Some theory about NiMH batteries:
The solar panels used here are rated 2V/200mA in full sunlight. I used 4 in series to that gives me 8V/200mA or 1.6W. Now, I want to use this to charge 4 NiMH AA batteries rated capacity C=2600mAh. How do we know the batteries will be full?
A decent NiMH charger checks the temperature and also voltage drop at the end of the charging. However, to be able to check for the small drop at the end of charging, the charging current must be something like C/2 (the capacity, divided by two, without the "hour").
In full sunlight I measured the short circuit current of the panel to be 270mA, so about C/10. This is the short circuit current, so at higher voltages (the battery voltage) the panel will charge the batteries at less current than C/10. Constantly charging batteries at low currents (compared to the capacity/hour) is called "trickle charging".
Now, it used to be in the past that NiMH batteries did not handle trickle charging well, if above C/20 or even C/50. However modern NiMH can be safely (for the battery I mean) trickle charged at C/10. Add to that the fact that the sun won't be up all the time, and we conclude that our charger can be very simple: one diode. The diode makes sure that the batteries can't discharge back in the solar panel, once the sun is down.
What about the load?
NiMH can take quite a load (up to 2C),but there is one thing that they don't like and that's deep-discharge. Deep discharge, meaning drawing current from the batteries when their voltage is below a certain point (0.9V ... 1.0V) will shorten their life time considerably. Obviously, there are two things you can do to prevent deep discharge:
1) use a switch to disconnect the load when the batteries are low.
2) use some electronic circuit to disconnect the load, once the voltage is low.
I used the second method in my battery box to prevent over-discharging the batteries: when the voltage is above 1.2V / cell (4.8V for the pack) the output is connected. Then, when the voltage drops below about 0.9V/cell or 3.6V for the pack, the output is disconnected, until the solar panel charges the batteries sufficiently again. In this way you do not need to worry about over-discharging.