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Jump starting a 12v from a 24v

nick_the_fish

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Today we finally managed to get the 105 out of a container in Durban after waiting for for 3 weeks down here.

The battery was very flat, which was annoying because it was a new battery and I'd hoped to get away with it for only 2 months of being disconnected. We tried everything to jump start it but nothing could get it to turn over. It was really flat.

In the end someone suggested we jump it from a 24v battery. I was very dubious about this as I was sure something was going to blow, but with no other choice we did and it seemed to work.

So... just how lucky did we get? Is jump starting a 12v from a 24v a total no no that you can get away with sometimes, or is it a kind of "try not to" ????

It'd be nice to know for next time.......
 

Chris

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Well it all depends how you do it Nick. You say that you had a 24v battery? It's not something I'd suggest doing every morning, but in a clinch, what choice do you have. Probably better that your battery was flat. I'd imagine that it acted a soak of some kind to absorb that. As a battery it's not kicking out perhaps quite the whallop as if you'd connected to a 24v vehicle with the engine running!

I guess the problem that you might encounter is blown fuses on things that are live at the time of connection that run12v. I'd try to avoid having to do it again, but if you have to, then find a 12v source to be safer. If you have to use 24v again then I'd keep the contact period as short as possible, ready, connect, turn the key, disconnect. But like I said, not the same as connecting it to a 24v running vehicle.
 

Grimbo

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24volt battery ?
That's a rare thing in the UK even ,
Most 24v trucks etc use 2x 12v batts so no problem to jump start a 12v vehicle by just going off the one battery on the truck ( the first in line from the earth side ) ....
You were lucky not to blow bulbs etc but as your batt was dead the peak was probably less than 24v at critical items ....avoid if at all poss .
 

nick_the_fish

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Thanks for the replies chaps,

yeah it was a bank of 2 12v batteries, but I was typing in a hurry and thought it easier to write '24v battery' than explain that it was 2 batteries which we used as 24v source rather than a 12v source etc.... I figured you'd all know what I meant

We tried using just one of the batteries but that didn't get it going, we gave the donor vehicle more revs but nothing could get the 105 to start.

We did exactly as you suggest Chris, kept the contact very brief, and it fired up first time.

Anyway, no harm done (I think) and all adds to the experienced based learning.

N
 
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Towpack

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Maybe, if the container has been stood in direct sunlight, the heat may have killed the battery. They must be like an oven inside in hot sun.
 

Towpack

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Obviously not a recomended practise but I'm with Chris's thinking that the onboard battery was so low that it acted like a sort of electrical sponge and absorbed the extra volts. With a brief connection of over-voltage I reckon you're in more danger of damaging onboard equipment or ancilliaries, especially anything electronic. Fingers crossed that isn't the case.:thumbup:
 

nick_the_fish

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How was the battery left during shipping nick, was it disconnected?
Yep, live was disconnected. It was done by the shipping agents due to long and boring reasons, personally id have disconnected both neg and pos but I don't think it makes a difference. Its surprising because it was a new battery, and I thought they were only supposed to lose 15% ish per month if there was no load, so I thought (hoped) we were going to get away with it.

Maybe, if the container has been stood in direct sunlight, the heat may have killed the battery. They must be like an oven inside in hot sun.
Yeah its possible.

Did you then try the other one?
Yep tried it off both batteries, plus another. None of them even came close to turning it over. We left the donor truck running for a good 10 mins connected with some revs to try to get some voltage into our battery but still dead as a dodo. We've done a few hundred miles in it now, and it seems to be taking and holding charge, so fingers crossed we haven't damaged it.

We've done a few hundred miles over the last few days, charging both the main, the leisure and various accessories, and it gets to 14.3v (after a while - presumably because of the amount of load).

Plus after being sat for 3 or 4 hours the main battery is showing 12.74v so it also seems to be holding charge well.

On a side note, very pleased with the new AGM leisure battery, and especially the Victron Smartshunt. Its really useful to be able to see how much load we're pulling and how much we can run the accessories for.
 
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