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Old Jul 1, 2005, 3:10 PM   #1
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Anyone understand the voltage rating system for lithium-ion batteries? Both my EN_EL 1 and NP-800 come off their chargers at 8.35 volts, at first I thought this a flash charge that would drop as soon as the battery was loaded but after a dozen or so shots with full power flash they still read almost the same. The A-200 red battery warning comes on with a reading of 7.4 volts which is the rated voltage of this battery. All readings taken with a Fluke DMM rated at .025% DC accuracy. Battery life seems to be OK.
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Old Jul 1, 2005, 4:52 PM   #2
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All batteries will have a current-voltage curve that gradually changes with time according to state of charge, but probably has a big long plateau on it. The open-circuit voltage shown by a DMM is at one end of the curve (zero current), and may not be representative.

You need to measure the voltage (or the current) under a standard load, rather than the open circuit voltage, to get meaningful results. Some multimeters have a 'battery test' mode that displays the current delivered through a standard resistor. My own cheap one has ranges for 1.5V cells, meant for AAs, presumably. and 9V PP9 batteries. A meter like this would probably be useful for these Li-ion batteries.

I've just tried it, and my Sony NP-F330 camcorder batteries show 8.35V on open circuit, and deliver 23mA through my meter's 360 ohm resistor. Ohm's law gives 8.28V as the voltage I'd measure underthat load if had two meters.
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Old Jul 2, 2005, 12:31 AM   #3
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My meter has no battery test function but I loaded the NP-800 with a 250 ohm resistor and got a reading of of 8.21, still seems high for a battery rated at 7.4 volts.
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Old Jul 2, 2005, 2:41 AM   #4
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The Lithium-Ion discharge curve is quite different to NiMH and is fairly linear from 4.2 down to 3.0 volt. For a two-cell pack this is 8.4 to 6.0 volt.

The "rated" voltage is 7.2 - just the average of these two extremes, unless the battery manufacturer wants to boast and calls it 7.4. ALL LiIon batteries use the same chemistry.

Unlike NiMH, LiIon don't hold a "surface" charge and start at 8.4 volt even when under load.

Unlike NiMH, LiIon have negligible self-discharge and 12 months after charging they will have 90% of their capacity.

For longest life, LiIon should be stored at 40% charge in the refrigerator (not freezer) in a plastic bag.

Mike


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Old Jul 2, 2005, 9:55 AM   #5
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Thanks for the info guys. I am a retired engineer but have been retired so long NIMH is new to me, alkaline and ni-cad were the hottest things going in my time.
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Old Jul 17, 2005, 12:41 AM   #6
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Your Li-Ion voltage sound normal to me. Two cells in serieswith an off the charge voltage of approximately 4.2V in series would give you 8.4V. That number is very close to your voltage. Li-ion cells normally run at a working voltage of 3.6V.

It sounds like your cells have a great capacity since you can use it and the camera see very little voltage drop.

Since Li-Ion cells store so large amount of power and the current drain is so constant with these cells then it could take a while for the cells to settle to the 3.6V working voltage.
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