I have a heavy duty 6 ton truck with 24 volt start batteries. I am building a house battery system, also 24 volts. I want to connect the house battery to the car battery with a switch, so that I can use the house battery in conjunction with starter battery when I use the diesel heater which is connected to the starter battery from factory.

The truck has a big old engine and hence a big old 4kW alternator. I have been warned by a person (not sure his opinion is correct which is why I'm asking here) that if I connect my house battery to the car battery and crank the engine, the voltage spike could burn up the cabling between house and starter battery, unless it is of the same gauge as the cable from alternator to start battery.

Does this sound correct? From my understanding electricity takes the fastest route which is through the thick 70mm² cables going from starter to starter batteries. Cables between starter battery and house battery are AWG 12 and hence the draw on the house batteries when cranking will be less.

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    You seem to have some faulty/incomplete understanding of how electricity works. Speed has nothing to do with it, and the ability if each battery to provide current (not voltage) is what determines which source will do so. If your starter battery is weak, the house battery will provide most of the current and you'll start your #12 wire on fire. – isherwood Sep 4 '18 at 14:29
  • You might consider asking at electronics.stackexchange.com. This seems beyond a home improvement question as outlined in the site description. – isherwood Sep 4 '18 at 14:30
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    So, to be clear, you're trying to use your 24V house battery bank to run the engine heater in your work truck? Or are you trying to use the alternator as a generator? – Chris M. Sep 4 '18 at 14:49
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    Current takes all paths at once, in proportion to their conductance. Conductance is 1/resistance. – Harper - Reinstate Monica Sep 4 '18 at 16:21
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    I think the smallest jumper cables you will find will be around #6 stranded, low voltage requires much higher current, for example look at a 200 amp battery charger, they plug into a standard outlet small cord but the dc output is high amperage requiring much larger cables. The longer the cable from your battery bank the larger the cable size will need to be. In some cases it would be cheaper to purchase a good charger than a run of heavy leads to your truck. – Ed Beal Sep 4 '18 at 16:56

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