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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 05-12-2007, 12:08 AM   #1
Jon The Buddha
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Default NiMh: Charged vs Discharged Voltage

I use a couple of Electrifly 8.4V - 3000mah NiMh batteries to run my Electristar. My questions is about the voltages I am seeing.

The last time I flew, I made sure I ran the plane until the engine cut off. Is this considered to be the point the battery is roughly 'fully' discharged? I measured the voltage out of both batteries and (I am going from memory here) I think they were approximately 8.5 volts. I have also read the voltage on the batteries after what I think is the proper charging time (36 minutes, charging at 5 amps on a basic Hobbico 900 timed charger). I think the reading was around 9.8 volts.

Do these numbers sound feasible? I am concerned about two issues...that I am running the battery down sufficiently enough between charges, and that, when charged, I am not exceeding some max voltage requirement. Until I can get a proper charger, I want to make the best use of the one I have and NOT damage my batteries.

Thanks gang...I am trying to learn this 'magical' battery stuff, but sometimes I get lost in all the info from the many posts and links to battery basics websites!
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Old 05-12-2007, 02:22 AM   #2
Matt Kirsch
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Technically speaking, fully discharged is considered to be .9 to 1.0 Volts per cell on a NiMH. Fully charged is between 1.3 and 1.4 Volts per cell. Unfortunately, you can't tell the state of charge on a NiMH by looking just at its voltage. The no-load voltage will be near full-charge voltage until the battery is almost empty. At 8.5V yours are on the steep slope to becoming completely empty, and do not contain more than a single-digit percentage of their total capacity.

Continue as you are until you can get yourself a nice peak charger, just monitor the battery's temperature. It should not get more than comfortably warm to the touch. Given a sane charge rate (5 Amps is pushing things a little bit, but it's not a totally crazy rate once the batteries have been cycled a few times) once the battery becomes warm, it's charged.
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