There are three different methods of charging NiMH-batteries:
- Voltage-monitored charging
- Temperature-monitored charging
- Timer charging
Voltage-monitored charging is the most common and most intelligent method of charging. During the charging process, the voltage of NiMH batteries rises until it reaches maximum. This means that the battery is fully charged. If you continue to charge the voltage will drop by a small amount.
If you string together all measured voltage values you will receive the typical voltage curve for those rechargeable batteries.
The switch-off of the charging takes place after:
- Turning point – the battery voltage is increasing but not as quickly as at the beginning (the rise of voltage decreases)
- Maximum – the battery voltage is no longer rising
- deltaV – the battery voltage has achieved its maximum and has already begun to decline
A timer-controlled charger terminates the charging process at a pre-set time.
Timer chargers are the easiest automatic battery chargers to use. As they are not able to determine the charging condition neither before nor during the charging process it is recommendable to only charge fully discharged rechargeable batteries with this type of charger. Otherwise, batteries will be partly overcharged.
This kind of charging utilizes the thermal behavior of batteries. At the end of the charging process, when the batteries are fully charged, the temperature increases quickly. There are three different switch-off criteria.
- The charging process is usually stopped when a battery temperature of 45-50°C is detected.
- After dT/dt: This means the charging process will end if the battery temperature rises within a certain time to a certain value. A common value is for example 1°C per minute.
- Only changes in temperatures are measured. Time is not a factor.
Normally, temperature-monitored charging is paired with a voltage-monitored charging for safety reasons.