Chinese BMS boards

A little update on tests on a eBay sourced 2 dollar Chinese protection boards.
- The board I used was rated for 25 Amps.
- As specified, It gives a short circuit protection and cuts of supply instantly in case of a short.
- The board does nothing below cell voltage of 4.19 volts while the batteries are charging.
- As the voltage of any of the cells increases above 4.19 volts, the shunt resistor of that particular cell kicks in which I figured out by soldering a small LED across the resistors.
- As the voltage keeps rising, eventually The circuit cuts off the connection of the charging voltage to the cells when the highest cell reached 4.28 volts. The charging resumes as soon as a load is applied.
- At the lower limit of voltage, the supply is cut off as soon as the lowest cell hits 2.50 volts. And will resume only when a charge is applied.

The shunt resistors drain about 40- 50 milliamps which is too insignificant when you are using several batteries in parallel. However with a bit of an add-on as drawn in the diagram, this voltage detection trigger can be used to activate a MOSFET or a small 5 volt relay to drain the battery through a bigger resistance of several watts.

Conclusion - With few mods, this PCB has the potential to become the heart of a working BMS for power walls.

Excellent work. I have been looking at cheap bms for ages. Following your idea, apply the theory of flying capacitor balancing, you could dump/retrieve charge from flying pack balancing. Very inspiring, especially if you couple with some well programmed Arduinos

Hi Do you have some reference about this cheap BMS ?

how hard would it be, if possible to change where the voltage range say for example, no less than 3.2v and no more than 4v? so you can get more cycles out of them?

aidynphoenix said:

how hard would it be, if possible to change where the voltage range say for example, no less than 3.2v and no more than 4v? so you can get more cycles out of them?

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I was looking for a similar setup. Unfortunately, the voltage detection circuitry is made up of small unknown components difficult to replace/ modify. I have a limited access to PCB design engineers and manufacturers here in India who might help. But a cheap PWM charge controller meant for 3S Li ions can help if used in combination with this.

kiwi said:

Hi Do you have some reference about this cheap BMS ?

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Check up 3s BMS on Aliexpress. Many there.

Hey Hillybilly. Do you think it would be possible to use a big schotky diode with a -0.5V drop on the input of the BMS to up the cutoff voltage to 3.0V instead of 2.5V? Would be great for overdischarge.

old thread but rather than a relay, possibly use optocouplers so not only you got isolation but can drive a FET quietly if the drain current on the optocoupler is too high. At least this would keep the circuit small in size and not need high voltage/current to drive a relay.
just a suggestion.

Ok , very old topic.
But you can also put external power resistors parallel to the originals.
The balance fet on most of those bms are rated for about 2 A.
I solder a few 15 ohm resistors in parallel and have now 350ma balance current

Interesting. I was thinking of taking one of those cheap $3 7S balance boards, putting 10W (or even 50W) ceramic resistors in place of the original ones, and some inductors on the voltage sense pins (to trick them to start balancing at 3.9V instead of 4.19V). Would that work? I think the board traces are too thin to handle more than a few miliamps, so I was thinking of just using whatever IC controls balancing, and breadboarding the rest. Anyone try this? Just for balancing. BMS would handle everything else.

Can you tell us exactly with what BMS you want to do this, because if the balance mosfets can handle the current, then it shouldn't be a problem, if not it will probably burn the mosfets.

Dallski and Ledschlucker, in the end my recommendation is - either you use these chinese type BMS unmodified as a last rescue, and make the Balancing with a different unit, or you leave them off.
I have about 6 different types of these Chinese BMS here in stock, have one type (6s) in operation, have analysed this 6s, and had a short look into 2 or three others.

All of them have worked as intended, and have the same features
- 2.5 Volt Cut-Off undervoltage
- 4,1 (something) as balance start
- 4.3 (something) as cut-off overvoltage.
That is exactly good as a last rescue in terms of BMS, but unusable as balancing mechanism for packs which shall swing in the 3.3 to 3.9 Volt area EXCLUSIVELY.

The cutof voltage is defined by a component intended for that, so little chance to change that, except you change it against another type, which has 3.3 cutof (or something, had to look that up.).

Cherry67 said:

Dallski and Ledschlucker, in the end my recommendation is - either you use these chinese type BMS unmodified as a last rescue, and make the Balancing with a different unit, or you leave them off.

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I was talking about these little standalone balance boards good for 66mA of balancing. https://m.aliexpress.com/item/32825...70GENsRXYgqaxkH83rBoCdD8QAvD_BwE&gclsrc=aw.ds
Hope the link works. Just search "7S balance board" if it doesn't.

BMS could be handled by a chinese BMS or any other BMS that doesn't handle your balancing needs if you have a really large parallel pack. I was just thinking you could modify them to handle a lot more than 66mA.

I know these, and have not bought them.

You get the same functionality together with the protection features ( under/Overvoltage, overcurrent) as one of the usual cheapo chines BMS, and the same problem, that they only balance at the top voltage.