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