Has anyone reverse engineerd those $10 BMSes from eBay?

[badboy1999]

I'm referring to the kind of bms that's used in this tutorial: http://www.ebikeschool.com/how-to-build-a-diy-electric-bicycle-lithium-battery-from-18650-cells/
Mine just arrived but it is seemingly DOA, the seller is shipping me a new one but I want one now
I'm guessing there's some kind of comparator action going on at each of the cells, and if the voltage is too high the optocoupler of the cell is shut off (on the 10s bms there's 20 optocouplers), and then al of those optocouplers in series need to be active for the FETs to conduct? But then how would the balancing work, you'd need communication on what cell is the highest voltage?

If anyone knows more about these BMSes that'd be very much appreciated!

 

[badboy1999]

Update, hooked it up to my lab power supply and shot a video.

How the bms works
My hypothesis is that every cell has a circuit for both OVP and UVP, and those go to a optocoupler, and all 10 OVP-charge-optocouplers need to be the same state for the charging to happen (when there's no cell at risk of overvoltage), and that the 10 UVP optocouplers all form a chain as well, stopping the discharge of all the cells when the voltage of just one cell gets too low. On top of that there's a shunt visible so there's probably over current protection, just like the ebay page promises.

Video
https://youtu.be/OT5hFV0Q4t0

Discussion, results
You can see the over voltage protection kicking in at approximately 4.30 volts, 30mA gets drawn from the cell, because the 150 Ohm resistor gets engaged. At approx 4.10 volts, it stops again, there's a very clear hysteresis happening. I'm guessing the OVP also means the charging optocoupler set to a different state, but didn't bother to measure it. I tested the connections for all cells but the one that didn't balance at all. All seemed to engage at 4.30v +-0.01V according to my multimeter. So the charging behaviour seems to be: charge the pack until one cell hits 4.30 volt, then discharge that one until 4.10, then wait for the other cells to hit 4.30. That'd certainly balance the pack albeit a bit slowly, a standard 8Ah pack (assuming 10% of the charge is between 4.30 - 4.10 volt
It would, though, lead to an infinite process of charging-discharging. At 30mA that would still take forever and probably won't damage the battery a whole lot but it seems reasonable to discharge the charger when you've charged it.

Conclusion
Overall, combined with the build quality, it seems to be a fairly decent BMS. Don't know anything about current limiting, or temperature stability, but if it's anything like the OVP it'll do just fine.
The only issue is that the cutoff voltage is too high. There's a reason 4.2 is the maximum charging voltage, it provides a good balance between cell life cycles and capacity. 4.3 halves the capacity according to what I've read on this forum, and also this primary source, they actually tested it: http://www.powerstream.com/lithium-ion-charge-voltage.htm
This doesn't mean that the cell pack life is halved. If you charge the pack to 42 volts, and one cell hits 4.3 volt because the pack is unbalanced, it's going to be a tiny bit damaged, so ultimately the whole pack will get the capacity of the lowest capacity cell. Not the best solution for balancing but a reasonable engineering compromise in my amateur opinion.

 

[ntr]

Thanks for sharing. Do you think there is a way to alter the OVP level? I have the same BMS but it is build in, so it's hard to do research on it. There is a little SMD component left to the connector with "431" printed on it. This could be a 4.3V zener diode (according to this site). Maybe repacing it with a lower voltage zener will lower the OVP level?

 

[badboy1999]

Or it's a tl431 precision voltage reference, and resistors are used to set the voltage, so you can replace 10 SMD resistors to set a new voltage
My chips have 5183u and D01ED printed on them so I guess your bms is just a bit different. Can't find 431 printed anywhere.

 

[DVDRW]

Anyone reverse engineered these cheap lipo alarm?
How isolation is achieved? No optocouplers.
What is the easyest way to connect it to Arduino etc?