Jump-Start the Vehicle Battery from the Leisure Battery?

[Bigtrucknuts]

I have my leasure 110ah AGM battery under the driver seat. It’s charged with a Ctek split charger also under the seat. I don’t have the room under there for the Smartpass unit.

But given that a hi amp cable is already there from the starter battery positive to the Ctek. Can I put just a high amp three position switch to disconnect the Ctek and connect to the leasure batter positive? So In that condition both are connected in parallel and I can jump start the van.

So would that work? Or is this a silly idea likely to result in explosions and death?

 

[Dellmassive]

Lol.... I've done this myself...kind of.

A switch won't work due to the massive current involved....but there are work arounds.

BUT that was on my T5.1 where I run my own 0awg cable from the starter battery. (Fused at 150amps) the starter in-rush current is many hundreds of amps and will blow fuses or even melt cables.... but what I could do was jump Start The SCR by energizing the relay and give it 5mins for the leisure battery to back feed the starter battery..... then disconnect the SCR and start like normal.

Now over To The T6... the factory SCR cable is fused at 80amps and only has a small 100amp relay......which is only energised when The van is running.

So you need to look closely at your Hi-amp Clable... is It The factory cable or aftermarket?

What's it fused at?..

You won't be able to jump Start from it directly..... but may be able to use The battery back flow method I used above.

Another option that I've used before is to use a purpose jumper cable from the pos of the leisure battery to the pos of the starter battery..... then jumpstart like normal...... but thats if you can safely get to the battery under the seat..... mine was under the passenger bench to easy to access.

 

[Bigtrucknuts]

Hay thanks for the reply Dell, I thought someone would have tried this.
The charging system and cable is all my own work. I don’t recall it’s rating right now but it’s as thick as jump start cables. It is fused to the Ctec with a 30amp trip switch. So I was thinking put a switch between the starter battery and the trip switch. Switch goes
1, starter to Ctek
2, starter to off or not connected
3, starter to leasure battery no fuse

I get that drawing high amps continually would heat the cable up but for quick stab or two I was thinking the existing cable would cope.
So only question seems to be what sort of big ass switch would I need... ?

 

[Dellmassive]

600amps Inrush kinda switch.

The current drops once the engine is spinning..

So you would need a battery isolater type switch to handle the current..


And that 30a fuse will just go bang lol..


But using The isolator switch for just battery to battery back flow.... that might be under 30amps.

Also both sides of the starter/leisure battery should be fused for safety.


Battery Isolator Switch - Removable Key





.

 

[Dilbert]

If you really want to do this, why not use the leisure battery to do a fast charge of the starter battery? A few minutes on a 20-30A fast charge and you should be able to crank the engine again.

You would need far less current capability but you would need to regulate the charge back to capacity of your cables/fuses/switches, etc.

 

[mmi]

A T6 diesel engine startup current measurement in thread Which Jump Starter?

For this purpose I would fuse both ends of the connecting cable using 150 A fuses. This would allow a surge of 300-400 Amps for at least 1 sec. This should be sufficient to start the engine even without any help from the main starter battery. But as stated above, charging somehow beforehand the main battery would ease the job.

The above is based on spec sheet of time-current characteristics of “Mega”-type car fuse at https://m.littelfuse.com/~/media/au...bolt-down-fuses/littelfuse_mega_datasheet.pdf

VW guidelines of cable vs. fuse sizes can be found in chapter 2.5.2.1 Electrical wiring/fuses in the document “Body builder guidelines Transporter” – available at Body builder guidelines: Transporter - BodybuilderDatabase

 

[travelvolts]

Or even better, avoid flattening your van battery to begin with!!!!

 

[Bigtrucknuts]

Is that my wife trolling me on a forum site?

Anyway thanks MMI that’s just what I’ll put in, for that just in case (oh shit) moment.

 

[travelvolts]

Should listen to the Mrs, sounds like she knows best!

 

[Bigtrucknuts]

Ya she would like to think that too.

 

[travelvolts]

don't they all!