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Help with recharagables Answered

First off I'm pretty expierenced with electronics, but I've not been using rechargables that much. I'm not sure how to build a charger for nimh batteries. I want to charge a 3.6v battery, 70 mah (3x 1.2 volts, 70 mah each, I know, it's a small battery for a small device) as cheaply as possiable. I also want it so that it can still power the circuit (takes about 20 maH to run the circuit) when charging. If possiable, I want to charge like 5 or 6 of these at once. Help please!

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guyfrom7upschorhr

Reply 11 years ago

thanx! I think I'll go with the second link. do you think I could get rid of the 330 hom resistor and the LED? Do you think I can get rid of the left most resistor and the "NTC"?

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schorhrguyfrom7up

Reply 11 years ago

Hello,
the LED seems to be just the charging status.
I am not sure about the other parts you are adressing. The datasheets of the IC will probably solve these questions.
If you aim for simplicity and you can live with NiCd cells, you should consider them. All you need is to charge them trough a diode so they cant discharge through the source (solar panels for example). Except for their stronger memory-effect, they are very robust. If you have regular charging-decharging cycles such as in yard lights, they last long.
Also, you could take appart a (12 v car or USB) nimh battery charger which are available cheaply anywhere these days, and maybee end up cheaper and smaller then building something yourself.
http://www.quasarelectronics.com/gpc588.htm
http://www.google.com/products?q=battery+charger+usb&btnG=Search+Products

Also, cheap dual nimh/nicd chargers you can buy that have a switch are usualy nothing more then a simple nicd charger, but the switch increases the charging time. If the power is low enough, nimh cells will survive this, but overcharge and heat are often a problem. I could immagine with a solar cell project it might work anyway.
More circuits; I read at http://www.rcgroups.com/forums/showthread.php?t=251291 that it works for nimh as well
http://homepages.paradise.net.nz/bhabbott/charger.html

Also, if you work with microcontrollers, or your project allready includes one anyway, you could check out
http://www.angelfire.com/electronic/hayles/charge1.html

http://www.maatel.com/en/produits/41/ni-mh-battery-charger/
If your project aims for small space and capacity, there are small lithium cells that will be about the size of a quarter and have 3v, 100mah.
Charging those needs a good charger thats especialy for lithium cells, else they can burst, burn or melt.
Lithium Charger
http://www.rcgroups.com/forums/showthread.php?t=130567&highlight=lithium+charger
Tiny lithium cells

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guyfrom7upschorhr

Reply 11 years ago

well my project that I'm working with (I'll post an instructable later) invloves constantly charging the battery, and sometimes taking away the charger, so if I went the nicad way I'd probably kill them, so that's why I need a good charger. Lithium batteries (i've had some experience with, but not a whole lot) would be overboard for the project, so I'm going to go be reading that data shhet, thanks for all of your help!

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Goodhartguyfrom7up

Reply 11 years ago

The emergency lighting system where I work are "constant charge ni-cad" but the trick to lengthening the life is to detect full charge and then, switch to "trickle charge".

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Goodhartguyfrom7up

Reply 11 years ago

It appears to me that the Negative Temperature Coefficient Thermistor is included to help prevent over-charging (in the presence of excess heat). The IC might work without it, but then again, you may fry the Ni-Cd if it is not there.

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Patrick Pendingguyfrom7up

Reply 11 years ago

It actually says on the title that those are optional (the NTC is a negative temperature coefficient thermistor). However, the minimum programmable charge current for this device would appear to be 400mAh, and this would be far to much current in this application. Cheers, Pat. Pending