I pulled it from the scooter, left it in the middle of the floor in my garage and let it cool down. All the cells now showed 4.4 volts!!! I then attached a load and pulled them back down to 4.1 volts per cell. That pack did suffer some loss of capacity, but no cell exploded or even puffed up. 2 weeks later it had it's own BMS. The other 3 12S packs that had BMS's were just fine. The cells despite the charger producing too much voltage were all sitting at 4.2 volts and soon were back down to 4.1 volts thanks to the BMS's.
I am absolutely with you that it needs very bad cell voltage differences to overcharge one group significant so that there will be a fire.
But think what happens if somebody has one 10s charger and one 13s charger with same charging plug and they get confused.
It would charge to >5,4V per cell and booooom bang fire, because the BMS cannot turn it off.