Top Balance done - Capacity test delayed

[wiseacre]

I never thought I'd be here but after a long time lurking in the forum I mustered up the courage to DIY.
24v 8s - JBD BMS
Lishen 202 Ah cells
Cells were called Grade B.
There was supposed to be cosmetic flaws - two had very minor dents (hardly noticeable) but the rest looked perfect with big pads and threaded posts
All arrived at 3.2+ v

Top Balance completed this morning -
Assembled the battery to capacity test and found the tester DOA ?
MakerHawk Electronic Load Tester USB Load Tester 150W 200V 20A Resistor Adjustable Constant Current Battery Capacity Tester Module Intelligent Discharge Resistance Power Tester

The assembled battery is just sitting waiting to be used
Current cell voltages after 10 hours sitting

1. 3.539
2. 3.537
3. 3.541
4. 3.533 -lowest
5. 3,552 -highest
6. 3.552
7. 3.551
8. 3.552

.019v difference between high and low -

I understand the cells dropping from 3.65 to lower voltages is normal.

Debating if I should just wire it up to the system and do a rough capacity test tomorrow using a load on the inverter.
I'm not sure what to think about all this so any comments are welcome.
Do's or Don'ts ?

 

[Steve_S]

That voltage drop after 10 hours is awfully Good ! You can do a Happy Dance IMO, good cells.
I do not see why you could not put that 8S Together and run a test through the Inverter with a Known Draw.
You did not mention if you have a Victron Smart Shunt or other type of shunt to be able to observe & monitor, so depending on your equipment and what you have on hand, you'll have to adapt a way to be able to monitor progress.

BTW: Devices like Heat Guns, Hair Dryers and Portable heaters are all good for a respectable "load" for testing BUT the labels suck ! Heat Gun might say it is 1200W but it won't be, you'll want a clamp meter or at least a Kill-A-Watt on the 120V socket to actually know what it is pulling.

Hope it helps, Good Luck

 

[RCinFLA]

There is a surface capacitance charge that builds up when charged above 3.45v and allowed to have a charge current drop off at a given voltage.

This surface capacitance charge will take several days to bleed off on its own with unloaded open circuited cells. Don't judge voltage difference with this surface charge. Its self discharge rate varies between cells.

You can bleed off this surface charge by placing a bleed resistor across the cell for about a minute. You should do it for about a mnimum of a minute to give some time for charge to migrate out of cell layers. Need to bleed off about 0.0001 x battery AH capacity. For a 202 AH cell that is only about 0.020 AH, which is 1.2 amps for about a minute. Only important thing is giving at least a minute to discharge and accomplishing 0.01% C discharge so can be longer for a given resistor value you have on hand.

After bleeding off the surface charge, and waiting 1-3 minutes for cell to reach equilibrium, should be about 3.45v if fully charged and the cells should match in voltage if top balanced.