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Say I need 600 Amp Jumper leads to start a diesel, can I use 2 sets of 400 Amp leads between the 2 batteries? That is 2 positive clamps on the positive posts and 2 negative clamps on the negative posts. Would they share the load?

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    you only need that much current if the battery is completely flat. Let it charge a few minutes and you greatly reduce the current the cables need to carry while starting
    – agentp
    May 16 '17 at 12:04
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Theory:

Theoretically they would share the load assuming an even contact on the clamps. Since metals in generally have an positive temperature coefficient of resistance the leads automatically distribute the current equally.

Practice:

My gut feeling tells me that a significant part of the resistance "happens" on the clamps, so it would be difficult to balance the load (every clamp contact in a pair of clamps needs to have the exact resistance). In practice I would stick with one set of 400A leads and be confident that it would work, but everyone needs to decide for himself if he will take the risk (Wear gloves in case something does not work as expected).

Anecdote

I once started an 3.5L diesel with a set of 16mm² copper leads, the leads got warm but the car started.

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    I started a car once by touching the metal bumpers to provide a ground, then running a length of wire (don't know the size, but it wasn't large) between the positive posts. It took a little bit, but was enough to charge the battery to get it started. Go figure. May 16 '17 at 18:23
  • @Pᴀᴜʟsᴛᴇʀ2 when you have nothing everything goes :)
    – Martin
    May 16 '17 at 19:41
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According to the science YES they will equally share the load, as long as the conditions between the two cables are the same. ensure they're both connected the same, both have the same length of cable, and are both the same tolerance.

According to the rules of catching things on fire, make sure the above precautions are met. Other than that. Two wires in parrallel will share a load

EDIT: DO NOT use just one set of 400A leads as the vehicle uses the most energy at startup, and jumping is when most of the cranking occurs, and thus most of all the current will be active during startup

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Most diesels need 800 - 1000amps for starting (2litres and more), so with the other answers the plan is ok.

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  • Sorry, but your numbers are flawed. 1000A would be more than 10kW! 100-200A is more plausible.
    – JimmyB
    May 20 '17 at 20:19
  • @JimmyB You don't know what you are taking about - think or better yet look at the thickness of the cables going to the starter on larger diesel engines - my jump cables are flexible 800amp welding cable and they work very well. Shame you downvoted my answer due to your stupidity or lack of knowledge.
    – Solar Mike
    May 20 '17 at 20:30
  • @JimmyB Go check the workshop manual for a Perkins 4108 diesel and it says it needs 900Amps...
    – Solar Mike
    May 20 '17 at 20:44
  • No need to get upset. I doubt that the marine engine you are referring to is representative of car engines. - See e.g. products.bosch-mobility-solutions.com/en/de/_technik/component/… : 2.5KW maximum for Diesels of up to 3 liters (@ -20 deg. C!) Probably half that at e.g. +10 deg. C. - I guess I only disagree with "most" and "2litres and more" in your answer.
    – JimmyB
    May 21 '17 at 12:40
  • As an ex- vehicle electrician I worked on small cars up to heavy earth moving equipment and all in between, so Bosch may only be refering to the latest generation of starters - older ones (less efficient ...) used on tractors hgv etc can easily exceed 1000A and I had to make new cable between batteries and to stsrter capable of dealing with that level of current. Do note the OP mentions 600A....
    – Solar Mike
    May 21 '17 at 12:46

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