Piston Slap: Jumping the gun on 6-volt jump starts?

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David writes:

Hi Sajeev,

I thoroughly enjoyed your article regarding the use of an 8-volt battery in a 6-volt vehicle! It made me think of an issue for which I’ve sought a solution. It seems that nobody produces a 6-volt jump pack. As you know, it’s not hard to find a 12-volt jump pack; lots of brands, sizes, power ratings, etc. But, as far as I can tell, nothing similar exists for a 6-volt vehicle.

We all know that vehicles with 6-volt systems are a bit strained when it’s time to start—especially when cold. Further, most of us with vintage vehicles (particularly those old enough to have a 6-volt electrical system) don’t drive them everyday. That can make starting even more difficult and more challenging to keep 6-volt batteries at full charge (battery maintainers notwithstanding).

On more than one occasion at a car show I’ve seen a vintage vehicle experience difficulty cranking and fail to start due to a relatively weak battery. For a 12-volt vehicle, the widely available power pack is a convenient and compact solution that gives the vehicle a boost and gets the driver heading back home. For a 6-volt vehicle, there is no comparable solution. (And finding another 6-volt vehicle for a safe jump start is not always possible.)

Power Pack Jump Start
Brandan Gillogly

So, I’m wondering if you’re aware of anyone providing such a jump pack solution for us vintage 6-volt vehicle owners? (Or, is the market considered too small for the known jump pack manufacturers to make such an effort?) Perhaps there’s another solution of which I’m not aware?

I’m not sure if others have had this question, but I wondered if it might be a good topic for your Piston Slap column. Thanks for your thoughts and consideration of this topic and for your truly enjoyable writing (and, your graciousness toward commenters who don’t always deserve that grace!).

Sajeev answers:

Thank you for your kind words and this fantastic question, David! You mentioned the benefit of battery maintainers, and those should be used as frequently as possible to reduce the need for a jump start. Nearly every maintainer I’ve come across has a provision for 6-volt trickle charging, so you are right when you suggest this isn’t the problem.

2022 Bonneville Car Show hotrod group
Brandan Gillogly

The bigger concern here is enjoying your vehicle outside of your property and getting stranded at a car show, strip mall parking lot, gas station, etc. You can’t use your handy 6-volt garage trickle charger in these places, so you need a 12-volt jump pack. There are two types of packs, one with an internal battery and another with a supercapacitor. From what I can find, the latter has yet to be tested on a 6-volt vehicle, but the general consensus is that using a 12-volt jump pack with a self-contained battery is safe. Well, provided you follow a few rules.

Keep in mind I have never tried this personally (as I do not own a 6-volt automobile), so try this at your own risk:

  1. Keep hand tools and a fully charged jump pack in the vehicle.
  2. Turn off all accessories: lights, radio, etc.
  3. Disconnect the negative cable from the 6-volt battery. (This removes the battery from the circuit.)
  4. Connect the jump box to the positive battery post and the negative cable.
  5. Use the jump pack as instructed by the manufacturer.
  6. Start the vehicle.
  7. Remove the pack’s positive and negative cables QUICKLY.
  8. Reinstall the negative battery cable.
  9. Hope and pray the motor doesn’t stall. (Just kidding … probably.) 

Again, I have never done this before. But step #7 is certainly paramount, as you want to minimize the time a 6-volt system has 12-volts being crammed down its throat. Fires, battery explosions, etc. are not worth it. If you don’t trust the wiring in your electrical system to handle this, just pay for a tow. (Some insurance policies offer free towing, and I’ve taken full advantage of that with multiple carriers with great success.)

Don’t fear the tow but also set yourself up for success: Replace old wiring, especially all the grounds and on the starter circuit. As we learned from Stu Tell in the aforementioned 8-volt Piston Slap article:

“The battery and starter cables themselves are usually internally corroded, and that itself causes you problems. Change them all out for AWG 2/0 cables along with grounding your battery ground cable right to one of the starter bolts or the engine block itself. From there run a jumper to ground your frame and a jumper to ground the firewall sheet metal. You will be amazed how well a 6-volt system will start with a new set of thick 2/0 cables, good grounding, and a good battery.”

Well said, Stu. And changing these wires is usually very easy on older vehicles, especially with the proliferation of home improvement stores stocking high gauge wiring that’s fuel and oil resistant. So this begs the question, will you even need a jump pack if the wiring is up to snuff in the first place? Probably not. 

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.comgive us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.



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    David, for what it’s worth, before I changed my Ford 8N tractor’s systems over to 12 volts, I often used “regular” battery jump start cables from my pick-up to give the old 6 volt starter some help on cold mornings. Never used a jump pack, but would think that a 12 volt jump is a 12 volt jump, so-to-speak. I agree that #7 on Sajeev’s list is really important. Since I didn’t have to be “in” the tractor to press the starter button, it was easy to be positioned where I could quickly pull the jump cables as soon as the engine caught. I would assume that wiring meant for 6 volts would overheat pretty quickly if exposed to 12 volts for an extended time.
    It’s interesting that many chargers and tenders have something as easy as a switch or button to change their output voltage. Seems like a jump pack manufacturer could provide something similar and charge a few more bucks to pay for it. There aren’t H>UGE numbers of 6 volt’rs out there, but in the collector/vintage car world, there is definitely more than just a few. Perhaps we need to start a letter-writing campaign to some of the jump pack companies!
    I also would like to second your opinion of our editor/author’s talents and level of patience/graciousness, and it was sure nice of you to point it out. The latter things especially are far too often overlooked in this world, and when we see them, we need to comment on them! 👍👍

    One thing about 6V systems is the amps are higher – twice as high for a comparable load in a 12V system, so wires in 6V vehicles are already sized to carry a lot of amps. It is generally insulation that becomes the issue when volts go up. I would suspect that the insulation on healthy 6V wiring can probably handle 12V without any issue, which is probably why this trick tends to work. It would be the accessories like dash gauges and such that would be my primary concern with putting 12V to a 6V system.

    Beefier cables and solid grounding are a big help, as is making sure your starter is in good shape, battery connections clean and tight, charging voltage where it needs to be etc. Carrying an extra charged battery in a stout container with thick jumper cables has gotten me out of trouble a few times. Highly recommended.

    This one got my curiosity going, and the first thing I found was there are a lot of folks out there asking the same question. This is odd considering product developers are typically looking for ‘white space’, and there is some here. I was thinking someone could modify a 12V jump pack, but it looks like the core of these things is generally a single 12V cell. It appears that part of the problem is that there aren’t too many folks out there making high capacity high amp 6V batteries, other than car batteries themselves.

    I saw that some folks solve this problem by using a marine 2-battery arrangement with selector switch. This is only going to really work if you have some sort of diode arrangement that charges both batteries while only one is outputting to the system, otherwise battery 2 will likely always be dead when you need it.

    I had a hard-starting 6V car shipped to me last year, and the (very experienced, highly regarded, 6V-savvy) shipper told me he had used a 12V jumper pack on it so he could drive it into his rig.

    Optima makes a very compact 6V “Red Top” starting battery, which is about half the size of the battery currently in the car (literally, like someone sliced the car battery in half). It is almost the size of an older jumper pack I used to use. If I can cobble together a case for it, it’s going to become my traveling 6V jumper pack.

    I have one of those Optima red top 6V in my 1937 Buick, highly recommended, it is going on 6 years old this spring and works without any issues. Starts every time.
    It is longer than the intended battery, I had to put it on a angle in the battery box under the front seat.

    I haven’t done this since I have good clean wires and connectors in my 1940 International S2 pickup,, but it would seem to me that instead of disconnecting the battery, one could disconnect the output of the generator and use one side of the 12V battery to replace that and the other side to chassis ground. Since the generator voltage varies directly with speed, it is the voltage regulator that determines the max voltage to the system. So the 12V would appear to be coming from the generator and be regulated down as required. This might vary by car, but fundamentally I believe this should work. When the car is running you would simply disconnect the 12v battery and reconnect the generator terminal.

    This article brought back memories of when I used to run a tow truck back in the ’70’s. When a vehicle would be a hard starter, we had a switch on the truck that would allow us to jump the vehicle with a 24 volt crank. It was a miracle I never wrecked a starter because it would really make them spin! Love your articles Sajeev!

    Oh wow, now THAT’s impressive! Yeah, sounds like you dodged a whole lotta bullets, but my research suggests that 24v is acceptable if used really, really quickly. Thank you for reading, I appreciate it!

    This question took me back to my youth when I would call for a jump start from the local service station on very cold (below zero) mornings. I recall the tow truck drivers would routinely use 24 volts (or more) to get a frigid engine to spin fast enough to start — before the days of high energy ignition or fuel injection. Never caused an issue afterwards with the electrical system in the car.

    Never thought about this before. It is interesting there are no 6 volt options out there for a jump.

    “Back in the day” as teenagers we never had a problem jumping from a 12v car to one of our oldies. Heck, batteries cost money! There is usually enough series resistance in the clamps and wires to limit the voltage somewhat. Also for this reason carrying a spare 6v battery around might not work as expected.

    Also don’t forget to check the POLARITY of the 6v system as most were POSITIVE (red terminal) ground

    Gel batteries are made in a myriad of configurations and 6 volt versions are especially common. If there isn’t one with a high enough capacity, several can be wired in parallel to make a convenient spill proof pack. Also: in addition to the previously mentioned maintenance, verify no oil has gotten onto the starter brushes and commutator. It’s a common and tremendous power sap on older engines.

    Yeah, you can jump start a 6V car with a 12V battery. I’ve been doing it since I acquired my WW II ‘43 Ford Jeep in ‘67 when I was 16. It had a short in the electrical system that I wouldn’t find until around 2012. (I know, sounds crazy but I used the Jeep daily during HS & college so the battery never ran down completely most of the time. And besides, with a manual transmission one only needed to either park it on a hill or have a couple of people give you a push!)

    I never thought to remove the negative cable but it makes sense & safe to do so.

    As noted by a few other replies, I highly. recommend the 6V Optima battery. Has better cranking power (which comes in handy in the winter) than a regular battery.

    I have a long history with various tractors. We have never had an issue jumping the 6v off the 12v. We did jump a buddies old 6v car too, and the electronics survived without issue. And most 6v cars don’t have much in the way of electronics.

    The battery in most non-lithium jump packs is usually pretty easy to replace. If one were that concerned, they could get a 6v battery and install it in the jump pack. Reuse the jump pack battery in your riding mower.

    Of course, if you are so risk averse to avoid jump starting a 6v off a 12v, you probably won’t be taking apart a jump pack.

    Overall though, it might be worrying too much about something that isn’t really a problem.

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