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Author Topic: Starting a 6-volt car with jumper from a 12-volt car?  (Read 445 times)

Offline jdemerson

  • 1952 Cadillac 6219X Vermont -- Emerson
  • Posts: 864
  • Very large collection of Cadillac sales brochures
  • CLC Number: 26790
  • Name: John D Emerson
Starting a 6-volt car with jumper from a 12-volt car?
« on: October 15, 2021, 09:05:10 AM »
This topic has been discussed, but the results are mixed and sometimes conflicting.

Suppose a 6-volt Cadillac is at a show an hour from home, and the battery goes dead (maybe the lights were left on). The car ordinarily starts quickly. No other 6-volt cars are around. Is there a procedure for safely jumping it from a 12-volt car without causing any damage?

From discussion in the Forum, here is a possible response:
1.   Do not have 12-volt car running when jumping. All lights and accessories off in both cars.
2.   Connect for several minutes with the key of 6-volt car off.
3.   Then try to start with key of 6-volt car; as soon as it starts pull off the jumper cable.


Is this likely to work?  Is it reasonably damage-proof? Can someone improve on it?

John Emerson
1952 Cadillac Sedan 6219X
« Last Edit: October 16, 2021, 08:20:20 PM by jdemerson »
John Emerson
Middlebury, Vermont
CLC member #26790
1952 Series 6219X
http://bit.ly/21AGnvn

Offline Jay Friedman

  • Posts: 2533
Re: Starting a 6-volt car with jumper from a 12-volt car?
« Reply #1 on: October 15, 2021, 09:31:16 AM »
I've started 6v cars using jumper cables with a 12v battery several times.  With all accessories in the 6v car off, connect the 12 battery to the 6v battery.  Immediately start the motor and then disconnect the cables quickly.  Since the 12v jumper battery is only connected  briefly, I've never had any harm done.
1949 Cadillac 6107 Club Coupe
1932 Ford V8 Phaeton (restored, not a rod).  Sold
Decatur, Georgia
CLC # 3210, since 1984
"If it won't work, get a bigger hammer."

Offline 39LaSalleDriver

  • Posts: 474
  • Name: J. Isaacson
Re: Starting a 6-volt car with jumper from a 12-volt car?
« Reply #2 on: October 15, 2021, 01:06:05 PM »
I like to keep this handy quick reference chart that i made on my phone for those "just in case" moments. Fortunately, I haven't had to jump my car in a long, long time.
Jon Isaacson
(formerly USNTar)

1939 LaSalle 5019

Offline Art Gardner CLC 23021

  • Posts: 1604
  • Name: Art Gardner
Re: Starting a 6-volt car with jumper from a 12-volt car?
« Reply #3 on: October 15, 2021, 04:50:11 PM »
One thing is for certain -- it won't damage the 6v starter.  I have converted a good number of 6v cars to 12v and just left the 6v starter in place.  It works fine. Sure, in the very long run i might reduce the life of the starter a tad.  But not worth changing.

Any bulb is at risk of blowing.  So turn everything off, as noted elsewhere...
Art Gardner


1955 S60 Fleetwood sedan (now under resto)
1955 S62 Coupe (future show car? 2/3 done)
1949 S60 Fleetwood sedan (restored 30+ yrs ago)
1958 Eldo Seville (2/3 done)

Offline wrench

  • Posts: 996
  • CLC Number: 27930
  • Name: Jim Cullen
Re: Starting a 6-volt car with jumper from a 12-volt car?
« Reply #4 on: October 15, 2021, 09:46:56 PM »
I’m still running 6v on my ‘51. I have used a 12v jump box on occasion. I turn off all the stuff and just jump it. I do hustle to get the thing disconnected once it starts, but that’s about it.

In theory, if you have a dead/down 6v batt and you hook up a 12v source, that 12v is never gonna reach past the batt. The dead batt is gonna suck down the voltage for a good while. I’ve never hooked a meter up while jumping, but I have charged a lot of batteries after starts while monitoring voltage. Once you kick that starter into the loop, overall system charge drops quite a bit under load.

Current definitely leaves the battery faster than a Gen or Alt can fill it back up.
1951 Series 62 Sedan
1969 Eldorado
1970 Eldorado (Triple Black w/power roof)
1958 Apache 3/4 ton 4x4
2005 F250
2014 FLHP
2014 SRX

Offline jdemerson

  • 1952 Cadillac 6219X Vermont -- Emerson
  • Posts: 864
  • Very large collection of Cadillac sales brochures
  • CLC Number: 26790
  • Name: John D Emerson
Re: Starting a 6-volt car with jumper from a 12-volt car?
« Reply #5 on: October 16, 2021, 07:43:19 AM »
Jay, Jon, Art, and Jim,

Thank you for these most helpful responses. I've taken Jon's diagram/instruction list, printed it, and added to my operator's manuals (one in glove box and one at home).

This is one more example where the Forum is exceedingly helpful! Thank you!

John
John Emerson
Middlebury, Vermont
CLC member #26790
1952 Series 6219X
http://bit.ly/21AGnvn

 

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