Problem with jump starting from 2nd Battery

[DiscoStew]

I have done much searching the old threads and found many people with dual battery systems stating that they have the ability to jumpstart their vehicle from the Aux battery if the main battery is flat.

While this seems logical if the battery is just flat but does anyone know if this will work if the main battery is actually completely stuffed.

I thought that attaching a good battery to one that has "collapsed" woul effectively drain the good battery very very quickly, hence making it useless for jumpstarting. I thought that was why you always started the donar vehicle before connecting the batteries ie to protect the donar battery but when using your own Aux battery you don't have that option.

Am I simply wrong about this (hope so)?? and if not do people have solutions already?

I am putting in an SC80 tomorrow and if I am wrong about the above will also install a momentary-connect switch on the dash that will put the two batteries in parallel while the switch is pressed.

Thanks

[County3.9]

Unless your starting battery is completely shorted it wont be a problem. You will need a massive solenoid to momentarily connect the two batteries. I just have a jumper lead handy. In 12 years have never used it. The amount of times you'll need to connect the batteries won't be worth the cost or effort.

[slipedisk]

Hi mate, had this problem in my last 4x4, a Landcrusier, I hooked up the duel batts system and the main did fail (dropped cells) down the Coorong I had a system, purchased two good size clamps (the red/black batt type) and smal 5mm cable the lenght from the aux to the main plus 12", the two batts are already earthed, clamp each end of the cable to the positives, of you go.
A mate I was fishing with flattened his batt, left cb/uhf/radio/int lights on, and didn't have enough to start, not the best but it did get him going, drove the L/C up to his bull bar so both touched = earthed, used the cable to joined my + to his + and she started, as said not the best but she did start without a problem.
I don't carry jump cables, only the one with the clamps.
I hocked up the L/rover the same and tuck the cable behind the l/h light, fits nicely.

[DEFENDERZOOK]

why not just swap the two positive terminals around......?

or even swap the batteries around if the cables dont reach.......and simply start up and drive away.....

[DiscoStew]

why not just swap the two positive terminals around......?

The aux is permanently set up behind the drivers seat for the fridge, which sits where the passengers used to. Too far for jumper leads and a lot of hassle to extract it for a jump start.

I might just terminate the wiring to the aux at some lugs near the main and then connect these to the SC80. I can then use jumper leads from there or do the swap as suggested.


or even swap the batteries around if the cables dont reach.......and simply start up and drive away.....

The Aux is a 120Ah and will not fit into the bracket for the main so cannot swap them.

Thanks for all your answers, means I don't have to setup anything complex.

[numpty]

I just turn the switch to the better battery, but that's because of the system I have fitted.