I have an antique truck using an 8 volt battery conversion from its original 6 volt because the starter turned too slowly to crank it. This protected the instruments from damage. If I need to jump off this 8 volt truck with a 12 volt modern car, what resistor do I need to achieve this? I'm not an engineer, so what am I missing here? The conversion made a huge improvement in operation of this antique truck.
How do I modify automotive jumper cables to safely jump an 8 volt system with a 12 volt car battery?
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\$\begingroup\$ I don't have any direct experience, but there are 2 V lead acid batteries used in fork lifts. You don't say what you think you need (would be nice to have the specs on the original 6 V and also 8 V batteries), either. So I can't say how doable any of this is. But I suspect it can work out, if you cannot otherwise find an 8 V (4S) lead acid battery. \$\endgroup\$periblepsis– periblepsis2026-02-28 01:33:17 +00:00Commented yesterday
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2\$\begingroup\$ What problem are you really trying to solve? You would not need a jump, unless there's a problem. Please edit your question with this information. Have you tried a new battery? And a charger to keep it charged? \$\endgroup\$TimWescott– TimWescott2026-02-28 06:18:09 +00:00Commented 20 hours ago
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\$\begingroup\$ Carburetor and ignition in this truck make failure to crank/ battery depletion in the field (away from chargers, etc.) a common occurrence. It's and inherited truck and to avoid theft, it does need to be abandoned away from home to get a battery. Some form of jump is the most practical solution. I don't have deep pockets to afford any and all alternative solutions. \$\endgroup\$Not too bright– Not too bright2026-02-28 14:23:01 +00:00Commented 12 hours ago
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\$\begingroup\$ The easier option is to convert the truck to 12V. \$\endgroup\$fraxinus– fraxinus2026-02-28 21:20:52 +00:00Commented 5 hours ago
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5 Answers
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A resistor will not help you because its voltage drop is dependent on current. Say you size the resistor to give 8V when the starter is cranking. When you first connect it, the voltage drop will be very low because only the instruments will be drawing power (before they are fried by the 12V battery). If you size the resistor to give 8V with just the instrument load, your voltage will drop to essentially zero when you go to start the truck. You need to jump the truck from an existing 8V battery. Yes, there are voltage converters that can create a stable 8V supply from a 12V input but at the power level of a starter motor they are cost prohibitive.
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\$\begingroup\$ Your answer is very helpful to me. Thanks \$\endgroup\$Not too bright– Not too bright2026-02-28 14:24:09 +00:00Commented 12 hours ago
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The currents involved for cranking the starter rule out any reasonable solution using resistors.
What might make more sense is to get another 8V battery and keep it topped up with a trickle charger like this one.
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\$\begingroup\$ It's a great suggestion, which I'm already doing. You may have seen my comment to a different thread: Carburetor and ignition in this truck make failure to crank/ battery depletion in the field (away from chargers, etc.) a common occurrence. It's and inherited truck and to avoid theft, it does need to be abandoned away from home to get a battery. Some form of jump is the most practical solution. I don't have deep pockets to afford any and all alternative solutions. \$\endgroup\$Not too bright– Not too bright2026-02-28 14:28:36 +00:00Commented 12 hours ago
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So, had to do similar with 12v and 24v systems.
What I did was to temporarily disconnect the supply to all the circuits except the starter and the ignition system. This meant that the other systems were safe and the only system getting s higher voltage was the starter.
Just make sure you know what is supplied from where as some may have made poor modifications in the past.
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\$\begingroup\$ I will have to see how difficult / time consuming this might be when away from home resources. This is a simple electrical system that I need to trace...and I'm quite an amateur, more of a Motörhead. \$\endgroup\$Not too bright– Not too bright2026-02-28 14:32:27 +00:00Commented 12 hours ago
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\$\begingroup\$ If engaging the starter disconnects the instrumentation, headlights, etc. then you should be able to locate that wire from the ignition switch and add a switch or relay in series with it. \$\endgroup\$Carl Rutschow– Carl Rutschow2026-02-28 16:59:46 +00:00Commented 9 hours ago
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\$\begingroup\$ @CarlRutschow not likely on what the OP calls an antique truck, that functionality designed into solenoids only came in after about 1980 based on what I remember about the wiring of cars and trucks at that time. \$\endgroup\$Solar Mike– Solar Mike2026-02-28 17:03:36 +00:00Commented 9 hours ago
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While jump starting this thing from 12V downconverted to 8V is somewhat a challenge, topping the 8V battery up for few minutes from a 12v vehicle could be easy.
Just use few paralleled 12V headlight bulbs. Connect the negative of the two batteries together (like one does in a normal jumpstarting) and the lightbulb bank between +12V and +8V. Leave the 12V engine running so you don'g get TWO vehicles with dead batteries, wait 10 minutes and try cranking. Cheap as hell and will work.
p.s. Converting the whole thing to 12V could be way easier than trying to service an exotic voltage system. If the vehicle was 6V system to start with, it is old and very simple in this regard.
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One option might be one of those lithium jumpstarter packs. Typically they contain three lithium ion cells in a '3S' configuration, meaning 3 cells in series give you roughly 12V.
If you were to modify the pack so that the output was only using two cells in series, that would provide you with roughly 8V, at the same current the jumpstart pack is advertised to provide.
There are two potential downsides to this:
You need to make a high-current connection to the tab of the second cell - ideally welded. This could be awkward.
The charging circuit may not be happy on seeing two depleted cells and one fully charged cell, and either shout an error or try to balance the cells (which could take a long time and/or cause the pack to get hot). This depends on how the specific pack is constructed.
An alternative might be a 3S LiPo pack as used by RC modellers, but you'd need to check the maximum rated current - pulling too much may cause it to overheat (and catch fire, in the worst case).
It might be safer just to have another 8V lead acid as your jump battery.
