"Double-charging" NiMH AA's??

IamMatt

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I was charging a nearly-new set of Duracell 2500mAh NiMH AA's (maybe the third or 4th time they had been charged) on an Energizer Accu Universal Charger. After they had been charged and the light went off (indicating they were fully charged and were trickle-charging), my hand slipped as I went to take them out and they sort of slipped back into the charging position, and the charging light went on. I left them in to see how long they would charge, and they went for several hours before the light went off--close to the time they would charge if they were fully discharged.

Is this normal? Is it OK? Does it mean anything?

Thanks for your patience with this newbie.
 

chimo

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I have that charger. IIRC, the charge cycle is timer based. It is normal for this charger, but not likely too healthy for your batteries.

Paul
 

bcwang

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It means that's not a smart charger. And it also means you probably just overcharged your batteries. :)
 

IamMatt

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Guess I need a better charger :-(

The specs say, "FEATURES: Charges NiMH cells in 5 hours (9V in 14-16 hrs.)" and "Output: 2x (500mA @ 2.8V)" So does that mean that a 2500 mAh AA should take 5 hours to charge? Kind odd because this charger was made WAY before there were 2500 mAh AA's.

Guess I'll have to see if the batteries' performance seems changed. I use them in a digital camera and when they camera shuts down with the "change batteries" warning, they still have plenty of juice in them to power a Streamlight Jr Luxeon for several hours.
 
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IamMatt said:
Guess I need a better charger :-(

The specs say, "FEATURES: Charges NiMH cells in 5 hours (9V in 14-16 hrs.)" and "Output: 2x (500mA @ 2.8V)" So does that mean that a 2500 mAh AA should take 5 hours to charge? Kind odd because this charger was made WAY before there were 250 mAh AA's.

Guess I'll have to see if the batteries' performance seems changed. I use them in a digital camera and when they camera shuts down with the "change batteries" warning, they still have plenty of juice in them to power a Streamlight Jr Luxeon for several hours.

Unfortunately, that charger won't fully charge a 2.5Ah battery. You need around 120-125% the rated capacity to get a full charge at C/5 rate. That charger should fully charge a 2.0 to 2.2Ah batteries just fine.
 

IamMatt

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Handlobraesing said:
Unfortunately, that charger won't fully charge a 2.5Ah battery. You need around 120-125% the rated capacity to get a full charge at C/5 rate. That charger should fully charge a 2.0 to 2.2Ah batteries just fine.
By that do you mean that the output as measured in mA needs to be higher, or that it can get by on a lower mA output if it charges for a longer time?

I found a Product Datasheet from Energizer that says the output when charging AA NiMH's is 420 mA when "quick charging" then it drops to 35mA when trickle charging. I know the charger is supposed to "fully" charge the battteries during quick charge, then switch to trickle charge when the indicator light go off. I am guessing from the data available that there is no smart circuitry, it just charges for 5 hours at the higher rate then switches down to trickle (though I am not sure if this is accurate, because sometimes if I put two pair of batteries on to charge at the same time, one pair's indicator light goes off significantly before the other pair's).

My bottom line is simply whether I can fully charge my 2500 mAh NiMH's on this charger without harming them (assuming they are fully discharged and assuming I do not put them through two full 5-hour cycles) by putting them on the charger, letting them get as much as a charge as possible in the 5 hours (presumably getting 2100 mAh by charging at 420mA X 5 hours), then letting them trickle for another 12 or so at the 35 mA rate to get the last 400 mAh of the 2500 mAh capacity?

I guess the other concern is that they seem to have significant capacity left after the camera gives me the "change batteries" message, so I could risk overcharging them if I put them through the 5-hour timed cycle when they have 400mAh or more still in them?

Thanks!
 
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