LI-ion BMS

Hello, i recently purchased a 2s li-ion bms protection board that i will link below. In the link below it shows a schematic page on how to wire it but not what voltage to give to it when charging. would 9v be a good bet? and also can i just hook it up to my variable`dc power supply and set it to (constant voltage mode)? https://www.ebay.com/itm/2S-8A-7-4V-8-4V-BMS-Protection-PCB-Board-for-18650-Lithium-Li-ion-LiPo-Battery/262863547887?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I’m not sure what you’re asking. it appears the module doesn’t include any charging function, just protection. You’ll also need a 2S charging module of some kind.

ShermanP: I'm not sure what you're asking. it appears the module doesn't include any charging function, just protection. You'll also need a 2S charging module of some kind.

It does , it says it on the first picture (charge+ , charge-)

Those are protection devices, they are to prevent overcharge and overdischarge.

They are designed to protect the battery in the circumstances when a connected charger fails, they are not designed as chargers.

If you were to attempt to use one as a charger then the circuit will limit the current into the battery to 15A.

tjones9163: It does , it says it on the first picture (charge+ , charge-)

No, it actually says Charge+ / Discharge+ and Charge- / Discharge-. Which means those are terminals that you connect your charger output or your discharge load to. The BMS will then prevent overvoltage or undervoltage when either charging or discharging. It does not contain a charging circuit.

Steve

They were lying to call it a BMS, basically, its a effectively just a sophisticated fuse.

thanks guys. I couldn’t read the IC. Does anyone know what type of IC you would use for that?

With a single cell, things are pretty simple. But things get more complicated at 2S and higher. Ideally, you want each cell to be individually charged correctly in-circuit. I wasn’t able to find a module that does that for 2S. Perhaps someone here can enlighten us.

i just wanted to add that depending on how much current your batteries need to supply, there may be an alternative which in the end is simpler and cheaper. That alternative is to use only one lipo cell which drives a boost converter to step up the voltage to whatever you need. Then you could use a USB-powered TP4056 charger module (one with protection built in if your lipo cell doesn't already have that) and an MT3608 boost module, both of which are small and cheap. Then if you need to run your device while charging the battery, you would need to add a load sharing circuit to allow USB power to drive the boost module while charging is taking place independently. That would consist of a p-channel mosfet, a shottky diode, and a resistor.