Cold comfort wrote:
> Hi everyone, the electical dept is not a strong point of mine so I
> would like to ask the following, I have two 12v 17ah batteries wired
> for 24v , when I go to charge it I unhook the wires so that it is two
> 12v batteries again because I only have a 12v charger. When I do this
> and I test the before charge voltage, both batteries give me different
> readings, a difference of 1 to 2v , should they not discharge evenly?
When connected in series, the two batteries will discharge at rates
proportional to their internal resistances. In fact, the 2 volt cells
in each battery will also discharge at rates proportional to their
internal resistance as well. With two new batteries, the resistances
are relatively the same, so discharge is the same. As they wear down,
the difference in resistance increases, as does the discharge rates.
> Also, what voltage should a 24v battery drop to before needing a
> recharge?
A normal 12v battery should be recharged when it reaches 11.8v, which is
roughly 1v below it's fully charged value of 12.8v. So, a 24v battery
needs to be recharged after it drops 2v. Note, when reading a battery
you really need to put a load across it to get a meaningful reading.
With just a voltmeter attached, the battery can show a high voltage even
though it can't carry a proper load.
Get yourself a heavy duty 1-2 ohm resistor and measure the voltage when
it is connected across the battery to get a better "loaded" voltage
reading. I have a "blue box" that contains a load resistor, momentary
switch, digital voltmeter, 10A analog meter and two connectors (in,
out). I can use the box separately to measure the voltage with/without
load and also put it inline with motors to measure their amp draw.
Frank P.
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