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Posted by D Sherman [72.47.153.24] on Thursday, May 17, 2012 at 10:23:35 :
In Reply to: Starter Motor Draw posted by Randy on Long Island [74.101.123.203] on Thursday, May 17, 2012 at 01:31:44 :
You could measure if you have a high-current shunt, but in very rough terms a starter motor is maybe 1 HP. Call it 1 kW electrically. They're series-wound so the current and speed vary wildly with load. Again very roughly here, 1kW at 12 volts is about 100 amps, or 200 amps at 6 volts. I can't imagine a 6 volt starter drawing 360 amps.
But now I'll repeat my opinion that I don't like high-current battery cutout switches. The point of the switch is either prevent parasitic loads from draining the battery during long storage, and/or to make it harder for the neighborhood punks to steal the truck. Both can be achieved just as well, with much simpler equipment, by switching everything EXCEPT the starter.
The starter is not going to "leak" any current just sitting. Put a 30-50 amp switch or relay in the lead going from the battery to all the rest of the equipment (sometimes this comes off the starter post rather than the battery post) and you've achieved the same goals as a high-current battery disconnect switch without the problems. I like to use a relay so that I can run small wires to a small low-current switch hidden under the dash. This also avoids having to open the hood to re-connect the battery.
Part of what I don't like about the high-current switches is that if they're open for a long time, their contact surfaces tend to corrode, and then when you close them, they don't make good contact. 12 volts isn't enough to punch through corrosion, and 6 volts would be even worse at it. At 6 volts you need to eliminate every possible milliohm of resistance between the battery plates and the starter brushes, not add more. I'm sure the expensive marine-type cutoffs are better than the usual type that is basically a wing-bolt screwing two pieces of metal together, but it's still unnecessary complication, expense, and resistance.
On a lot of my old rigs that are rarely used, I just disconnect the battery ground cable from its post when not in use, and push it back on tight without tightening the clamp bolt when I need to use the rig. This seems to work fine, with a little wiggling each time, and avoids wearing out the clamp and having to hassle with clamping and unclamping it.