Battery ChargerTemperature compensated

L200 Charger Circuit Schematic Circuit Diagram

Creating an Impromptu NiMH Battery Charger: The Circuit’s Origin

This particular circuit originated from a pressing requirement for a NiMH battery L200 charger. Faced with a lack of an immediately available dedicated IC, the author ingeniously repurposed an L200 regulator and a 4.7 kΩ NTC thermistor. With these components at hand, a charger framework was constructed, focusing on a cutoff condition triggered by cell temperature increase, deviating from the conventional approach of relying on negative delta V detection.

L200 Charger Circuit Schematic Circuit Diagram

Utilizing the L200 with Thermistor Feedback

In this configuration, the circuit employs the L200 regulator in tandem with the thermistor within the feedback loop. When the batteries are in a ‘cold’ state, the regulator’s output voltage settles at approximately 1.55 V per cell. As the batteries warm up to a temperature ranging from 35 °C to 40 °C, the output voltage decreases to about 1.45 V per cell, while the thermistor concurrently exhibits a resistance of around 3.3 kΩ. This temperature-sensing mechanism is sufficient to prevent overcharging of the cells. A charging voltage adjustment is facilitated by P1, while R2 serves as a limiter for the charge current, capping it at 320 mA. To manage heat dissipation, a small 20 K/W heatsink is attached to the IC since it dissipates approximately 1.2 watts during operation.

Permanent Connection for Hassle-Free Charging

This charger circuit offers the convenience of permanent connection to the battery pack. Charging initiation occurs upon connecting a ‘plug top’ adapter to the charger’s input. The author’s utilization of an unregulated 12 V supply unveiled an open-circuit voltage of 18 V, which settled at 14 V under a load. It’s crucial to note that despite a reduction in the charge voltage upon completion, it’s inadvisable to leave the cells continuously on charge. The author has successfully employed this circuit to charge a battery within a torch. Impressively, after three years and around 150 charge cycles, the cells have exhibited no signs of capacity deterioration.

Nickel Metal Hydride Batteries: A Rechargeable Option

Nickel metal hydride (NiMH) batteries fall under the category of rechargeable batteries. They share a similar chemical reaction at the positive electrode with nickel-cadmium cells, featuring nickel oxide hydroxide. However, the negative electrodes employ hydrogen-absorbing alloys instead of cadmium. NiMH batteries offer advantages such as higher specific energy, containing fewer toxic metals, reduced memory effect, and the ability to generate high peak power. Additionally, they excel in deep discharging and are considered environmentally friendly. Nonetheless, NiMH batteries are relatively more expensive, experience higher self-discharge rates, and exhibit lower efficiency compared to lead-acid and Ni-Cd batteries.

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