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Old 10th Aug 2020, 9:53 am   #4
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default Re: "Battery Manager" for a Li ion Battery

That looks like a fairly standard 'battery management system' (BMS) for Li-ion cells. All it will do is prevent damage to the cells by overcharging or overdischarging, and, looking at the PCB, it looks like it has some attempt at balancing the fully-charged voltage between the cells.

It'll do the job, but it doesn't include any charge regulation circuitry - you're essentially dealing with the naked cells. You need an external charger which will limit the charging current, probably to around 2A for 18650 cells, and detect when the fully-charged terminal voltage (12.6V in this case) is reached, then shut off once the current falls to a reasonably low value. Good chargers also detect undervoltage and gently bring the cells up to their normal operating range, before cranking up the charge current to charge them quickly.

If you have no other power management in this application, the BMS should be set to cut off discharge at a cell voltage of 3.0 V for Li-ion cells. Any lower than that starts to risk damage to the cells. The spec says 2.3-3.0V which is rather a wide range, so presumably it's configurable or you can choose it when you buy the module.

A side note on balancing: all cells, even nominally identical ones, are slightly different, so when charged in series, some will reach full charge before the others. A good BMS will then start to bypass those cells, shunting the charge current they would otherwise be receiving, so the remaining cells in the series chain can quite rapidly come up to full charge as well. This needs big, hot load resistors or cunning circuitry to shunt the current aside. I think this simple BMS has neither, only very small bleed resistors to provide some limited balancing once charging has completed. That means that as one cell reaches the cutoff voltage, the BMS will likely shut off the connection to the charger altogether and only reconnect it once the bleed resistors have taken the edge off the full charge on that cell. Then it'll reconnect the charger and the cycle will repeat until all the cells are charged. This can take a long time if there's a significant mismatch between the cells, and the on/off cycling can thoroughly confuse some chargers.

Chris
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