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Old Jan 10, 2006, 10:53 AM   #1
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Join Date: Nov 2005
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I know this is kinda off topic byt anyaway: Can you post some links or tell me how to build a circuit for the testing the charge level of a NiMH battery. How do cameras know when to display the "low battery icon"? I was told that a simple voltage measurement with a voltage meter means nothing? So how do i do it? Does it have something to do with the current which you can drain from the battery at that moment?
Any help appreciated! THX
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Old Jan 11, 2006, 9:09 AM   #2
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Measuring the state of charge (SOC) of NiMH batteries without discharging them is not easy. Indeed open cirquit voltage, i.e. just using a voltmeter, is nearly meaningless. If it measures over 1.3 V the cell is full or nearly full. If it measures below 1.2 V it is empty or nearly empty, everything in between is uncertain.

A bit better (but by no means highly accurate) results can be obtained under load. You can measure the voltage under some load. Providing that you have the full discharge curve under this load for this particular cell you can estimate SOC with 10% accuracy at best
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Old Jan 11, 2006, 4:57 PM   #3
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I note that the Sony BCG34-HRMD that appears on this site (and I own) includes charge detection. Presumably this works for any AAA or AA rechargeable NiMH cell rather than just Sony (well Sanyo) cells supplied with the charger. Does anyone know how it does that?
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Old Jan 15, 2006, 7:53 PM   #4
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Im not sure what you mean by charge detection but i do know how peak chargers work. I think the way NIMH is a little different.

With nicad as the battery reachs the "full" level the battery starts to heat up and once it gets hot the voltage drops. The charger detects the peack voltage level threw the charge duration and when it drops back .3 volts it kicks off.

I think NIMH may work with resistance. I think once the NIMH batteries reach the full level they increase in resistance and the charger detects this and shits off. I think this is why you cant put alkalines in aNIMH charger, the resistance of the alkalines are different and it requires more power to charge.

There are also different types of charges, like hard and soft charging. I think NIMH needs a soft charge.
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