I'm trying to design a circuit that lets a load will get power from a solar panel or a battery (in case the solar panel is not delivering enough power).

The question is about connecting the charge controller and the BMS-2S to the load.

1. 12 V solar panel connected to charge controller as input
2. 2S (3.7 V each) Li-ion batteries
3. RPI Zero as load. Will draw about 150~200 mA

About the BMS: it says that pins P+ connect to charger+ and load+, and P- connect to charge- and load-.

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Won't connecting it that way, with the BMS and the load parallel to the charge controller, interrupt the charge controller main goal?

I wonder how it will affect the Constant Current and Constant Voltage mechanism.

Im not sure that the load is supposed to be connected that way. I'd be happy with some guidance.

  • \$\begingroup\$ Did you find your answer? I'm also wondering if load and BMS in parallel impacts charging. \$\endgroup\$
    – rrmoelker
    Commented Mar 14, 2021 at 19:12
  • \$\begingroup\$ The charge controller will probably never finish its charge cycle because the RPi will draw a significant amount of current. This will confuse the charge controller into thinking the batteries are not yet full. The LiIon batteries will therefore be under constant charge, which will kill them in a short amount of time. \$\endgroup\$
    – StarCat
    Commented Jan 7, 2022 at 9:16
  • \$\begingroup\$ @StarCat: That's why the current-sense resistor is an external component and not measured inside the charge controller IC. Current should be measured on the path dedicated to the battery, after the load current has branched off. \$\endgroup\$
    – Ben Voigt
    Commented Feb 17, 2023 at 22:14

1 Answer 1


When your energy source is a solar panel, the constant current is not constant, as it depends on how much current your panel delivers.

This way, the "constant current - constant voltage" charging algorithm transforms into "limited current - limited voltage" charging. Batteries are generally OK with this, as long as your BMS behaves.

The load will subtract from the available charging current and it is up to you to get large enough solar panel if you want the batteries to be fully charged at the end of the day.


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