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I'm using a 2s BMC to charge a 2s2p lithium battery pack (7.4v, ~6Ah). The output of the charging circuit is the same as the input - they're not separate. As a result, when I want to charge the batteries, I just hook up 8.5v to the +P and -P terminals and let the BMC do its thing.

Is this bad practice? Is there any potential damage to the batteries if I am charging them AND the power from the charger is powering the circuit (the circuit normally powered by the batteries) at the same time?

I don't believe so off-hand, but I've heard that too much ripple on the voltage supply for the BMC can cause issues where it has a hard time detecting the charge status of the Li-Ions.

Thanks!

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  • \$\begingroup\$ If the battery pack and BMC are from reputable vendors (not a no name ), it should be fine. You should also know the BMC charge rate. I'm not a fan of the quick charge. Check out battery University, They have a section on charge methods. But first understanding Li-ion is a good start if needed. And if you use no name products read the section on safety. batteryuniversity.com/learn/article/types_of_lithium_ion \$\endgroup\$ – Misunderstood Oct 18 '18 at 0:01
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The BMC measures and controls the voltage and current that is provided to the battery and do so quite precisely to improve the life of the battery (depends of the quality of the BMC of course).

If you connect a circuit, while charging, some of the current the BMC would think goes to the battery will actually go to your circuit.

This might create issue the BMC charging algorithm, especially to detect the end of charge of the battery, since you are still drawing current at a voltage the battery shouldn't.

There are different lithium chemistry, and charging profile also depends on the chemistry of the battery, some has some small fluctuation that the BMC will pick up to detect the end of charge.

To answer the question, it might depends on which battery/chemistry is used and what BMC is being used. In some case it will be no problem, in some case it might fail to detect the end of charge and go in fault.

In case of Li-ion and Li-po, it would probably not damage the battery as the end of charge is constant voltage.

A better solution would be to power your circuit from a source before the BMC, this usually can simply be done by adding a diode between the battery and the circuit, or better with an electronic switch.

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  • \$\begingroup\$ I just want to make sure I’m understanding what you’re saying, and you’re understanding my layout: I have charger, 8.4v, goes to “+P” and “-P” on the BMC. Those same connections continue out to the circuit the battery powers. When the charger is unplugged, the batteries power the circuit (through +P and -P), and when plugged in, I feel like the 8.4v should power the circuit AND the BMC as it charges the batteries. I don’t understand how the BMC would think it was getting power that it wasn’t... \$\endgroup\$ – Helpful Oct 18 '18 at 3:33
  • \$\begingroup\$ As an additional note, the batteries themselves are not connected to +P and -P, they are connected to 3 other pads on the BMC. \$\endgroup\$ – Helpful Oct 18 '18 at 3:33

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