Just today we had to move a Deutz D40.2 (1962 or so) out of the way and the battery (something like 12V 130Ah or so) wasn't up to it. So I first tried helping out with a just-charged car battery (12V 36Ah, used for electric fences), then with a running car (old Mercedes). I finally unconnected the tractor starter battery (which probably had just gobbled up most of the previous attempts) completely and just used the running car (with someone pressing on the accelerator) and basically gave up when one of the starter cable clamps' metal started glowing and melting through its plastic.

The starter motor of the tractor was at best moderately impressed, with an action that would have corresponded to a mostly empty starter battery of the tractor.

So what's the actual starter current like here? I know that with a fully charged starter battery, you'll have not all that more than a few minutes of actual starting before things get ugly. The pre-glow current is something like 3x10A and I think one is better off doing this a minute too long rather than having two bad 3 second starting attempts.

I don't have a current measuring clamp myself and certainly no current meter that I could viably put in the middle. I think I measured the voltage during start at some point of time, and it dropped something like 2V or so while starting.

Are there any hard numbers regarding the starting current? Because this feels a bit excessive to me. It does start as long as the battery is in reasonable state.

  • Are you sure you attached the jumper cables correctly? From your description of "starter clamps' metal started glowing and melting" you were in series instead of parallel. This doesn't answer the overall question, so leaving it as a comment. Mar 25, 2017 at 12:44
  • @Paulster2: yeah, cables were correct. I mean, for the second attempt the tractor battery was off anyway (its ground contact off), and the clamps at the tractor were right at the starter (for plus) and on a chassis part (for minus). Wasn't really better than with the tired battery on. And not much to do wrong on the other side either. The clamp might not have had the best contact, but for starting to glow you need a solid current anyway. And the starter moved, just not enough. And after charging with 6-8A for few hours, the tractor started fine.
    – user26697
    Mar 25, 2017 at 20:39

1 Answer 1


As the diesel engine is about 2.5 litres, you will probably need between 800 to 1000 amps to start it - as you saw by the clamp glowing and melting plastic - most likely because the contact area was small for the current flow.

It is VERY bad practice to use a running engine to try to start an engine - the high current demand can damage the alternator, I am sure there are people who will reply saying "I did this and it was fine..." but I have changed alternators for people at their cost and they moan "all I did was try to help someone..."

You mention in your later comment that after charging for 6 hours or so it started fine - due to the battery being able to supply sufficient current.

  • I'm not sure what you're talking about by "VERY bad practice to use a running engine to try to start an engine" ... it's is the way you're supposed to do it. When people fry an alternator from jump starting, it's usually caused from the jumpers being put on incorrectly (putting batteries in series instead of parallel). Mar 25, 2017 at 21:19
  • "It is NOT the way you are supposed to do it" - if you check out the control circuitry in an alternator you will find it contains diodes - these cannot support the current surge during a starting operation (600 - 1000 amps) but can deal with maximum charge situations of 100 to 150 amps, advising people it is good may permanently damage their alternator. I have always, and will continue, to tell people to use their car to charge the flat battery for 10 minutes, then turn the engine OFF - then and only then attempt to start the flat vehicle.
    – Solar Mike
    Mar 25, 2017 at 21:25
  • You say people make the mistake of connecting in series - I have NEVER seen anyone do that - to make that mistake would mean that they would have to disconnect one battery cable and few ordinary people have the tools around to do that when they are in the supermarket car park for example.
    – Solar Mike
    Mar 25, 2017 at 21:27
  • 800 to 1000amps: ok, that explains a lot. We've used starting aid on this tractor a few times successfully but it would appear that the tractor battery would then still have been good for most of the current, just not all. Good point regarding the alternator current. Hope we didn't fry it. Guess we'll find out in a few days.
    – user26697
    Mar 25, 2017 at 21:46
  • @Solar Mike: if you use one battery the wrong way round, you get both batteries "in series" with the first cable and then shortcircuit that series with the second cable. Maybe that was meant?
    – user26697
    Mar 25, 2017 at 21:54

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