NiMH battery pack charging circuit using a solar panel

Question

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:

1. 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.
2. Are there any design considerations I have missed in the circuit which I should be looking into keeping the circuit as simple as possible?
3. 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?

Answer 1

What diode should I choose to connect in series with Solar panel to prevent reverse current when Solar panel produces no current? I prefer not to go with traditional silicon diodes since the voltage drop is high and cannot afford any voltage drop.

First, the regulator you are using will prevent reverse current. Although most panels come with diodes pre-soldered. Even if you add a diode, You are still using a regulator to drop the voltage to 8V. what difference does it make anyway?

Secondly, If you are worried about voltage drop and efficiency, you shouldn't have considered using a voltage regulator to convert 12v to 8v. If you are still going to use the regulator, do add an appropriate heat sink as it will loose ~ 1W in heat.

To answer about low voltage drop diodes anyway - Did you consider Schottky diodes? Also, check out the HSMS-2805 series they have an ultra-low voltage drop.

The better way to do is the reverse voltage protection circuit if you are ok with a little complexity.

I intend to use a load which will draw about 250mA from the battery, is it safe to use such load provided it will be ON simultaneously when the battery gets charged using solar panel?

Your load is 250mA and your charger limits/charges at 210mA. you will drain the battery instead.

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 am sure you can make it better but I can't think of a simpler way.