Battery Not fully charged ?!

Hello folks I recently built a 12S4p 30q samsung, I’ve been having an issue with that battery not fully charged. It always charges up to 49V. The Charger LED turned green ( which mean charged). I open up the battery and examine each series, only one always at 4.15v and the rest range around 4.07v. I’ve seen topics like this but yet no finest solution. Im using Bestech Charge - bypass only bms, the charger model picture below. Any comment, solution is highly appreciated . Cheers !

Which bms do you have? Seems cell unbalance is making it hit total voltage early and then dropping

A pic of the BMS would help, burned resistors can cause this

Previously I used SuPower BMS, I parted that one out and build on top of this bestech bms

Double check balance lead from bms, make sure they are all connected and in the right spot.

Also how many amps is the charger

Sounds like a two-fold problem. First, the charger isn’t going to 4.2/cell. Second, because the cells aren’t reaching 4.2, the BMS isn’t balancing them.

The charger has a couple blue potentiometers on the board. One controls the final voltage, one controls the charge current. You should open it up and twiddle the pots until it outputs 50.4 volts, then charge the pack again and see if that solves the problem.

it 50.4V, 4Amp , I usually Unplug the charger when the LED turned green, and the pack at 49V. So i;m not sure what make the charger stop charging ( or it still charging but LED says otherwise )

Take a meter and check the voltage of the charger without anything plugged into it.

For the future though you should probably not take the charger out until 50.2v.

Also maybe try a different charger if you have one. It could just be a bad charger as well

Need a pic of the actual BMS to see if there’s damage

  • This is battery output without load

  • this is when plug in, and LED green turn on ( charged), meter measure from battery output

BMS before install

Individual series voltage

1 x 4.18v 11x 4.07v

what charger you reccomended

I’d go with a standard 3-5 amp ebike Charger.

Lol need to see the BMS as it is now :wink:, specifically the PCB underneath with the balance resistors

Look for something like this

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Hey mate, I think @MysticalDork is right here. And what is happening is when you plug it into charge the voltage is dipping slightly down to 49v. Oh and Merry Christmas everyone


This is definitely his charger being undervolted

You need to turn one of the blue pots inside to make the end voltage 50.4 when the light turns green.

With the charger unplugged from the pack -Remove the top

-hook up meter to the plug

-find which blue pot changes the voltage, if the pot doesn’t change the voltage, put it back and move to the next

-once you find the next that does voltage, hook up the meter to the pack and play in the charger. Once the charger turns green if the pack is still under 50.4v, turn the pot in the direction 1/4-1/2 turn at a time, and allow the pack to charge until the light is green, if the voltage is still not 50.4 repeat. Do this until the pack reads 50.4v, and the charger is green.

-put hot glue or something on the pot to keep it from moving, close the charger back up. Your charger may read 50.6 or 50.8 while unplugged, but will only supply 50.4 under load due to the resistance of the pack. All changers should be calibrated after you buy them before completing the board

Example of the end result


It could be also that, but for new cells to get that unbalanced something has to be draining them unevenly

i’ve read somwhere here that the bestech BMS starts balancing when its finished charging (basicly liek every lipo charge does). so if you leave it on, it might actualy solve your inbalance problem automaticaly. no idea if this information is correct though

It can/will balance when it hits full charge, assuming there’s nothing wrong with the actual BMS that caused the unbalance