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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