Probably Totally Stupid jump start question

[mike65]

Having a land Rover FFR (with 2 batteries) I have been doing something that is dangerous (thinking) and have come up with an interesting thought.

When you have the radio batteries installed you have 4 in total, 2 for the radios and 2 for the Landy..

Therefore if the Landy batteries are flat and it will not start can you use the other two to jump start yourself?

Is this a totally stupid idea or has somebody tried it?

 

Mike

[fv1609]

Mike. It could work but you have to be cautious. Although the two sets of batteries share a common earth there is a problem. Bear in mind quite a heavy current is drawn in turning over a starter motor.

 

The vehicle battery is earthed with a short strap to chassis, not so with the radio batteries. The -ve lead for that is considerably longer & will be earthed at the shunt box. So you will have a voltage drop on the thinner -ve cable as it was not rated to run a stater motor.

 

So I would put a jump lead from -ve radio battery directly to -ve terminal of vehicle battery.

 

You want to avoid the risk of the +ve radio battery supply being drawn through the BCK battery relay contacts in the Generator Panel No.9 terminals to jump the +ve of the vehicle battery supply. Otherwise you might severely damage the relay contacts.

 

But you could disconnect the +ve lead to the radio batteries & stow the end of it on the terminal provided so that doesn't short to earth & melt your BCK relay when then the generator starts & the BCK relay closes. Then a jumper lead from radio battery +ve to vehicle battery +ve.

Edited March 6, 2010 by fv1609

[ruxy]

I have not managed to get a good look at a winterized FFR 109" - so don't know for certain but I think they would have made use of the radio batteries for the SB-ORD "Inter Vehicle" slave start..

Possibly there was pacility to quickly change the slave socket supply cables from the starting batteries to the radio batteries.

 

24 volt are probably like the 12 volt slave start sockets - a few different methods , it seems not all 12 volt GS carried the extra tractor battery in the tub. One of the Ex-MOD jump start cables I have is brand new (I think not modified) it has a pair of croc clips at one end instead of a slave start socket at each end giving more versatility.

 

The helistart versions with side reeler - some photographs I have seen seem to have a Anderson plug or sililar for the heli end.

 

http://www.ttabconnectors.com/index.php?page=products&internal=SBORD%20Inter-Vehicle

[mike65]

So it seems my idea is not that stupid after all. although it maybe safer just to swap batteries over.

Need to sort that part of the electrical plumbing out later.

 

Mike

[Stone]

Also bear in mind the radio batteries will be intended for low-current deep discharge over many hours, as opposed to starter batteries which are intended for brief periods of high-current cranking. You'll probably get away with it if you don't do it too often (or if they used the same grade of batteries in both locations!) but it's not ideal for battery longevity.

 

Stone

[ruxy]

This is the older carried for radio - The No. 4 Mk3 (UK6TN) 12 volt 100 Ah - they are in fact starting batteries for tanks , so not a deep cycle leisure battery.

 

 

Normally two would be carried - the Lightweight "Unitary Kit" in fact has a spare storage tray to carry two additional , the very latest specification is the "Armasafe" , obviously if you are paying - then that is the only problem.

[mike65]

OK so you could possibly jump start yourself.

However is it possible to jump start a 12v car from a 24v Land Rover?

Is it as simple as just connecting jump leads to one battery

Only asking because a friend had a flat battery so needed help to get the car running.

I played safe and connected a battery charger for short while.

 

Probably guessed by now tha I have little experience of 24v systems so need to learn as much as possible.

 

Mike

[fv1609]

Is it as simple as just connecting jump leads to one battery

 

Yes.

 

But Mike you should later reinstate the charge in the battery that was used to do the starting. In the 24v system the two 12v batteries are in series, your vehicle's charging system cannot preferentially give more charge to the more discharged battery. Even if you run the engine for a long time one battery will be undercharged. You then run into the problem that people can have where they tap off 12v to run a CD or radio etc.

 

So best to charge each battery individually later.

[mike65]

Thats useful.

It is possible to do but I would then have to charge both my batteries, seperately, later.

Mental note, when friend with flat battery (in automatic) car calls needing a start and is away from mains electric, move Land Rover and get Astra out of the garage, it is less hassle.

 

Why can't they fit starting handles to modern cars?

 

Thanks

 

Mike