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I am just buying some LiPos (3.6V) and charging circuits for a project. The batteries come with overheating, over-voltage, over- and undercharging protection circuitry. In the reviews, someone mentions that you can use any source to charge it due to the protection. I googled it but it seems people make a difference between protection and charging circuitry, and I cannot find an example of someone using it without charger. I see why, but wonder if a protective circuit would work as a make-shift charging circuit, at least it won't explode, right?

In short, can I just hook the battery on the Vin on my D1 mini, supply the board from battery and charge the battery when hooking the board onto USB?

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No, don't. A BMS is not a charger.

  • A BMS can't limit the charging current, let alone provide a constant current for the CC charging stage, it can only stop charging altogether when the current is too high;

  • It doesn't terminate the charging properly, but, at best, keeps the battery floating indefinitely at its maximum voltage;

  • Maximum voltage on many BMSes is set at just over the maximum recommended voltage for charging, and this will overcharge the battery;

  • Etc.

Even if you get away with it (as in "no immediate fireworks"), it will seriously limit the cycle life of the battery, and possibly not charge it at all if you don't limit the charging current.

Also, your ESP is 3.3V, and you can't charge LiPo with that, and simply connecting a 5V voltage source from the USB to a LiPo battery is not a good idea, BMS or not.

There are plenty of cheap ICs and modules (with USB on their input side) around for charging LiPo batteries; you should use one of those. The BMS is there as a last-ditch protection when things go wrong.

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