I am using a Solar Module to charge a LiPo battery. The current supplied by the solar module goes into a battery charger IC and from there to the battery. This works fine. Now I want to add an external usb charger which will also charge the battery.

The first problem I have is: What would happen to my solar module if the voltage from another power supply will be connected to its terminals ?

The second part of the problem: I need a selector circuit that will disconnect the solar module's output while the usb charger is connected and will reconnect it when the usb charger has been removed.

Later Edit

Addressing the OR'ing diodes suggestion: Here's why I don't want to use a diode on the Solar module's output: The battery used on this project is a Li-Ion 2800mA Type. There's a microcontroller on board which sleeps most of the time. Once in a while (for a few minutes every 2-3 days) it wakes up and drives a motor (an Hbridge is used). The power consumption is not very big but ... the solar module delivers a top 100mA /6V. This is a slow rate of charge. If I place a schottky to isolate the solar module from a second power supply I will miss those .1 - .2 volt (forward voltage) dropping on the diode. Now let's assume the sun doesn't shine very bright for a few days or a week, and the battery discharges to around 3.5v. That's the lower limit for the LDO which provides 3.3V for most of the logic onboard (including the motor driver, the microcontroller and the motor itself). That's the reason I wanted to add the second recharging method. So basically I'll use a 5V usb charger to recharge when necessary. I thought about using a DPDT switch (half will switch power on/off and the other half will comute between the two input supplies for the battery charger). But I have some issues with the dimensions and honestly I'd preffer the electronic switch in this case.

Addressing the electronic switching: I saw an app note from maxim. The MAX6326 seems a good choice for this scenario. But a bit expensive. I would preffer a simple approach (as in: as few components as possible) using mosfets. And I need help with this approach and choosing the right mosfets. Or perhaps someone has another suggestion to solve this in a more elegant way.

Thanks a lot

  • \$\begingroup\$ Why not prioritize the solar energy - that's the one that is "free"... \$\endgroup\$
    – Solar Mike
    Commented Oct 19, 2018 at 16:09
  • \$\begingroup\$ it's beeing able to charge at night or when cloudly that concerns me \$\endgroup\$ Commented Oct 19, 2018 at 16:16
  • \$\begingroup\$ Seems if you solve the second requirement, you've made the first requirement superfluous. \$\endgroup\$
    – mike65535
    Commented Oct 19, 2018 at 16:27
  • \$\begingroup\$ Do you know anything about MPT theory and ESR matching? \$\endgroup\$ Commented Oct 19, 2018 at 19:04

1 Answer 1


If you want the charging selection manually done, get a SPDT switch and and select between the two chargers.

Not knowing the voltages and currents involved makes the automated switchover suggestion tentative, but you can put OR'ing diodes on the outputs of both solar and USB charger output. Highest voltage wins each diode prevents current from going back up the other charger (generally a bad thing).

If you want less loss than a Shottkey type diode (usually 0.3 to 0.6V loss, depending on current and diode), you'd have to put together a MOSFET switch selector, that would get you nearly no loss on the charging paths.

Make sure your components can handle the charging current, even a relatively low current 1-2A will heat up a small diode to an uncomfortable temperature, FETs are considerably better that way since they have very little voltage drop when properly biased.


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