Ni Cad chargers help

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Buford
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Re: Ni Cad chargers help

Post by Buford » Thu Jul 17, 2003 11:25 am

Several years ago, I was interested in building a charger for a 9v rectangle rechargeable replacement, and did a little study of the subject. NiCADs really are much more quirky than anyone might suspect at first glance. I mean they've been used for years and years in a host of consumer and industrial applications, right?
Generally, as some of the posts have indicated, they can be slow-charged with a constant current source at 1/10 of the rated current capacity, and kept on "trickle" after reaching full charge. Some of the other factors to consider about NiCADs are their notorious "memory", the need to avoid overcharging -- I believe fast chargers generally incorporate a temperature sensor into the design, whether the NiCAD is of a sealed or vented type (the small ones with which I've had contact are sealed), the fact that NiCADs are susceptible to shorting within the cell, and of course that for a reliable judgement of charge status the terminal voltage must be checked under a load.
Also, I recall from my reading that although many chargers such as the simple design with the LM317 appear to work just fine, optimal NiCAD charging actually calls for a half wave rectified DC "ripple" supply. This is the type of scheme I used.
Just a couple of ideas to explore...

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