24volt jumpstart

[Desiderata]

My daughter has an old lorry, two 12v tractor batteries in series to give 24v . It hasn't been started in a while and the batteries a have got a bit of power left but just not quite enough to start it.
The best thing she has to jump start it with is a tractor with a single 12v battery. Loads of oomph in it.
Best way to connect the jump leads?
1. Across one of the 12v lorry batteries?
2. Across the two 24v outlets?
Or
3. Don't do it, you'll blow something up.
I've given her my advice, just wanting to see if the more techy on here agree.

[GreenV8S]

Desiderata said:

My daughter has an old lorry, two 12v tractor batteries in series to give 24v . It hasn't been started in a while and the batteries a have got a bit of power left but just not quite enough to start it.
The best thing she has to jump start it with is a tractor with a single 12v battery. Loads of oomph in it.
Best way to connect the jump leads?
1. Across one of the 12v lorry batteries?
2. Across the two 24v outlets?
Or
3. Don't do it, you'll blow something up.
I've given her my advice, just wanting to see if the more techy on here agree.

Option 1 would give you a slight chance to start it but the second battery will probably prevent you from achieving enough voltage and current to start it. It would be easy to reverse charge the second battery and is not a good idea.

Option 2 is likely to set things on fire.

The right answer is to use two healthy charged 12V batteries with the right capacity and current rating. If the two existing batteries are basically healthy, they could be charged separately using a 12V charger.

[Matt_E_Mulsion]

Put the jump leads from the tractor onto one battery and leave it connected and the tractor running for fifteen minutes or so at a fast idle to put some charge into the first battery.

Then do the same with the second battery and leave it for fifteen minutes or so again. Then whilst it's still connected to one battery see if the lorry will crank over and start.

[Desiderata]

Matt_E_Mulsion said:

Put the jump leads from the tractor onto one battery and leave it connected and the tractor running for fifteen minutes or so at a fast idle to put some charge into the first battery.

Then do the same with the second battery and leave it for fifteen minutes or so again. Then whilst it's still connected to one battery see if the lorry will crank over and start.

More or less what I told her, I did say to give it a very quick try across one battery first but if it didn't start immediately, charge them up one at a time before giving it another go.