NiMH battery charger circuit and production

Photocoupler

I recently came across a M9000 camera battery that's widely available on the market. Most of these batteries are made by connecting nickel-metal hydride (NiMH) cells in series, typically rated at 12V with either 1.8Ah or 2.1Ah capacity. However, when using the original camera charger, the battery tends to overheat—reaching temperatures above 50°C. This issue becomes worse with frequent charging, leading to rapid decomposition and evaporation of the electrolyte. Eventually, the battery pack becomes unusable, and even after charging, it often doesn’t hold enough power.

The original charger is designed for maintenance-free lead-acid batteries, with a nominal output of 12V or 14V at 1.5A. It’s not suitable for NiMH battery packs because the current is too high and the voltage is too low. Last year, I built a custom charging circuit that has worked reliably for over a year. It’s used to charge several NiMH battery packs for different cameras. The charging parameters are: a current-to-capacity ratio of 10:2, a voltage ratio of about 1:1.5, and a total charging time of 5 hours. The battery charges fully without any overheating.

As shown in the diagram, the mains supply is rectified by D1 to D4 to generate around 200V DC, which is then fed into an oscillation circuit consisting of VT, C1, C2, R5, R6, L1, L2, etc. The pulse voltage from the secondary winding of BT (L3) is rectified and boosted by D7 and C4, resulting in approximately 18–19V DC. The current is around 500–600mA, which is sufficient to charge a 12V NiMH battery pack with a capacity of about 2000mAh. The transformer BT can be salvaged from an old fluorescent lamp ballast. L1 is wound with 120 turns of 0.18mm wire, L2 with 10 turns of 0.18mm wire, and L3 with 25 turns of 0.26mm wire. The transistor VT can be a C2271, C1507, or 3DA87E with a BVceo rating of at least 350V. C1 and C2 are ceramic capacitors, and resistors should be 1/4W. Diodes should be selected according to the schematic. If the output voltage is too low, simply adjust the number of turns on L3. Soldering is straightforward, and mistakes are unlikely.

The PCB can be designed and manufactured based on the provided diagrams (note that the transformer is not mounted on the board). This DIY solution offers a safe and effective way to charge NiMH batteries without the risk of overheating. It’s especially useful for older devices like the M9000 camera, where the original charger may not be suitable. I’ve tested this design multiple times, and it works well under various conditions. If you're looking for a reliable alternative to the original charger, this circuit is worth trying.