3S 40A BMS generic board - question

Hi everybody,

I’m experiencing problems with my generic 3S 40A BMS.

It cuts off current (stops charging/ turns off) much before expected.

As much as I know - it should charge until every battery reaches approx 4.2V, but in my case - it stops charging when my batteries are around 4.13V.

The power supply is set to 13.5V / 1.1A.

I soldered another board, but the problem still exists.
I can manually charge every battery to 4.2V using my power supply, but not with this BMS.

And ideas on what could be wrong ?

are batteries very different in terms of “age” ? were they balanced before you started charging ?

Does your board offer “LiPo balance charge mode”?

One possibility is that the battery holder (being of riveted construction) has significant
contact resistance, which is distorting the the cell voltages it is measuring. Any form of
high-current charger requires rock-solid connection between the cells (not just rivets).

In my experience those battery holders aren’t good for high current and trying to solder
the connections for better connection just melts the plastic. Perhaps solder wires to the
original metal foil contacts?

My calculator says 4.13 / 4.2 is 0.983333. Less than 2% below.

Looks like an usual resistor tolerance issue. But might also be “second choice” devices sold.

“are batteries very different in terms of “age” ?” - No, I bought them a month ago, and all 3 can be charged at 4.2V using a power supply.

“were they balanced before you started charging ?” - yes.

“LiPo balance charge mode” - yes, my BMS has it.

MarkT - I’m also thinking similarly, since I had contact problems (the solder gun melts the palastic, then i had to add more solder etc. etc. it was a mess. But If i proble the first battery - and last battery + I do get 12.36V)

ZinggJM - hmmm…sorry but I still believe that it should charge up to 4.2V. There are plenty of tutorials with the same bord - charing the battery to 4.19/4.20.

IMO the problem is not with the board, but the batterly holder. I’ll make a different holder.

Thanks for the fast replies !

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