Help- 48v 20ah battery not balanced.

Hi all,

I'm new to lithium battery technology. I bought a used V-Power 48v 20ah 13s lithium battery. The battery charger is going to green although the battery is only getting up to 48.9 Volts. I checked the 13 wires going into the bms and calculated the differences in volts. It looks like the 5th cell group is causing the bms to shutdown the charge and preventing an overcharge. I would like to know the best process to either discharge the 5th cell or possibly replace the bms (if that is causing the issue)? I tried leaving the charger plugged into the battery overnight but the battery voltage still only gets to 48.9 volts.

3.5
3.7
3.6
3.7
4.2
3.9
3.5
3.6
3.9
3.9
3.4
3.5
3.4

Those cell voltages are all over the place, I wouldn't hold out too much hope of getting it working properly. They don't go like that for no reason. You need to look more closely at that high cell group. Most likely one or more of the welds are broken or one or more cells are leaking.

the first question is "What kind of chemistry are you running? Is it Li-ion, LiFePo4, Lipo?

As stated those cells are really charged all over the place. An over night charge might not be able to balance them. It might take longer.

What I would do first is go for a short ride, like 1/2 or 1 mile to discharge some of the top end, check the voltages (write down the results for compassion later) then put it on the charger over night. In the morning, check the voltages again and again write down the voltages for comparison.

Also, some pictures could help. Just make sure they are in good focus and under 512k file size for easy posting.

Here is some pictures of the setup it is a lithium ion battery.

I am not sure how the wiring is setup under all the tape. It is in a hard box that came from the factory so it should have been protected from damage. I counted 9x13 cells wide for a total of 117 cells.

People have suggested to wire a bulb to the cell groups that are higher volts to discharge. Can I wire a bulb from the bms wires? Another person suggested hooking up the battery to a balance charger. Can someone suggest a cheap balance charger that I can hookup and how would I go about doing that?

I am a noobie but thanks for the help so far guys.View attachment 2View attachment 1

If your cells are Li-ion then you do want 4.2v per cell. Are you sure your charger is charging at to at least 54.6v?

The first thing I would do is to plug that battery into your charger for 3 or 4 days. If that doesn't balance the cells then something is wrong. Either the cells are bad, or the bms is bad. Just leaving it on the charger is easiest way to get the answers you want, provided your charger is good.

Normally when people need to drain individual cell groups they open the wrapping and access the individual cells and their connections.

If you want to try to drain some of the high cells, your creativity is what will do it, unless you blow it.

If you disconnect the white wires from the BMS, connect a alligator clip to the black wire nearest the connector on the bms (without shorting any other wires) on the BMS, insert a small wire into the first hole of the connector, meter the clip that is connected to the black connector and the first wire in the bms connector (if you get over 4.2v then you are starting on the wrong side of the connector.) to see what the voltage is. If it is higher then the lowest voltage of the metered wires of the connector, drain it down to something close to the lowest voltage without going under 2.75v.

I have a small LED light that I pulled from a flashlight that pulls about 1 amp at 4v.




If you want to use a balance charger, they are expensive it you want to go 13s.

I would just leave it on the charger for 3 or 4 days to try to balance the pack so you don't accidentally blow your bms.

Thank you e-beach. I think I may just leave it plugged in I read that balance charging is extremely slow but after a single night of charging I did noticed a small change in the cell voltages. Notice the first 3 cells are even now. If after about a week I don't see an improvement I will try to lower the higher cells using a similar bulb you posted. I tested my charger with a voltmeter it is outputting 55.2 should I be concerned? I may replace it soon a capacitor inside is starting to swell up slightly.

3.6
3.6
3.6
3.8
4.1
3.9
3.5
3.5
3.9
4
3.6
3.6
3.4

I think I may just leave it plugged in I read that balance charging is extremely slow but after a single night of charging I did noticed a small change in the cell voltages.

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Make sure that the pack is in a fireproof (not "fire-resistant") location. You may want to build one in your garage using solid patio blocks, double thick, to make a sort of enclosed 'non-fireplace'.

I will do that thanks. I actually removed the battery from charging until my new charger gets in I don't trust the old one I have. I will definitely find a spot to charge safely for the moment the battery is in some thick plexi type enclosure.

Paco007 said:

I will do that thanks. I actually removed the battery from charging until my new charger gets in I don't trust the old one I have. I will definitely find a spot to charge safely for the moment the battery is in some thick plexi type enclosure.

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I know it's hard to believe without seeing it, so search YouTube for "lithium battery fire." The plexiglass would likely do nothing more than add more smoke...