The most simple trick to prevent overcharging seems to be: make sure your solar panels... won't overcharge the batteries! If you have sufficiently big batteries (enough mAh) the solar cells will just never be able to fully recharge them and thus overcharge them.
To estimate the number of hours needed to fully recharge batteries, use the simple calculation: I / C * 120% where I is the batteries capacity (in mAh) and C is the solar panels output (in mA). The 120% is an approximation that we will lose about 20% of the charge. Of course, for a solar panel, we also need to consider the efficiency of the cells under the sun (e.g. in cloudy weather the efficiency drops).
You can alternatively use an online charging calculator such as this one to ease your work.
Simple trick
The most simple trick to prevent overcharging seems to be: make sure your solar panels... won't overcharge the batteries! If you have sufficiently big batteries (enough mAh) the solar cells will just never be able to fully recharge them and thus overcharge them.
To estimate the number of hours needed to fully recharge batteries, use the simple calculation: I / C * 120% where I is the batteries capacity (in mAh) and C is the solar panels output (in mA). The 120% is an approximation that we will lose about 20% of the charge. Of course, for a solar panel, we also need to consider the efficiency of the cells under the sun (e.g. in cloudy weather the efficiency drops).
You can alternatively use an online charging calculator such as this one to ease your work.
I've read elsewhere that it's possible to limit overcharging by simply adding a resistor between the diode and the batteries.