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Old 25th October 2009, 02:53 PM   #1
diyAudio Member
Join Date: Apr 2009
Default Smart charger for NIMH

I have designed an intelligent charger for NIMH batteries, but I just have one question to ask before I complete the software for it.
At what voltage will one single AA NIMH-cell(rated 1.2v) be fully charged?
I looked on a charger and I could see the output was 1.4V, so I'm thinking that, when the battery reach 1.4v it's fully charged?

Thank you,

Simon H.A.
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Old 25th October 2009, 09:19 PM   #2
dangus is offline dangus  Canada
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Location: Vancouver Island
Sometimes there's a voltage peak and then a dip. That was the case with NiCd cells. Temperature rise is another sign that they're full. Take a look at the data sheets for "smart charger" chips, like the Telefunken (now Temic) U2400B. There could be smart charger chips hiding in junked laptops, if you have some around that originally used NiMH battery packs.

I spotted a bunch of Rayovac smart chargers marked down to $9 (complete with 2 NiMH AA cells) at a Walmart store. While the case is poorly thought out - smooth, tapered and rounded, so it's very like graspng a slippery bar of soap to unplug from the wall - it does appear to be a true smart charger and charges 1 to 4 cells individually. LEDs for each cell indicate "charging", complete, or failure. If I can open it up and identify a chip, I'll post here.

Last edited by dangus; 25th October 2009 at 09:24 PM.
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Old 25th October 2009, 09:30 PM   #3
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My design is, that I have made a current limiter, that feeds the battery with a current of lets say 100mA. Then I have a microcontroller that read the voltage on the battery, and when the voltage reach the peak voltage, it will turn off the charger.
But I found some information, and I can see the peak voltage is about 1,52V, so I will try that, and then do some testing to see if it is fully charged.

Thank you,
Simon H.A.
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Old 25th October 2009, 09:46 PM   #4
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The NiMH "AA" cells I have all say 270mA for 16 hours

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Old 25th October 2009, 09:48 PM   #5
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I will use a lot more current, it was just an example
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