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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.
Wattsup, Korishan, zag2me And 6 others like this post
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 Smile 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?
(10-03-2017, 01:48 AM)aidynphoenix Wrote: 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?
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.
(10-02-2017, 11:46 AM)kiwi Wrote: Hi Smile Do you have some reference about this cheap BMS ?
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.
The power of lithium ion is in our hands!
We'll show them what we're made of!
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.
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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.

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