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.
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
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