Date: Wed, 05 Apr 95 08:14:00 MST
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: M.Radtke@az05.bull.com
Subject: Re: Big Battery = Destroy Starter
Hello,
I don't know the answer to the original question about whether the parallel
batteries will fry the starter. This is mostly because there are a lot of
factors at work, and I don't know how to consider them all. However, there
is certainly a possibility of damaging the starter. Let me consider a
couple of things that were said so far:
1) The 12.5 volt thing. Actually, the starter is designed to run on
about 8-10 volts. At the high starting currents, voltage is
dropped across the cables, and more importantly, the internal
resistance of the battery. Thus, parallel batteries reduce the
effective internal battery resistance and produce higher voltage at
the starter. Of course, it will not exceed the 12 volts, but even
that is higher than the design voltage of the starter.
2) The starter resistance thing. A DC motor can be either parallel or
series wound, or maybe even a combination of the two. I don't know
how a starter motor is configured, so I can't show the specific
equivalent circuit for it. However, as was pointed out before, the
equivalent circuit from the point of view of current draw is a
generator that depends on starter speed, and some resistance that
is only dependent on the wires in the starter. In general, the
faster the starter turns, the more this generator generates, and
the current goes down. For a locked rotor, the generated voltage
is zero and the current is determined by the starter resistance
alone. The locked rotor current could be huge since the starter's
internal resistance is very low. And, because of #1, two batteries
make it even larger.
In conclusion, I would feel OK about paralleling batteries if everything is
normal, or one battery is low, as in the case of a jump start. However, if
the engine is difficult to turn because of an internal problem, parallel
batteries could certainly fry the starter. In other words, fix the engine.
Mike, I used to be an electrical engineer, Radtke
Michael A. Radtke - Z74 Voice: 602-862-4897
Bull FAX: 4853
13430 N. Black Canyon Hwy.
Phoenix, AZ 85029 Email: m.radtke@bull.com
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