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Safely Jumping a 6 Volt Battery with a 12 Volt

September 23, 2010 | Troubleshooting & Repair

By Ray Knight, Geoff Fleming, Harry Pellow, and Steve Proctor

 

Ray Knight: I have no experience with Optima, so my comments are regarding 12V jumping of 6V cars. I will try to separate what is written from my own experience. There are several problems and/or potential problems with jumping a 6V car with 12V.

  1. You can or will fry any lights (so turn them off) and consider pulling the fuse for the clock or disconnecting it.
  2. You can or will fry any radio or digital electronic gadgets (so remove or turn them off).
  3. You can explode a battery. This chance is very low, but higher if the battery is near totally dead or if the weak battery is frozen. The explosion is related to gases given off in the battery and sparking, so the risk is less if the final ground connection is made remote from the weak battery. Explosions can and do occur with same voltage jumping. Explosions are truly rare, but the resultant burns can be painful, and eye damage can be permanent.
  4. All current safety bulletins say rather emphatically to jump only with same voltage source. (quoting frying electronics and safety)
  5. In the pre-electronic era (when I was in high school), the wrecker jumper system was 24 volt, routinely used in the industry at that time to jump 12V cars. Maybe the industry was foolish at that time, but that was what was routinely done for decades.

With these cautions you have to make your own decision. I will not jump a DEAD 6V with a 12V, I will try charging first. I do however rather routinely jump weak 6V with 12V, have for 35 years. I do wear glasses or goggles, I do make the last connection to ground remote from the battery, and I always look away from the battery, and have good ventilation. I don't leave it connected very long (remove as soon as starts), and I do turn radio and all lights off.

I always thought it would be nice to have a set of cables with inline sealed switch, but not aware of any being made. If you accept the risk and precautions to jump with 12V you then have the option of carrying one of the new portable boost battery units, which I now do.


Geoff Fleming: It is done all of the time and has been for decades. Just use normal precautions and once the car starts, disconnect the jumpers. Be sure that all power using appliances are off during the process so no excess voltage is transmitted to them. It is best to apply both cables to the 6 volt battery first, or one to ground and the other to the positive termnal then make the attachment to the 12v source.


Steve Proctor: I have always gotten a bit of a spark when connecting or disonnecting the jumper cables.  Therefore, since batteries can generate hydrogen gas, you want to make sure the last connection you make is not at either negative battery terminal.  Connect positive to positive, then negative to the 6V battery and finally, negative to a frame member of the 12V car.  That will keep the spark away from either battery and is much safer.  I have exploded a battery before in a different scenario and it is a scary experience.

Second, the battery cables may get warm during this process, so once connected, I move quickly to start the car and to disconnect the cables as soon as possible after the car with the sick 6V battery is started.


Harry Pellow (July 23, 1999): Sometimes it IS necessary to Jump Start a 6-volt 356 with a 12 Volt Battery. And Sometimes you must do it YOURSELF, like when you're alone, so that you don't have the Luxury of having two people. I got my early Teenage experience in the Old Man's Junk Yard. Ever try to start a 6-Volt Ford with a 12 Volt Battery when the Ford is POSITIVE Ground? You learn fast in the junk yard.

My one-person, 12 Volt Jump to a 6 volt Battery procedure:

  1. Have the Positive (+) Terminal of a good 12 Volt Battery within reach of one's left-arm when one is sitting in the Drivers Seat.
  2. Attach THREE of the 4 Jumper Cables! Hook up both cables to the 6-volt battery (Red to (+) of Battery, Black to (-) of Battery. Aside: Junkyard Knowledge - note that the (+) Terminal of any car battery is a bit LARGER than the (-) one. Don't believe me? Take a CLOSE look. Leave off the 4th cable which would have gone to the (+) Terminal of the 12 Volt Battery.
  3. Sit in Driver's seat. Make sure naked (+) Positive Terminal 12 Volt battery is within reach of left arm/hand. Place 4th Battery Cable in left hand.
  4. Insert key into ignition. Turn to start.
  5. Then and ONLY then, attach that last Cable to the (+) Terminal of the 12 Volt Battery on the floor by the Driver's Door (use the door as a shield if you must. Brawwk. Brawwwk.)
  6. Engine will crank over like a Modern Car, highly unusual for a 356, and almost always starts up amazingly quickly.
  7. As soon as it begins to light off, command your Left Hand to disconnect that Cable it just connected.

Voila! So far, about 100 12 Volt Battery Starts of a 6-volt car. No 6 Volt (or 12 volt) battery explosions yet!

Praise the Voltage gods. And: Keep the 356 Faith Maestro

6 Comments

Profile missing thumb
Rich Simmons
October 27, 2010 at 12:20 AM
I e-mailed you but I am not sure that the E-Mail link is working ?<br /> I'd like to buy these parts<br /> thanks ...Rich<br /><br /> rsim494202@aol.com
Aa493c9f9299cdb0d51a6d201e2b655c 10664
John Taylor
June 18, 2013 at 8:44 PM
Question re using 12-volt battery: If I turn off all accessories and/or pull the fuses can I then temporarily put a 12-volt battery in place of the 6-volt and safely use it to crank the engine? It seems this would alleviate the potential risk of battery explosion, but are there other problems with this arrangement. Thanks. <br /><br />John Taylor
Profile missing thumb
Anders Daniels
August 09, 2013 at 10:41 PM
I am thinking of connecting a 12V jumper to my 6V Optima battery and connecting the ON button leads on the jumper to a relay which is energized by a wire from the starter wire at the key switch. So that the 12V to the battery is only on when the starter is energized. Would this work? <br />Anders Daniels
Profile missing thumb
Rich Simmons
October 27, 2010 at 12:20 AM
I e-mailed you but I am not sure that the E-Mail link is working ?<br /> I'd like to buy these parts<br /> thanks ...Rich<br /><br /> rsim494202@aol.com
Aa493c9f9299cdb0d51a6d201e2b655c 10664
John Taylor
June 18, 2013 at 8:44 PM
Question re using 12-volt battery: If I turn off all accessories and/or pull the fuses can I then temporarily put a 12-volt battery in place of the 6-volt and safely use it to crank the engine? It seems this would alleviate the potential risk of battery explosion, but are there other problems with this arrangement. Thanks. <br /><br />John Taylor
Profile missing thumb
Anders Daniels
August 09, 2013 at 10:41 PM
I am thinking of connecting a 12V jumper to my 6V Optima battery and connecting the ON button leads on the jumper to a relay which is energized by a wire from the starter wire at the key switch. So that the 12V to the battery is only on when the starter is energized. Would this work? <br />Anders Daniels