Twin starter battery wiring on Mk5s

[Noctule]

In anticipation of some winter travels I am considering a second starter battery. What would be the best way to connect that battery to the existing circuit?

Positive to first battery, starter or alternator?

Negative to first battery or starter?

I've already discounted using the chassis as an earth. I've used it in the past when the leisure battery has served to jump start in winter, but that's now at the back.

Thanks all

Mat

[MinorMatt]

Ford connected the 2nd battery to the same points as the main battery

I.e. to starter motor bolts

[Noctule]

Ford connected the 2nd battery to the same points as the main battery

I.e. to starter motor bolts

That's kinda what I was guessing, the most efficient route for starting.

Thanks Matt

[dumper]

We had a few with twin batteries at work and when one of the batteries was done for they wouldn’t jump off but if you disconnect the dead one the better battery of the two would start the van on it’s own without a jump leads usually in the end we just removed the second battery and return them to single battery.

[HairyFool]

DSC_0127 4.JPG

Mine is the full twin battery system from Ford when they did away with the Accessory battery idea but kept both batteries.
Standard 2 terminal starter solenoid with a junction block on top. Primary battery positive in from the right with the negative below onto the starter bolt.

Secondary positive in from the left, can't see the negative but I think it is on a gearbox bolt.

Leisure battery in the back under the sink.

[V184]

We had a few with twin batteries at work and when one of the batteries was done for they wouldn’t jump off but if you disconnect the dead one the better battery of the two would start the van on it’s own without a jump leads usually in the end we just removed the second battery and return them to single battery.

Same