The charging of a battery is made possible by a reversible chemical reaction that restores energy in a chemical system. Depending on the chemicals used, the battery will have certain characteristics. For a Li-ion battery or Lead acid battery charger, the basic algorithm is to charge at constant current until the charge current has dropped to a lower limit then termination is triggered. Unlike Li-ion or Lead acid battery, when a NiMH battery is charged, there is no significant change in charging current when the battery is fully charged. Instead, it’s voltage begins to drop. On the other hands, Ni-MH batteries are damaged from overcharging. It is therefore important to do accurate measurements to terminate the charging at exactly the right time. In this respect, Ni-MH charger is more complex than that of lead acid and lithium-ion..
So what make a Ni-MH battery charger different from others, for a main consideration, is the Termination Methods.
Ni-MH battery will self-discharge at a rate of C/30 to C/50. If the battery is over-discharged or not used for long time, larger current charge can not fully recover the energy capacity, so the battery need to be pre-charged with small current about 1/30-1/20C to activate the battery. After the voltage of battery rise up, then can enter fast charge stage with large current about 1C to charge. The charging current keeps constant.
When one considers safe charging, time-efficiency and low cost factors, the MCU controlled charging method can be used as an ideal solution. MCU controls the whole charge process and guarantees the battery pack is fully charged, and also avoids battery pack over charge.
Intai’s Ni-MH battery chargers use the following termination methods.
- The use of voltage drop (-dV/dt) as primary termination method. The charge termination is based on a negative slope in battery voltage.
- With absolute voltage as backup. Charging is terminated when the voltage rises above a percent upper limit.
- Time control: A simple and secure way to measure when to terminate charging.
- Automatic detection of shorted/ bad battery.
The battery chargers made by Dongguan Shijie Intai Switching Power Supply Factory fully implement the latest technologies . | 
