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A Fast Charger For NiMH & Nicad Batteries

This is a truly versatile charger. It can charge up to 15 identical NiMH or Nicad cells. You can build it to suit any size cells or cell capacity and you can set the charge rate. It can fast charge, trickle charge and has safeguards, including temperature sensing, to prevent overcharging.

By John Clarke

Standard NiMH and Nicad chargers are available just about anywhere, from hobby stores to supermarkets, the service station and even your local chemist or newsagent.

However, they usually only charge two or four AA cells and at quite a slow rate – typically they will take between four and 15 hours to charge. But what if you want to charge at a much higher rate or more than four cells at a time? Or cater for C and D cells or battery packs?

The only complete answer is to build the new SILICON CHIP Fast NiMH Charger.

Click for larger image
These 1500, 1700 and 1800mAh 'AA' NiMH batteries were considered "state of the art" in our last NiMH charger (November 2002). Now 2500mAh are quite common (we've even seen claims of 3000+ 'AA'). Our new charger will handle these as well as C and D cells and even battery packs.

It can charge from one cell up to 15 cells simultaneously and battery packs up to 18V for both NiMH and Nicad types. Charging can be set from just a few milliamps up to 2.5A and it includes a reliable end-of-charge detection, with extra safeguards included to prevent over-charging.

Safety is important when charging NiMH and NiCad cells and batteries because they can be destroyed, or have their life seriously shortened, if the charger is left on for too long after the battery pack has reached full charge.

To see why over-charging can destroy a battery pack, have a look at Fig.1. This shows the typical voltage, temperature and internal pressure rise of a cell or battery pack with charge. Once charging goes past the 100% point, the temperature and internal pressures rapidly rise and the voltage initially rises and then falls.

Continual overcharging will damage the cells due to the elevated temperature. This accelerates chemical reactions that contribute to the ageing process. In extreme cases during overcharging, excessive internal pressure can open their safety vents to release the pressure. These vents will re-close after the pressure is released but the cells will already have been damaged.

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