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Question about BMS


I ordered bms from the bottom link. It is working ok with 5 batteries but i cannot get it to work with 4 batteries. If you look at the pictures, 3S, 4S and 5S look completely the same, same elements on same places. I ordered both 4S and 5S version and like i said, i cannot find a single difference between them so i dont know if they are identical and dont even know which one is 4S and which one is 5S. Is it possible that it is basicly only 1 version of BMS, but that it works with either 3, 4 or 5 batteries ? Anyway when i connect it like its on picture attached, i get 1.5V from the bms, instead of 16.6V.



Hmm i would still like your input but i think i answered my question. 4S and 5S look almost the same but there are some tiny changes. I included image in attachment for you to see and also to maybe help someone in the future. Whats also funny is that my 5s that i ordered looks different then the one in an image. Some resistors or what are those tiny elements are not on the same places.


Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, quite probably the same boards, and you add links, or resistors, or whatever to configure the number of batteries - obviously THEY should do that, according to which board you order.

Read the number off the chip, and download the datasheet for it, that should make it clear.
Thank you for your reply.

There is no number on the board, cant read what it says on the chips and doubt it would help me, i am not an expert. I will just keep those boards for 5S battery packs and i ordered 4S and will wait until it arrives. I also found another picture of difference between 3S, 4S and 5S.


Nigel Goodwin

Super Moderator
Most Helpful Member
There is no number on the board, cant read what it says on the chips and doubt it would help me, i am not an expert.
The board number (if it had one, wouldn't mean anything - but with the number off the chip we could check the datasheet, try using a magnifier and a bright light, at different angles - you can usually make it out if you get the light just right.


Well-Known Member
Most Helpful Member
Since they are cheap Chinese no-name-brand then nobody knows which one is which. I guess it would cost too much more to mark them.
Just wanted to say that i got my 4S board today and it is indeed working and i was right, its different than 5S board :))

I completed my nicd to lithium conversion. It's 14.4V drill and while it usualy has around 16V at full charge (1.35V * 12), it now has around 16.8V at full charge (4,2V * 4), actualy about 16.5V since my charger stops when cells are around 4.15V. Well actualy i dont know whether its the charger or the board that stops, but i am glad that its on the safe side.

So first impressions ? I am using 20A samsung cells and while i can stop my nicd/nimh pack with my hand, i certanly couldnt stop my lithium pack. I also installed a fan into the case and set it to run when cells reach 45C (i plan to change this to 60C as soon as i get the right thermistor). Pack is much lighter now and so i am glad. Though it still buffles me how this tiny BMS board can handle such high currents. It is supposed to handle 25A but for the love of god i dont know how it continue to work with such high currents, its so tiny.

Nigel Goodwin

Super Moderator
Most Helpful Member
Glass half empty / half full :)
Sorry, I've no idea how that applies? - your old batteries were so absolutely shot that you could stop the drill with your hand - so you can't really compare it to new Li-Ion batteries. Replacing with new NiMh would have given a similar impressive improvement.

I also have upgraded my drill, and as you mentioned a HUGE difference is the lower weight.
I actualy hand picked the 12 batteries, its old nimh/nicd from drills, and i tested them, on average they are about 1000mAh. But yeah, i agree that new nimh/nicd pack would be the same. Still, i am happy with the result.
As for half empty half full, from other topics it seems that i am usualy happy with the result but you, a bit less. So thats how we roll :)
Could anyone explain to me how this BMS works to see if i understand it correctly:

Ok, so when the cell reaches 4.2V or so, it will put a load on the battery, which will stop charging it IF the charging current is slow enough (42mA or slower). And that means the 4.2V battery will no longer charge, but it will continue to charge other batteries.

Ok so that is ok. But what is it in the description of this board saying there will be no balancing if we only use 25V. If cells are badly unbalanced, 1 could still reach 4.2V and others would continue to charge, only they will never all reach 4.2V cause well, not enough voltage.

I might have a problem since i will charge 6S batteries with charger for 6S that is only about 25V. Batteries dont go out of balance over the night so i was thinking. How about i normaly charge the batteries with charger. And then like two times a year i first charge batteries with 25V and then attach it directly to 25.6V with a big resistor, which would limit the current to about 20mA. And then it should work normaly balancing the cells. Would that be ok ?
Ok i tried this. I charged the 6S cells to 24.7V with li-ion charger and then i connected them directly to 25.5V DC source. Current is around 30mA, which is inside 45mA. This means theoreticaly it should charge all cells to 4.25V, or rather 4.2V if it really cuts off the cell when it reaches 4.2V. Could someone confirm this ?

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