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Old 08-14-2013, 04:41 PM   #1
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Question for Battery Gurus

My battery reads 12.6 volts with nothing turned on. Same thing if I switch the battery switch off. If I turn on lights the voltage drops to the point that it read 12.2v with 8 lights on.

I know that those eight lights are drawing a fair amount of current, but what would explain the voltage drop as measured at the battery?

In theory, with sufficient current capacity the voltage at the source should remain constant.

Does that mean that the open circuit voltage is a false reading and that the true state of charge is indicated by the voltage when under load?
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Old 08-14-2013, 04:43 PM   #2
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a battery will almost always read a higher un loaded voltage vs a loaded voltage... just normal battery life... the load will always draw some of the voltage down...
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Old 08-14-2013, 05:00 PM   #3
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I hope your camper isn't plugged when you took the readings.
Because it should read higher voltage.

T
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Old 08-14-2013, 05:04 PM   #4
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Right. 13.7 when plugged in.
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Old 08-14-2013, 06:02 PM   #5
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Every battery has a certain amount of internal resistance. You can think of your battery as being an ideal voltage source with a small (well under one ohm) resistor in series. As you draw current, the voltage drop across the internal resistance is what you are loosing.

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Old 08-14-2013, 06:08 PM   #6
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I guess that makes sense.

8 lights = approximately 12 amps, 0.4 drop = .03333 ohms internal resistance.

I imagine that resistance goes up as the battery discharges, compounding the problem!
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Old 08-14-2013, 09:11 PM   #7
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I found and posted in a previous thread taking battery stuff. Try that for starters. Its actually pretty informative.
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