Do I need a solar charge controller for low watt/low battery voltage situations? This site says that they are generally not needed:
http://www.solar-electric.com/solar-charge-controller-basics.html
For instance, I want to use a 2w 5.5v panel to charge 3X 1.2v AA batteries.
Do I need to use a steady state voltage to charge the batteries, or can I get away with just a zener diode to limit battery flow to the solar panel at night?
With that small panel the batteries will not over charge. Check the current draw to the panel in the dark from the batteries to see if you need a diode.
The accepted answer is ~= 3 years old - but is wrong.
A 2 Watt 5.5V panel will supply 360 mA in full sun.
About 2000 mAh on a 6 hour summer day in many locations if optionally positioned and moved - so let's say 1500 mAh and could be lower.
2 x AA NimH in series rated at say 2500-3000 mAh an charged at up to 360 mA wil require about 1.5V/cell for full charge. The 5.5V available gives about 2V headroom (even with series diode). This is more than enough to ensure that the panel will continue to charge the cells when they are fully charged.
If left on charge and not used for several days in bright sunlight will overcharge the cells. Modern NimH cells are unable to tolerate this level of charging.
This will rapidly damage and too soon destroy modern high capacity AA cells.
Some form of charging controller is required.
