First of all thanks for sharing 'real life data', am I correct in assuming you used a granny EVSE with the aim to balance the cells? Did you by any chance check the result min/max cell voltages aferwards?
I would be grateful if some of the 'gurus' can help me understand what I'm seeing:
- for starters let's assume that there are no losses and all the energy goes into the HV battery (400VDC @100%SoC, 200Ah cell capacity).
This one is unlikely, the 400vdc refers to and architecture, 400v, 800v 1,000v are the common ones in commercial EV's, DIY started as low as 72vdc.
As an example, the LFP MG4 51 is a 400v architecture, 104 cells, nominal voltage 3.2v, 330v, it will range between 3v @ 0% SOC, 312v and 3.6v @ 100% SOC 375vdc ..... but all of these voltages as still give or take a bit, absolutely fully balance at the end of charge would be 375vdc, and highly unlikely with resistor type balancing, it would be an absolute fluke for that to ever occur.
As the battery approaches 0% SOC as the BMS sees it, a warning will come up and performance will be reduced.
The same goes for 0% SOC, the battery
should cut supply if any cell drops to 3v, this will be the cell with the least capacity at the time, the other cells will be higher than 3v, but as soon as the supply is cut, that 3v cell voltage will climb ......
this part is a guestimation by me, the supply is reconnected, but the available current is much reduced to get that last bit out of that low cell to get you home or at least off the road.
Passive balancing of the type used by the majority of EV makers that are built to a price, only top balance, so what is left will not be shared across all 104 cells, a good active balancer will, all but very slowly, they rely on voltage difference between high cell and low cell voltage to transfer capacity from one cell to another.
- as long as SoC is below 100% the current into the battery is about 3.75A
Probably not, the total supply current divided by the number of cells, less and electrical energy lost as heat while operating all the BMS and conversion from AC to DC and heat generated in any cabling .... and anything that is drawing power at the time, such as battery heating or cooling, charging the 12v battery, any lights using power out of the 12v battery, the air con if it's running, etc .... At 6 amps 240vac charging and the air con running, the supply is not keeping up with the demand ..... the losses are considerable
As the assumed SOC approaches 100%, the charge current will taper off to avoid cell voltage cut off over run. Cells with a higher internal resistance than others, will have a lower terminal voltage at a lower charge current, this means SOC while charging can not be assumed by voltage, there are a lot of factors involved.
- however after being 'fully charged' 3.75A is still 'flowing' for another 35min (2.2Ah)
Not likely, once a cell reaches the high voltage cut off, in these simple systems, charging stops, the power used from the mains is simply to power the BMS.
- the current then sharply drops to about 0.6A afterwards for approx 25min (0.25Ah)
No, in the cheaper systems, there is not trickle charging to attempt to reach 100% SOC.
- the remaining 2.5h (balancing?) are averaging around 125mA (0.3Ah)
The balancing resistor is only active on the high voltage cell/s to bring them back to 3.4v, any cells less than 3.4v, tough luck, maybe the owner will do another balance real soon to give those cells a chance to get closer and maybe exceed 3.4v so they can be balanced.
It is important to note here, with LFP cells, at a rested 3.4v, attempting to push even milliamps into the cell will increase the voltage to 3.45v min, the more current that is attempted to push into the cell, the higher the terminal voltage will climb.....
A cell at 3.5vdc, rested for 12 hrs, has a 100% capacity voltage of 3.4v, and a surface voltage of 0.10vdc, that has no real capacity, a 1 amp load will burn that surface voltage capacity off within 30 secs or so, you can not have a cell charged to 105%, only 100% plus a surface charge voltage .... I could explain how that is stored, but for another time.
That is the major questions answered all the rest as so variable, they can not be answered definitely .....
What triggers the initial current reduction:
a) the highest reported voltage cell is reaching cut-off voltage
b) the highest reported voltage cell is getting close to reaching cut-off voltage
c)?
At what stage are the parallel bleeding resistors (post #127) activated?
a) after SoC 100%
b) after 1st current drop
c) during the 2.5h (balancing) section
d)?
Assuming that to increase the cell voltage by 1mV, approx 150mAh of energy is required, there doesn't seem to be any scope to significantly 'lift' low reading cells during the balancing phase?
No, this is a wrong assumption, a cell voltage will increase 0.05v before any charging current will be pushed into the cell, just part of physics and the cell chemistry resistance to change ..... maybe that is why humans can be so stubborn at times, a lot of chemical reactions going on in one of them
I hope that helps and doesn't just add to the confusion, the learning curve is very steep for how LFP cell function compared to Lead acid or any other chemistry electricity storage device
T1 Terry
Yes, I didn't make it up 
As you've explained it above, it's important to be clear that NMC EV car fires are very very rare compared to petrol & diesel car fires. The UK AA & RAC motoring organisations carried out a joint 2yr study of car fires here in the UK, near 3yrs ago in Europe, USA, Australia & New Zealand & some other countries that escape me at the moment. The very comprehensive result show that EV cars are 80 times less likely to burst into flames than petrol or diesel vehicles. Similar less large studies around the world report EV's as being anywhere between 50 & 70 times less likely to catch fire. Pretty good I thought & shows the lie being spread by anti EV propaganda.
