I have been working on a NiMH battery pack (7.2V / 2100mAh) charging circuit powered by a 12V solar panel. With some research I found that charging NiMH takes different forms by measuring change in voltage, change in temperature, voltage across the battery.
I need my charging circuit to be simple, so I opted for measuring voltage across battery method and slow charging at the rate of C/10.
Using this I designed the below circuit:
I have used an LM317 to give an output of 8V which will be used to charge the battery. I have also used an LM311 comparator to activate charging and discharging through an IRF510 MOSFET. The current is limited to 210mA to charge the battery at the rate of C/10.
I have set the hysteresis of the comparator at VL at 7.3 and VH at 7.8 so when the battery across battery crosses 7.8V it stops charging and it starts charging once the battery voltage drops below 7.3V.
Now my questions are:
- What diode should I choose to connect in series with solar panel to prevent reverse current when the solar panel produces no current? I prefer not to go with traditional silicon diodes since the voltage drop is high and I cannot afford any voltage drop.
- Are there any design considerations I have missed in the circuit which I should be looking into keeping the circuit as simple as possible?
- I intend to use a load which will draw about 250mA from the battery. Is it safe to use such a load provided it will be ON simultaneously when the battery is charged using the solar panel?