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I'm an amatuer. I have a Model A, positive ground, 6v system. I want a jump starter I can safely use with it. I have used a 12v starter grounding on the block, with the negative cable going to the starter pole. I worry that I'll damage the system doing this.

A friend said I could use a resistor and fabricate a 12v powered stepped down to 6v system. He's a chemical engineer, and neither of us know how robust a resistor we should use. Any help will be appreciated.

Can't believe no one makes a 6v jump starter!

  • Nobody makes 6V jump starters because no one makes 6V cars. –  Sep 25 '16 at 13:06
  • Your starting point should be a 6V lead acid battery. Dropping 6V through a resistor is going to need an awful big resistor. –  Sep 25 '16 at 13:19
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    A Model A what? Please don't assume everyone knows what you're talking about. If it's a 6 V car then it should have a crank handle. Seeking product recommendations or where to buy them is off-topic and will result in question closure. See Don't ask. If you change it into a design question it may survive. – Transistor Sep 25 '16 at 13:23
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    @Transistor British Postage Stamps [tm] do not have the country name on them . They invented stamps. They don'T NEED names. Other contries can use them if they wish. So too, a Model A IS a Model A. Anything else is a "Model A xxx". Not this ! :-) [[PS: Ford]]. – Russell McMahon Sep 25 '16 at 13:35
  • @IanBland: He HAS a 6V battery -- that's the whole point! He's looking for an external charger/booster for it. – Dave Tweed Sep 25 '16 at 13:59
  • Doesn't this car come with a cranking handle? – Steve Matthews Sep 26 '16 at 09:34
  • @Ignacio See my answer for some available products. – Russell McMahon Sep 26 '16 at 12:44
  • @DaveTweed He has a "flat" in car battery which the jump starter charges OR substitutes for. 12V ones often use small 12v SLA's. – Russell McMahon Sep 26 '16 at 12:45
  • I am going to have to dig out one of my "DEAD BATTERY" portable jump starters and open it up. Should be able to replace the dead 12V with two half size 6V batteries. Add a switch to go from 12V for charging it and 12V jumping use to two 6Vs in parallel for momentary jump charge use on the old car. Or just make it 6V only and redo the charger/meter on the box. – spicetraders Sep 27 '16 at 01:44

5 Answers5

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If I was doing this I'd be likely to design my own. As Ian says - a 6V lead acid battery would be the logical starting point. And, a 12V battery and buck converter may be a better one.

In many cases starting units do NOT provide cranking current directly - instead they transfer charge to the battery that is "flat" and it then provides the starting current. So a power supply that charged 6V batteries from 12V may do what you want.

My experience is that using a 12V battery to charge a "deadish" 12V battery in a car is a far slower process than if somewhat more than 12V is available. When jump starting from another car, starting the engine to raise the source voltage greatly improves transfer rate.
In the case of your 6V system, if you are certain that the 6V in-car battery is in place and it will clamp the voltage to not much above 6V worst case, then charging from 12V via a resistor may be viable. The target battery MUST be in place. If not, the 6V system will rise to about 12V and significant damage may occur.

If only 1A flows then power in the resistor = V x I = (12V-6V) x 1 = 6V x 1A = 6 Watts. Easily done.
If 10A flows then power in resistor = 60 Watts. Nichrome wire from a cable supplier can be used to easily do this.
if 50A flows (getting serious) then resistor dissipates 300 Watts. This is STILL within the capabilities of a nichrome air cooled spiral wire resistor. I operate a 500 Watt version of this as a load for exercise machine testing. That's getting into serious power dissipation but is doable. Not overly good for a battery probably.
So at say 10A, if you need 400 A cranking for 3 seconds to start and assuming half is lost in system. Need = 400A x 3s x 2 = 2400 Amp.seconds.
At 10A that would take 240/10 = 240 seconds = 4 minutes. I suspect that in most cases you'd need less than that.

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I recently attempted to jump start a 12V car using several old but fully charged 12V batteries, 1 at a time. Charging happened but the car "needed something more". With somone ready to hit the starter instantly I connected 2 x 12V batteries in series using longish jumper leads. Go!. It went!. Very effective. As with your 12V to 6V method, there is a potential risk to the system.

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However, Never say never: Battery booster 6 V, 850 A | SOS 6V-850CA.

enter image description here

Also - Dutch - 6v/12V Trickle Charger Norauto NX501 6 / 12V

Amazon 6V/12V $US29.94

Looks familiar

Russell McMahon
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If you used a resistor then it would have to be a Shunt resistor that is 50% of your total load and that would get very, very hot (would work, but not a good idea).

Ray Henderson
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Jump starting a 6v system with 12v would almost certainly fry most of your electrics/electronics, but looking on the good side you would have somewhere to warm your hands!

Ray Henderson
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A resistor would have to be rated at the same power as the starter motor, which is on the order of 1 kW. Resistors with that rating are available but cost thousands of $. And you'd have to match the resistance value to that of the starter motor. Not an option I'd recommend, the jump starter is a much better option.

Hobbes
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Thanks for all the replies. I'm checking with a foreign manufacturer of a 6v jump starter. Hopefully they'll have a US distributor (I can't find 'em, though). The need for such a device lies in the old car hobby. Tens of thousands of Model A Fords are still in service. There are probably 1o00 thousand other cars still running using 6v systems. We can jump using a 12v external battery grounded on the block. The cars start easily, but we have to be quick. The cars have no electronics, so no worries there. The convenience and safety of the purpose-built jumper is desirable. BTW, the Ford does have a crank. Its 80 year old owner doesn't enjoy using it.

user22707
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