Issues with top balance.

I bought a charger that takes way too long to charge to 3.65 v. I think it's been on the charger for about ten days and it's only at 3.37ish. I set it to CC instead of CV to get it to charge quicker. I checked it this afternoon and it was at 3.7, yikes. Pulled it off the charger and after a bit it dropped down to 3.37. tried it again and had the same results. Is this just surface change that'll burn off? Just seems odd to me.

Also I was wondering about where I need to fuse and what sizes etc. Really the only thing left I need to buy. I'm running 8x 280ah 3.2v cells in a 24 volts configuration. 100a overkill BMS with a tririon 4215N charge controller.

ppzzus said:

I bought a charger that takes way too long to charge to 3.65 v. I think it's been on the charger for about ten days and it's only at 3.37ish. I set it to CC instead of CV to get it to charge quicker. I checked it this afternoon and it was at 3.7, yikes. Pulled it off the charger and after a bit it dropped down to 3.37. tried it again and had the same results. Is this just surface change that'll burn off? Just seems odd to me.

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Was 3.7V reported by the charger display, or was it measured at the battery terminals with a meter? The only accurate voltage is at the terminals.

It just depends on the cells and their capacity as to how rapidly their voltage will fall off. The 280Ah are usually pretty slow to drop below 3.4V and may dwell above 3.5V for at least a few days, sometimes even longer.

ppzzus said:

Also I was wondering about where I need to fuse and what sizes etc. Really the only thing left I need to buy. I'm running 8x 280ah 3.2v cells in a 24 volts configuration. 100a overkill BMS with a tririon 4215N charge controller.

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With a 100A limitation on your BMS, 125A fuse/breaker between battery and load.

Ty for the advice. Yall are always so helpful. Yeah I was checking at the terminal. I have it set to 3.7v now and only charging at around 5a @ 20ish watts. Going to take a while to finish but I'd rather not burn these before I even get to used them.

The charge leads are junk on all of those units and some have poor internal resistance. Running additional wires and replacing alligator clips with ring terminals can make a big difference in the voltage drop.

Did you follow the top balance guide and 8S them with the BMS installed to charge at higher power before parallel balance? With the BMS protection in place, you won't damage the cells, and you charge at nearly 8X faster than 8P.

I didn't no, when I started top balancing I didn't have the BMS in yet. Kind of running out of time to get on the road so I just went with a parallel setup to get it going. I just got it in today so I may move over and do that. I'm not in a great neighborhood so I'm reluctant to setup my panels until I have them mounted on the bus lol. Hopefully this weekend though.

If you're in a time crunch, you probably have no choice but to rewire in series w/BMS installed. Charging at 10A @ ~28V is so much faster AND The higher voltage minimizes the voltage drop effects.

The hour it might take you to reconfigure the battery will cause you to lose 3.35V * 7Ah = 23.45Wh of charge; however, upon putting them in series, and assuming you don't get anymore amps, 3.35V * 7Ah * 8 = 187.6Wh of charge the next hour and every hour after that until the BMS trips. At that point, all cells should be at a high state of charge, and you've cut your total charge time by 80%.