Best way to test battery charge? - RC Groups
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Dec 21, 2001, 02:59 PM
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Best way to test battery charge?

How can I tell if a battery is fully charged? I have a multimeter to check voltage. Is this the most accurate way? (or is it even accurate at all?) If it matters I am charging small cells 3.6v 300mah with a small solar panal. Normally I just use a peak charger for charging which is why I have no clue. The panal is 6v 100mah output in ideal conditions (4v ish in less light)
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Dec 21, 2001, 09:47 PM
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Voltage itself is not a good indicator. One way to do it is to observe the voltage as the battery charges. If the voltage goes flat or starts downward (assuming the solar cell output is constant) the battery is charged. You may also check for battery warming. The battery starts heating up when the charge is finnished.

Dec 22, 2001, 12:37 AM
Registered User
Hmm is theres some kind of meter that could do the job?
Dec 22, 2001, 02:29 PM
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Unless you have a constant current from your solar panel, there is virtually no way to tell when it is full. If you can make sure to have a constant current supply from the panel, all you need to do is watch the battery voltage. When it stops rising it's essentially full. A little after this you would see the votlage drop at this point the pack is overcharging and the charge should be terminated. With the 100mA charge rate the 300mAh cell may not show a peak. You should stop charging when the voltage stops rising. The voltage of the panel itself is irrelavent in this case as long as it is always over 4.8-5V.

Last edited by gkamysz; Dec 22, 2001 at 02:37 PM.
Dec 22, 2001, 06:16 PM
Registered User
Hmm, is there anyway to tell how much electricity will be required to fill a battery (or at least a close estimate) It almost seems to me that a 300mah battery charged at 100ma should be done in 3 hours? But, this is based on my logic which really doesn't mean much
Dec 22, 2001, 06:24 PM
Registered User
In theory that is correct but two important facts have to be considered besides "just the math".

1. The battery most likely will not be totally "empty".
2. Charging (of any kind) is not 100% efficient. Some energy is lost as heat.

Given the two unknowns above, it's hard to tell unless you follow what others have suggested above.

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