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I have a 1985 Mercedes 300d, with a battery with a cold-crank rating of 1000 amps. The battery is pretty much dead (and pretty much new). So far, I haven't been able to jump-start it with another car that has a 550-amp cold-crank rating on its battery, after letting the battery charge for about a half an hour.

If I let the jump car run long enough, will the Mercedes's battery eventually charge enough to start it, or do I have to find a charging source that puts out a higher amperage? If the former, about how long would I need to let the jump car run before trying to start the Mercedes?

  • Welcome to Motor Vehicle Maintenance & Repair! Why don't you let the charger do its thing and charge the battery? It most likely needs to be on there longer than a 1/2 hour. Also, try disconnecting it from the car (the negative terminal) so there's absolutely no draw on it. Also, when you say it's dead, how dead is that? Newer chargers won't charge batteries if they are completely dead, because it needs to sense there is a battery there first. – Pᴀᴜʟsᴛᴇʀ2 Nov 11 at 0:40
  • @Pᴀᴜʟsᴛᴇʀ2 Thanks for your response. I don't have a charger; I just have another car, a little old 4-cylinder Mazda van. I'm asking whether the Mazda battery can charge the big diesel's battery if I leave the Mazda running long enough (with jumper cables hooked onto it), or is the Mazda battery too weak to be able to charge it at all. – BobRodes Nov 11 at 8:25
  • If you want your van to charge both batteries, then the engine needs to be running at about 2000rpm to provide sufficient charge - if you leave it at idle the alternator does enough to just put the warning light out. – Solar Mike Nov 11 at 10:24
  • If you have a vehicle that stands unused, its battery will need charging periodically and the simplest solution is to buy a battery charger. They aren't very expensive. Being a new battery won't prevent it losing charge over time, even without the standby electrical circuits that a modern car has. – Weather Vane Nov 11 at 16:02
  • Thank you, Solar Mike and Weather Vane for your information. I shall keep both of these points in mind. – BobRodes Nov 12 at 3:30
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No, you need the full amperage power to crank the diesel at sufficient speed to start . It is unlikely to start if cranking at low RPM ; A gasoline engine is more flexible in this respect. Most have heard a gasoline engine barely turning over but then the cylinders start firing and help the crank speed . A diesel generally won't do that. I though of getting a diesel for a cold climate and asked an automotive engineer about pros and cons; he said starting a diesel in cold weather is basically a battery test. If the proper battery is in good condition it will start, a weak battery will not start it.

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I have successfully jump started my diesel Transit (with a great big lump of a 2.4 litre engine) from a small car, more than once. Things that help are: warm ambient conditions (so you can start without preheating using the glowplugs); and revving the helper car well above idle (but no need to redline it) for a good few seconds before trying to start. If you were trying to charge the battery using a small car idling, you won't have charged it much, similarly if you used a mains battery charger.

Keeping the battery topped up is a better idea. My approach is a 20W solar panel on the dashboard, with a charging regulator that automatically switches to trickle charging. This save messing about with mains cables (and I had the panel for topping up the secondary battery - it's a campervan).

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