Battery Failing to Charge

Hey guys, I just completed my first ever build - putting a BBS02 on my Felt MP. This thing is a blast to ride (video to follow soon!) but I've run into one problem... the charger treats the battery (a 20ah from Aliexpress) as if it's fully charged, ie. LED light remains green, it only trickle charges, takes about 3-4 days to give the battery a half charge.

First thought was a faulty charger, but I tried a new one from Dillenger, and am having the same problem. The charger is showing 54.6V on my multimeter; the battery is showing 51.9V and is powering the bike fine.

Any thoughts/wisdom you can impart would be much appreciated!

What is the battery voltage when the charger is hooked to it? The website doesn't say what the chemistry is, but a 48v LiFePO4 battery should have max charge voltage set at 58.4 (3.65 v per cell), but it should be pretty close to fully charge even at 54.6 volts. When you say it is only charging half way, how are you measuring the state of charge?

jimw1960 said:

What is the battery voltage when the charger is hooked to it? The website doesn't say what the chemistry is, but a 48v LiFePO4 battery should have max charge voltage set at 58.4 (3.65 v per cell), but it should be pretty close to fully charge even at 54.6 volts. When you say it is only charging half way, how are you measuring the state of charge?

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Thanks for the reply! I'll check that voltage when I get home later tonight. To clarify, that was charging from a partially used state (there's a crappy LED meter on the battery itself that showed "half" when it first arrived - probably wildly inaccurate, but I figured it at least shows that the battery should take some charge) to full. Once I was confident the battery was fully charged (after a few days of what seemed to be trickle charging), I took it for a 10 mile round trip, and then reattached the charger - charger was still not showing the 'charging' indicator, seemed to be still a really slow trickle charge.

Alright, so the battery shows about 0.5V out when the charger is hooked up. After riding to work and back today with some heavy throttle, about 11 miles, the battery was down to 49.8V (LED indicator on side of battery shows half-charge). Still no charging indicator on the charger - leaving it hooked up now so I can get a more accurate reading on how slow it's charging.

Given the charger voltage of 54.6V you could assume a 13S li-ion battery.

I would stick a power/watt meter or your DMM set to amps in series to actually measure the current going into the battery.

It may just have one cell much higher V than others and the BMS trying to balance the cells, by discharging the high cell slowly (50-80mA). With a 20Ah cell that can take ages. If you get a slightly higher final charge voltage each time you leave it on charge overnight it would indicate the balancing is going on, slowly.To speed that up would require opening the battery to measure cell voltages, and manually discharging the high cell to speed up the process.
Failing that measure the charge MOSFET to ensure it is switched fully on.

Returning the battery to China is probably out of the question.