Re: 6 volt starter with 12 volt system

Posted by lee on January 17, 2005 at 11:47:36 from (24.24.95.246):

In Reply to: Re: 6 volt starter with 12 volt system posted by Andy Martin on January 17, 2005 at 10:44:47:

I tried to keep it simple. It is actually more complicated as you suggest. The starter is as you say a motor, so the current will vary with load.
The cranking load is the same at any given cranking RPM no matter what battery is used. And it makes sense to assume that the load would increase with increased cranking RPM. You have to work harder to crank faster. I did not elaborate on my earlier post that what I stated only applied to a special case, a locked armature condition. Only at the very instant electrical contact is first made (the armature stationary) will the amps be double. It's called locked rotor current. A better way to look at the whole mess might be to look at the electrical power applied to the starter. here goes.

For electrical power in WATTS

WATTS = AMPS x AMPS x RESISTANCE

or for the same thing another way

WATTS = AMPS x VOLTS

Since I don't have actual numbers I'll just use words and say that the power applied to a 6 volt starter with a 6 volt battery is much less than that applied to the same starter by a 12 volt battery. If it were not the starter could not crank the engine faster as we all know it does with a 12 volt battery. The Volts we know have doubled from 6 to 12. The AMPS may even be less (than for the 6 volt case) as you say (I'm not sure) but they are certainly not near half so the power delivered in watts is much greater for the 12 volt case. It's the power that heats up the starter and can cause it to burn up more quickly. I'll stop here.

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