- Trickle charging (=charging very slowly): this means we use a charging current (around a tenth of the battery capacity). It such slow rates, there is no risk of "overcharging" the battery since the battery can handle overcharge if it come slowly. Many "overnight" chargers use this technique to avoid having to rely on intelligent techniques (and therefore more expensive electronic components).
I however recommend avoiding chargers using this technique: some of them try to come as close as possible to the trickle charging limit (to charge faster). This causes your batteries to age faster. These chargers can easily be recognised as they usually require you to charge your batteries in pair. Remember: charger manufacturers are also battery manufacturers and it is in their best interest to make you buy new ones.
- Voltage delta charging: this technique allows faster charging. It requires a charger charger capable to charge at a constant current and monitor the voltage of the battery. It will stop when the voltage drops slightly (5 to 10mV).
- Temperature delta charging: this technique is similar to the previous one (the charger uses a constant current to charge the battery) but this time the charger will stop the charge when the battery temperature starts to rise quickly (e.g. more that one 1°C per minute) or reach a threshold (60°C).
Bottom line: quality chargers can usually let you charges your NiMh one by one and say in their specifications that they have a temperature sensor. You should look for these ones.