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I have a solar input charger (CN3791) and USB input charger (BQ25300). Is it possible to charge the battery attached with both at the same time?

The battery needs to be charged at a constant-ish rate, which the solar charger can do in certain weather conditions, but not always. The idea would be to always charge with the solar charger, measure its output current, and adjust the BQ25300's output current to get the desired overall charge current. For this solution, both chargers need to be able to work "together", charge the battery at the same time.

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  • \$\begingroup\$ Both chargers need to know what current the other is providing, in order to terminate the CV phase properly! Unless perhaps you limit the CV phase to 4.0 V rather than 4.2 V, so the termination current becomes moot. \$\endgroup\$
    – Neil_UK
    Jul 14, 2023 at 13:01
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    \$\begingroup\$ You will be compromising the termination current. Apart from that, I don’t see any problems. \$\endgroup\$
    – winny
    Jul 14, 2023 at 15:05

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Yes, you can. As far as the battery is concerned, it's OK.

(As long as the Li-ion battery includes a protector BMS.)

Other people's concerns are unfounded:

  1. There is no issue with turning off charging when the battery is full. If each charger is set to turn off at a current of 0.1 C, then one charger will turn off first (when its current drops below 0.1 C), and the other charger when the battery current drops below 0.1 C. But that is overshadowed by the presence of a load, if any.

  2. There is no issue with CC capability. The two chargers may have different CC capability. Each will charge the battery with a current it is capable of, until it reaches its CV setting. No problem.

  3. There is no issue with CCCV switching. The two chargers may have different CC capability but both will have nearly the same CV setting. When the battery voltage reaches the CV setting of either charger, that charger with switch to CV and its current will start dropping. The other charger will wait until the battery voltage reaches its CV setting and it too will switch to CV. No problem.

The two chargers may not like being paralleled without a battery. But once the battery is connected, each charger only sees a battery, not the other charger.

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