PeachParts Mercedes-Benz Forum - View Single Post

Jeremy5848 · #4 06-27-2006, 01:05 AM

Thanks for the additional details. Your relays and fuses will be fine. As long as the fuse and the wiring can handle the inductive load of a starting DC motor (current drawn is maximum at stall, that is, when the motor starts), the charging system won't mind. Bear in mind that, any time the load exceeds the charging ability of the alternator, the excess comes from the battery. You are "overdrawing" the charging system but the battery is there to prevent problems -- in the short term only! The charging system must be able to keep up with the load while the motors are running. That depends on the size alternator in the vehicle. Make sure your alternator can keep up.

With each motor on a separate circuit and fuse, all I would be concerned about would be the wires. Make sure they can handle the load without excess voltage drop. I'd use at least 14 gauge, which is rated at 12 to 15 Amps, for each motor. Choosing too small a wire gauge is a "penny wise, pound foolish" error that we all have made. Too small a gauge cheats the motor and risks not only damaging the motor but, in extreme cases, starting a fire.

The right capacitor size is a function of the inductance of the motor, which is not easy to find out. You can easily experiment by buying some inexpensive electrolytic capacitors from a Radio Shack or other electronics store. Plain aluminum electrolytic caps will do fine; don't let the salesman sell you up to tantalum (expensive) or to an AC motor starting capacitor, which is a completely different animal. Get a series of values, for example, 1, 10, 100, and 1000 microfarads, at 100 volts (at least). Even though the motor is running on only 12 Volts, the back EMF generated when the relay contacts open will be much greater (that's why it causes sparks).

Try starting and stopping the motor with the relay and observe the sparking. Then put a capacitor across the relay contacts, with the (+) wire of the capacitor on the relay contact that is connected to the (+) battery terminal and the (-) wire of the capacitor connected to the other relay contact (sorry if you already know this). Try different values to see which one makes the least sparking and use that value. If the relay doesn't spark much without a capacitor you can get along without one but experiment and see.

Jeremy