I would like to charge the NiMH batteries, when there is excess power from the solar power and use the power stored in the batteries, when I need it (to drive a motor and at night). I have the following circuit:

IC1: LM317
IC2: LM7805


On the right side, there is a uC, which needs to be powered all the time (through day and night) and drives a heavy load (motor cca 2A) for a while once a day.

The problem is, that I am not sure how the circuit will behave when there is power from the solar panel (SOL+ and SOL-), but it might not be enough to drive the motor (which will be connected to the +18 line) - ideally the voltage should drop and the MOSFET Q1 should open, supplying the motor power from the batteries. But I am afraid, that the battery and solar voltages will get to some balanced state, leaving the MOSFET half open and frying it.

What is the common solution to this problem? Would simply changing the MOSFET for a diode solve it better? Also is the diode D2 needed (I put it there to stop the LM317 from trying to source current when powered from batteries).

Thanks a lot for your ansvers!

  • \$\begingroup\$ What is your NiMH battery pack voltage? If it is driving an 18V line, I assume it is 18V? What is your solar cell full sun voltage and current capability? \$\endgroup\$ – Joe Apr 10 '14 at 12:33
  • \$\begingroup\$ Actually the NiMH battery pack voltage is 7.2V, the 18V is an approximate maximum on the line. The specs for the solar panel are 21.5V open voltage and 3.88A shorted current. But I don't see how this is important... \$\endgroup\$ – Matouš Vrba Apr 10 '14 at 20:48

I have resolved this problem by replacing the Q1 MOSFET with a simple Shottky diode. All is tested and working well.

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  • \$\begingroup\$ My first thought when I read just the title. \$\endgroup\$ – Kaz May 15 '14 at 3:33

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