Battery charger par excellence

[bugwhiskers]

If anyone is after a battery charger I can highly recommend the Swallow ac/dc 2. I purchased one on ebay a few weeks back, new, for $125 Aus. What makes them special is they can charge NiCd, NiMH, Lithium Ion/Polymer and lead acid batteries. For the NiCd and NiMH there are also discharge/charge cycles up to 5 cycles to get the old sluggish ones moving again. You can even charge your 12v car battery with it. It also has a seperate input to power it from your cars battery if you are out in the country.

When it arrived I dug out all my old Nicads, one of them a 400 mAH is at least 20 years old. The multimeter read a big fat zero. The first time through the discharge/charge cycle it got to about 20 ma. Each subsequent cycle saw it creep back up and eventually it got back to 400ma. Aint if funny what a battery will do to avoid being thrown away.

regards
bugwhiskers

[mbiondo]

Discharge cycles

Your note got me wondering about the optimum discharge cycle of the various families of rechargables. Does it make any difference how fast the discharge is? In other words, is there a optimum current to discharge rechargable batteries?

Thanks...

Michael-

[bugwhiskers]

Hi mbiondo,

The mAH rating of batteries is what you can expect to get if you discharge them at exactly at that rate. Example, if you have a NiMH battery rated at 1000mAH you can pull 1 amp out of it for an hour. Lets call 1000mAH "1C" because often manufacturers specify charge and discharge rates using this terminology. If you discharge your battery in 30 minutes you have discharged it at "2C"...starting to get the picture? Often the max discharge rates are specified as multiples of "C" especially with the newer Lithium batteries as charging and discharging them within manufacturers specs is mandatory.

You could discharge NiCd at "5C" or more and many of the old slot car racers often did...but they get very hot and can die early.

When it comes to charging I prefer slow as the battery doesn't get hot. The small change in battery voltage with NiMH and NiCd called dv/dt when they are charging that indicates battery is full could be swamped and missed during a fast charge.

regards
bugwhiskers

[mbiondo]

Interesting stuff

Bug...thanks for the input. As a result I went to energizer.com (DOH! I should have thought of that first...before I asked the question. Sorry!) and found some very interesting stuff:

http://data.energizer.com/PDFs/nicke...ide_appman.pdf

The one thing that came as a real shocker was Over Discharging. In the past whenever my rechargables seemed to be losing some recharge capacity, I would just pop them into a flashlight, turn on the flashlight, and let it sit for a few days. My thinking was that I would totally discharge the battery to condition it. Never realized that you could over-discharge a battery.

What I found in the above application guide was that over discharge can actually cause one, or more cells within the battery to reverse polarity thereby significantly shortening the life the battery. So much for thought of making a simple discharge circuit out of a resistor!!!

I don't know if I'm dreaming this, but seem to recall I recently ran across a complete battery charging system in an IC. Don't know if it had discharge cycle circuitry within it. I'll go back in search of it to find out more...if anyone knows of such a circuit off the top of their please let me know.

Thanks again

Michael-