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Stone

Jump-starting 24V vehicles

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Bedford was flat the other day (someone left the isolator on, not me!) - we ended up taking the batteries out and charging them individually, but what's the correct procedure for jump starting a 24V veh from a pair of 12V vehicles?

 

If you just parallel each battery with a car battery (still in a car) using two sets of jump leads, if an electrical connection is made between one car and the truck chassis or both cars, at least two batteries are shorted! Could remove chassis earth connections from the cars but then why bother with jump leads, might as well remove the batteries.

 

So, have I missed something? :)

 

Stone

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Suppose you could use 1 x car and jump leads to parallel whichever truck battery is flat. If both flat and you want to use 2 car batteries and jump leads -

 

Neg. post car 1 to pos. post car 2 - neg. post car 2 to neg. post truck (earth lead) - pos. post truck (24V ouput) to pos. post car 1.

 

Only need to remove neg lead on car 1 battery as car 2 and truck (the one with earth lead) neg.s are both to earth.

 

Always get a grownup to check my advice - the usual disclaimers apply

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I need a slave start in my Foden wrecker once and had no other military vehicles around, a bloke stopped in his Golf and I took the opportunity to throw my slave lead onto his battery, 12v to 24v nearly sucked the life out of his battery but started my wagon, no dramas with his vehicle after that.

 

Edited to add this is not something I would recommend but it got me out of a spot of bother.

Edited by recymech66

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I need a slave start in my Foden wrecker once and had no other military vehicles around, a bloke stopped in his Golf and I took the opportunity to throw my slave lead onto his battery, 12v to 24v nearly sucked the life out of his battery but started my wagon, no dramas with his vehicle after that.

 

Edited to add this is not something I would recommend but it got me out of a spot of bother.

 

Erm! don't think that is a good idea!

 

Suppose you could use 1 x car and jump leads to parallel whichever truck battery is flat. If both flat and you want to use 2 car batteries and jump leads -

 

Neg. post car 1 to pos. post car 2 - neg. post car 2 to neg. post truck (earth lead) - pos. post truck (24V ouput) to pos. post car 1.

 

Only need to remove neg lead on car 1 battery as car 2 and truck (the one with earth lead) neg.s are both to earth.

 

Always get a grownup to check my advice - the usual disclaimers apply

 

Totally Agree you need to connect to one battery only, if only one 12volt source is available. Then it must be Positive to Positive and Negative to Negative.

 

If you are trying to jump start a 24V motor from a 12V motor, then connect the jump leads across one battery only on the 24 vehicle. (if you have a voltmeter, try and find which of the two 12V batteries, used on the 24V vehicle, has the lowest voltage and jumper the helping battery across this one). The 12 V battery on the helping vehicle will raise the voltage of one of the assisted vehicles bateries, and may be enough to get it started.

 

On no account concider connecting 12V across 24V. The 24V battery will try to discharge itself into the 12V, possibly raising its voltage to a point where equipment is damaged on the 12V vehicle. Remember the 12V battery will still be connected to the 24V system if and when when the engine starts, and if using any revs the Alternator on the 24V vehicle will cut in and will be pushing 28V (or more) around the system.

 

If you wouldn't put a 24 Volt battery charger across a 12Volt battery in a car, because of the obvious risk of damage, why are you allowing a 24V alternator to run its output into a 12 Volt battery?

Edited by antarmike

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I forgot to add when I do jump starts which is often, the Renault Master has a big battery and the Pajero is 2 x12volt heavy duty batteries(not 24v)

I wouldnt like to try using a Golf too often:-D lucky he was passing by and never to be seen again:cool2:

 

Christian

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This thread has reminded me of a time when I was at the Rotunda Museum in Woolwich, working on vehicles for them, a Tracked Rapier was parked in the driveway, when an Esso tanker arrived. The Rapier was in the way so I went to start it only to find the batteries flat, the tanker driver, to my surprise, offered to jump start it off his tanker. He must have noticed my concern and said it was reasonably safe he was delivering heating oil ! Anyway we did it and he was able to continue his delivery.

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Only need to remove neg lead on car 1 battery as car 2 and truck (the one with earth lead) neg.s are both to earth.

Hadn't thought of only removing one lead :D

 

Erm! don't think that is a good idea!

Well, so long as the jump leads are thick enough for double the current, you just have to worry about setting their battery and radio on fire when the alternator kicks in :rofl:

 

 

If you are trying to jump start a 24V motor from a 12V motor, then connect the jump leads across one battery only on the 24 vehicle. (if you have a voltmeter, try and find which of the two 12V batteries, used on the 24V vehicle, has the lowest voltage and jumper the helping battery across this one).

Makes sense to me. Probably easiest if we just carry a single spare battery around then - we can swap it over to get it started and then recharge the duff one on the generator, and not too much extra stuff to be carrying. Not a bad idea having some jump leads in the back, though!

 

He must have noticed my concern and said it was reasonably safe he was delivering heating oil ! Anyway we did it and he was able to continue his delivery.

Rather you than me! :shake:

 

Stone

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Makes sense to me. Probably easiest if we just carry a single spare battery around then - we can swap it over to get it started and then recharge the duff one on the generator, and not too much extra stuff to be carrying. Not a bad idea having some jump leads in the back, though!

 

 

Rather you than me! :shake:

 

 

 

If you have one duff battery and are carrying a spare, then it would be a better idea to make sure that you are using a pair of good batteries of equal voltage, rather than carrying on like that, because it will drag the other battery down eventually.

 

Regarding the tanker, not a problem, no worse than the proximity of a 100 gall. fuel tank on a truck.

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I forgot to add when I do jump starts which is often, the Renault Master has a big battery and the Pajero is 2 x12volt heavy duty batteries(not 24v)

I wouldnt like to try using a Golf too often:-D lucky he was passing by and never to be seen again:cool2:

 

Christian

 

As I said earlier, not something I would recommend,:nono: and it got me out of a spot of bother, no problem with his Golf after the slave start either.:cool2:

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My Toyota Surf has two big 12v batteries (it's called a mountain pack). A few months ago it would hardly start and out the volt meter came. The bad battery was holding 12 odd volts and the other 13 something. This was enough for it to cause problems, only a volt or two in it. Although it's called 12 and 24 volt, they actually charge and hold more than this. In a 12v system it's around 13-14 volts. In a 24v system it's around 26-28 volts.

 

My work truck puts out 28 volts.

 

I just went out and bought two brand new batteries. Don't bother changing just one as this will make the previously good battery suck the life out of the brand new one, change them as a pair.

 

My Defender is a 12/24 volt system. 12 for the vehicle and 24 for the radios. I've disconected the 90 amp generator as I don't need it, but you can re connect it and use the terminals at the shunt box to provide 24 volt power. Just make sure your jump leads are up to the job.

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