Float Charging NiMH Cells


I recently found an interesting circuit worth sharing that covers a few handy pointers for charging NiMH batteries.

Float charging has the advantage of keeping the cells fully charged and ready to use without the potential damage of long-term trickle charging or the cost of low-discharge cells. This approach works because NiMH cells do not have the memory problems associated with Nicads.


The Circuit

Based around the LM317 voltage regulator, resistors R2 & R3 and potentiometer VR1 set the maximum output voltage between 1.3V and 1.4V per cell. VR1 should be adjusted to obtain 1.35V per cell at the regulator output. Resistor R2 is fixed at 240Ω. In-case your interested, the formula for the output is Vout = 1.25 x (1 + (R2 + VR1) / R2)

Diode D1 protects the circuit from reverse polarity of the plugpack or in the event the plug pack is removed while the circuit is still connected to a charged battery pack. Resistor RCL and transistor Q1 limit the maximum current in the event of a short circuit or the connection of a well-discharged battery pack. LED2 provides an indication of voltage input to the charger.


The 680Ω resistor and LED1 provide the same function for the charger output, while also providing a minimum load for the regulator when the battery is nearing full charge (prevents the regulator output from drifting up and damaging the batteries).


More Cells?

The above variant of the charger is suitable for four NiMH cells rated at 2.5Ah with a peak charge current of 500mA. The circuit can be modified like as shown below to charge 1-10 series cells, with peak charge currents of 200mA-600mA:

Number of NiMH Cells Plugpack Voltage R3 VR1 R4 R5
1 6V 100Ω 270Ω (replace LED1
with a wire link)
2 9V 220Ω 200Ω 180Ω 1.5kΩ
4 12V 680kΩ 500Ω 680Ω 2.2kΩ
6 15V 1.1kΩ 500Ω 1.2kΩ 2.7kΩ
8 18V 1.5kΩ 1kΩ 1.8Ω 3.3kΩ
10 20V 2kΩ 1kΩ 2.2kΩ



Max Current 200mA 400mA 500mA 600mA
R1 1.2Ω 0.6Ω 0.5Ω 0.4Ω
RCL 3.3Ω 1.5Ω 1.2Ω (0.5W) 1Ω (1W)

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