Hi shocker
Sorry for late reply.
Ok:
No it's not ac but tbh there's no room taken up in the rad gallery to speak of so won't be a problem either way.
Heat? Well mines a v8 which gets stonking hot compared to a Tdi andeven at 110 degrees or whatever a v8 can run at when roasting, it's nowhere near the failure temps for modern plastic wiring and componentry etc.
Not double cable runs as such.
On the original battery, you have the original battery positive and body negative. There is an AWG4 cable running to one side of the split charge relay, which is a 120A durite version. This is protected by a 100A maxifuse next to the battery.
There is a AWG4 cable running across the engine bay connecting negative on both batteries.
On the second battery, you have the link cable mentioned above, and the negative cable for the winch, which is an AWG2.
There is the positive cable for the winch which is also AWG2 and the winch is protected by a 150A maxifuse (as per manufacturers specification)
There is also an AWG4 connecting the secondary positive to the other side of the relay. This cable is also protected by a 100A maxfuse next to the battery.
Yes the secondary battery is actually of a higher spec than the primary. The v8 is not hugely difficult to spin over like a diesel so Halfords standard spec 072 manages admirably.
I fitted Halfords high spec 072 for the secondary, because it solely powers the winch and therefore does not deep cycle. It in unlikely it would ever deep cycle, it never has, but it does need to be able to supply max amperage for short bursts (winching is similar to cranking an engine over of course)
Interestingly the winch is fused at 150A as per factory recommendation so the battery is likely to be understressed anyway.
And the relay is alternator switched, that way I know the batteries are only connected if the system is charging. Also, running the engine when winching allows the primary battery to assist, and the alternator provides amperage to help out too.
I have a pair of simple battery monitors on the dash, easily wired one to each side if the relay and common earth, I can monitor battery levels, and when the engine is started if they are not showing the same (both maximum) I know for some reason the system is not charging and/or the batteries are not connected.
You can just see them on the left of this picture
Cheers