Piston Slap: “Watt” we forget when re-charging 6V batteries?


But wait! Before we get to that question:

Hey Sanjeev (sic), thought I’d update you because I installed the SE electronic ignition in my 67 Camaro as per your recommendations. It took 15 minutes and the car started up immediately. It definitely runs smoother, and I dare to say, a little peppier too. I also installed one in a friend’s ’62 Cadillac, which now runs great considering the old points were at 25 degrees dwell. (Double victory for me! – SM)

John writes:

I have a 1947 Jeep CJ 2A, which has a 6-volt, negative ground system. I want to try to safely jump start it from the 12-volt negative ground system on my pickup truck without damaging either vehicle. Could I basically bypass the 6-volt battery in the Jeep, connect the positive cable to the positive side of the starting motor and the ground cable to the Jeep’s frame (first ensuring the Jeep is in neutral)?

Second question: How do I jump start a 1947 Ford 8N tractor that is a 6-volt positive ground from my 12-volt pickup battery?

Sajeev answers:

Battery jump starting can be a bit dicey, especially on vehicles with wiring/lights/gauges old enough to collect Medicare benefits. And I wouldn’t make a habit of bypassing the battery and running 12 volts directly to a 6-volt starter, either. Though the Jeep’s starter might survive if you get lucky. 

Assuming you can’t push start the Jeep, there’s a smarter, less stressful play: abandon jump starting altogether, instead buy a smart charger that works on both 6 and 12-volt batteries. I checked on Amazon and a decent 1-amp charger compatible with 6-volt batteries is under $30.


Since 1-amp makes for very slow charging, spend a little more for one with a higher amperage if you’re in a rush. No matter, the right answer is to hook up a charger, and wait for the LED light(s) to give you the all clear. If the batteries are too far away from a 110v wall outlet (i.e. parked in a field), remove them and recharge in your garage.

If you’re in a rush and just gotta do a jump start, well, you’re playing with fire. (Figuratively and literally.) Consider making things a safer by disconnecting the positive and negative cables on both the Jeep and the tractor, and just let the 12v battery recharge the 6v for a few minutes. With the vehicle’s wiring out of the equation you can probably recharge either the 6v Jeep or the positive ground tractor with a 12v truck that is not running (i.e. not compounding to this problem by adding high-amperage juice from an alternator). If you need to charge it up quicker, start up the 12v truck at your own risk. But I wouldn’t risk it, so do the smart thing and ignore this paragraph.

This is also a good time to note that traditional batteries need their water levels checked, and refilling with distilled water ensures an ideal recharge, no matter the method. Best of luck!

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