I started thinking about using the cell top BMS to load the higher voltage cells at the start of the charging process after i read post by harlequin2. Harlequin2 mentioned that some people had balanced their cells by leaving them in reduced charge current state for a few days. That was probably an extreme example but it made me think, the low current period of cell balancing is burning up kwh's whilst possibly waiting for just one cell to balance.
Quote:
Originally Posted by harlequin2
...
I balanced all mine with an individual cell charger made from an old 5 V/ 20 A computer power supply modified to put out 3.6 V instead of 5 V. Might take a while if you have a few hundred cells!
Also, weber and coulomb on the AEVA balanced theirs with only 1/2 A just by leaving the charger in balancing mode for a few days - they have about 220 cells.
|
1/2 Amp * 220 cells * 72hrs = ~28.5kwh
I would imagine it would depend on how out of balance the pack is as to whether holding the charger at a reduced current until all the cells caught up would use more power than reducing the charge current only to the highest voltage cells for the whole charge process.
The total charge time would be reduced and i get the feeling that it would actually use less power to load the high cells during high current charging. Tthe calculations to work it out hurt my head.
But say your high current charge period was 8 hours then loading the highest voltage cells during that period would at most consume 8hrs * cell count * cell voltage * BMS load current.
For the example quoted from harlequin2 and assuming the high current charge period is 8hrs then if the cell balance was achieved during high current charging. Then it would be an absolute maximum of 3.16kwh rather than 28.5kwh burned up by the cell top BMS modules. As not all 220 cells would be loaded by the BMS, only the highest voltage ones, it would actually be considerably less than that.
This is all purely guess work so it may be very different in practice.
Another thought i had was that as more and more people take up electric cars it is likely that the cars will be treated more and more like a regular appliances. The users will be able to drive the car but have no idea of the mechanicals. Eventually it wont just be enthusiasts using them.
At that time having a reduced current balancing period at the end of the charge cycle may cause some very short lived battery packs as the users just plug them in and then when they are almost fully charged they unplug and drive. Because what difference does 98% full to 100% full make? So cell balancing may never actually occur and the pack drift more and more out of balance. If the balancing was done during the high current period of charge then no matter how short the charging period was cut, the pack would end up more in balance than it was before the charge.
I am hoping to build some of harlequin2's cell top bms modules and then play around with the software and see how it works in practice.