Posted by David Sherman on Tuesday, January 04, 2005 at 11:07PM :
In Reply to: Is it safe to jump start a 6V with a 12V? posted by Mike Kopek on Tuesday, January 04, 2005 at 10:25PM :
You can do it in a pinch if you're careful. 12V won't hurt the starter, and the mechanical regulator should survive 12V being temporarily imposed on its output. Obviously any 6V lights or radio left on with 12V applied will get blown, so make sure absolutely everything is turned off except the ignition. With the ignition on, gages will be energized, but if you're quick about it, they probably won't burn out. Disconnect the 6V battery while jumping or it will suck a huge amount of current from the 12V battery. After that, the procedure is: hook up the 12V battery with the jumpers, start the truck as quick as you can, disconnect the jumpers, and quickly reconnect the 6V battery (quickly, so as to avoid damaging the regulator). On old trucks with no fancy electronics, this is fairly safe. If the 6V battery is extremely dead, you might even be able to get away with not disconnecting it if you're quick.
Better to put a charger on it, or push-start it. Bear in mind that when push-starting, the battery has to have enough juice to run the ignition. If it's below 3 volts or so, push-starting is unlikely to work. By the way, most small battery chargers current-limit at 5-10 amps, and can safely be used to charge a 6 volt battery in a pinch even though it only says "12 V" on it, so long as you disconnect it before it over-charges.
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