Top Balancing LiFePO4 battery pack

[aidan.f]

Why buy a battery box when you can make your own..? Low profile for 4x220Ah cells.(pic)

Question please..
I'm top balancing the cells. When delivered cell currents were very different (grade B), after 10 hours at 10 Amps currents had equalised. Realised charging was inefficent and prolonged so connected as 4S with BMS and charged to 90% (cell voltages within 2mV)
I'm balancing again, has taken 6 hours to get from 3.48 to 3.50 Volts, individual cell currents are now: 2.8,2.20,2,6,2,8 Amps, slowly dropping.
Should I just let current taper to zero at 3.5 Volts, which must be very close to 100% charge or as webby instructions suggest go for 3.6 Volts? Apart from the risk of damage it's going to take time...

Having fun..

[aidan.f]

To answer my own question 2.5 A per cell is like watching paint dry..
back to 4S 13.5 volts @ 10 Amps tapering nicely, cell voltages within 2mV

[Tod28]

How are you balancing in series? Active or passive? 3.6v for 100% for first off balancing charge if actively or parralel balancing then set max to 3.5v (14v for 4s)

"Off grid garage" has a good take on the whole gamut of LiFePo4 charging, balancing & use.

With 220 ah it does take time with very low charging voltage differentials between the charger and the cells (fractions of a volt) at the top end.

I built a 304 ah 4s pack to fit under the van seat base last May with a JK BMS with 2a active balancing. Initial charging to 3.6v to calibrate the BMS took ages (a day or so) for the last 5%. Don't allow charge above 3.5 v now operating (as I don't need the capacity)

Off grid garage's youtube treatises on voltages/state of charge/charge rate, myths etc and testing of BMS's etc is very informative. Having 3.5kWhr of battery capacity is awesome for lack of anxiety re how many days we can run without ehu/solar or running the van. Recharges happily at 65-80A off the alternator when we move off.

[Tod28]

https://off-grid-garage.com/

[grams]

You can get active balancing boards that continuously equalise the voltage between cells. They don’t care about chemistry or SoC or what the voltage is, they just swap charge around to make it equal.

Rather boring I suppose.

[aidan.f]

Thanks for the advice, I do have an JBD BMS with active balancer, but AFAIU it  just maintains balance only shunts a 100mA so. Initial balance has been done in parallel, wimped out at 3.55V all cells withi 1 mV and now discharged to 80% 2mV cell delta