Re: 12V accessory in 24V vehicle?


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Posted by David Sherman on June 23, 1998 at 00:40:50:

In Reply to: 12V accessory in 24V vehicle? posted by Peter Hewko on June 22, 1998 at 23:22:11:

I can't imagine how what you propose to do would cause a problem. A battery is a more
or less constant-voltage device. Clearly, the CB is drawing current from only one battery,
but while they are charging, both batteries have approximately the same voltage across them
(just like all the cells in the battery have approximately the same voltage across them).
The cells (or battery) which is more discharged has more "charging" chemistry going on in it,
but this is not harmful except in the normal way that all batteries wear out. If you doubt
this, you can always monitor the voltages on the two batteries with a digital voltmeter and
check the electrolyte in all the cells with a hydrometer.

Definitely don't try putting a resistor in the power lead. For one thing, there's no such
thing as a "12V" resistor. Resistors are rated in ohms. The amount of voltage drop across the
resistor depends on the current flowing through it. The amount of current the CB draws is not
constant. If you picked a resistor which gave a 12 V drop when the CB was idle, the voltage
would drop to near zero when you keyed the mike. There are fairly simple electronic circuits
which will drop a constant voltage, but they all will dissipate (heat up) as much power as the
CB draws. There are other circuits (switching regulators) which will drop the voltage without
getting as hot, but they are very complicated. If you only needed a tiny amount of current,
you could use what is called a "zener diode", but the CB will draw too much current. In summary,
just hook it across one battery (the grounded one). At least that's what I'd do and I'm an
electrical engineer. If the part about it ruining the batter is more than folklore, I'd like to
know why.

---David Sherman




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