NiMH and NiCad batteries are 2 of the most challenging batteries to charge properly and safely. These nickel-based batteries do not allow you set a maximum charge voltage, so overcharging can result damage if you are unaware of the proper charging methods for nickel batteries. Learn how to charge NiMH batteries so you can avoid potential charging problems.
Wall mount charger / Laptop adapter
Use a wall mount charger or laptop adapter to charge the battery overnight. Charge at C/10 (10 percent of the battery’s rated capacity per hour.) For example, if you have Toshiba pa3536u-1brs battery, you would charge it at 10mA for 15 hours. Your battery is likely to contain an oxygen recycling catalyst to prevent overcharging, but this feature is not functional if the battery is charged over C/10.
To preserve the life of the battery, do not allow a wall mount charger or laptop adapter to charge a NiMH battery for more than 13 to 15 hours.
Power dissipating package
Use a power dissipating package to charge the battery at C/3.33 for 5 hours if you cannot wait the 15 hours required for the overnight method. Before charging, fully discharge the battery to eliminate battery memory. You can drain the battery by allowing the charger to discharge it at 1 volt per cell. Once the battery is completely discharged, turn on the charger for the required time of 5 hours.
Microprocessor controlled battery
Use a microprocessor-controlled battery charger to charge the NiMH battery at 100 percent of its capacity for 1.5 hours. This type of charger contains a temperature monitor so that venting or overheating does not occur. It also senses voltage and current and adjusts itself accordingly. To end the charging session based on unsafe temperature increases, set the dT/dt value at 33.8 to 35.6 degrees Fahrenheit (1 to 2 degrees Celsius) per minute.
Tips
A microprocessor-based 9-volt transistor radio battery charger is an inexpensive alternative to a power dissipating package if you prefer the 5-hour charging method that requires a full discharge prior to charging.
Warnings
No matter which charging method you use, it is important to prevent overcharging. Use a timer to prevent this.
You cannot charge NiMH cells and batteries in parallel. Some cells or batteries may take more current than the others because each cell is not necessarily the same resistance. Instead, you must charge with a separate circuit for each string. Alternatively, you can balance the circuit by using a resistor that controls the current so that each cell or battery receives equal current.
NiMH and other nickel-based batteries do not charge efficiently in comparison to other types of batteries. The coulometric charging efficiency is only 66 percent, which means you have to put in 150 amp hours to get 100 amp hours out of the battery. The efficiency is even lower if you charge the battery faster than the standard 15 hours at C/10.