Starter problem?

[JohnEd]

Well I spent a few minutes with my local starter rebuilder on Friday.  Had this question about starter voltage that I kept forgeting to ask.  This relates to so many posts that I have seen recently about engines that don't want to start, crank a lot or the engine that "eats" starters.

The hot skinny:  Starter manufactures call for a absolute min of 9.6 volts across the starter in a 12 volt system.  That is measured at the starter posts, + and -.  Less voltage than that will cause such things as welded contacts in the solenoid, burned brushes and commutator, burned windings and all things associated with over current and high temp.

The bottom line is that if you have 12.6 volts at the terminals before you hit the starter AND your battery is good, and all the connections in the circuit are good then you will see more than 9.6 volts across the starter when the engine is turning over.  The closer to 12.6 the better but you will never get full voltage across a good starter because the battery has internal resistance and some voltage is always dropped there.  At 200 amps draw I don't know what a 8D will drop but I suspect it is something like a volt.  That means we have 2 volts MAX we can drop over the remainder of the start circuit and still have the 9.6 left for the starter.  R=E/I, resistance = 2 volts/200 amps or .01 ohms of resistance spread across the solenoid contacts and the bat terminals and the wires.  Everything has to be really bright clean and tight.

In a defective starter you may have resistance in the solenoid contacts or the brushes or the windings.  In that case you will see close to 12.6 volts across the starter but it will have little power to turn over the engine.

If you see any holes in this argument please point them out.

Thanks,

John

Rick said his pet peeve is the guy that thinks the frame will carry the current so they don't run a second cable for the neg side back to the battery.  Those starters don't last long or work well.  He said the rule is if it is a longer run that 10 feet to the battery you need a second wire....in cars.  Mounting the start battery in the engine compartment seems to make even more sense given the reduced cable length.

[makemineatwostroke]

John,  C amps play a important part of the cranking process DD has a minimum amount (950 above 32 to 1250 below 32) on the 24v starters double the amps for 12v you can have 9.6 volts on a lawnmower battery for a flash but it is not going to start the engine.Some try and use  2 grp 31 1000 cc amps batteries wired in parallel for 12 volt thinking they have 2000 cc amps in reality they still have 1000 cc amps just more amp hours you need the amps to go with the voltage same when you wire batteries in series you double the volts not the amps