Nissan Patrol 4x4 (Africa)

I am busy installing a battery in the rear compartment. Was just wondering where best place is to route the cables through to the engine bay. I am planning on using 25mm2 cable which has a double insulation and is quite thick. I will take both + and neg to the front. I have measured the distance and will require just short of 5 meters of cable.
I found that a good place to run the cables is along and under the door sill. In the front driver side I do not find a rubber grommet in the firewall, but there is a drain hole in the floor approximately under the accelerator. Problem with this is - how to seal it again if I run the 2 thick cables through that hole?
Another question I have is whether I will be able to use the second battery in parrallel to the main to add extra amps when winching - will the 25mm2 cable be good enough?
I also intend placing a Circuit Breaker (not fuse) close to the aux battery - was thinking of 150amp - ??
I also plan to install a CTEK 250 Dual DC-DC charger in the rear, which can also take a solar panel input.
Any comments or suggestions from you guys out there?

25mm cable is good for 125a to small for the winch especially at 5m you need at least 35 or 50 would be better . Run it next to chassis in sprec and keep away from carpets hot carpets makes fire....you need a big KA breaker and that will be very big in size biggest single pole avalible is a 125a the dont like sand and dust .still think a big switch or soliniod will be the answer I will get you s 315 fuse .remember breakers font allow for start up preak amps it trips .

Right Tinus, however, I will definately not use the rear battery alone to power the winch. It will always be in parrallel to the main battery and with engine running. I therefore dont think it will ever pull more than around 100amp from the rear, but would like to test that theory if I can find a suitable amp meter. Same for starting, if main is too weak to start engine, the wires from the rear battery is basically same as a jump cable., and this 25mm2 looks more capable than the average jump lead.

I have done some interesting tests – “… om the meet is om te weet...! ”

I have tested this by shutting off fuel pump and then run the starter through the 5m cable for approx 20 seconds (in cycles of 5 second on 5 seconds off – this more than enough – my 4,8 always starts in a split second anyway), and there is no noticeable temp rise on the cable at all.

I have done a similar test with the winch. Both batteries connected and run the winch under heavy load for approx same time it takes to load by boat – again, no perceptible rise in temp in the wire.
The reason why I was thinking of using the second battery in parallel is that after a couple of days winching the boat onto the trailer, the main battery losses most of its capacity as my vehicle and boat stay on the beach and I do not drive around enough to recharge the battery and there is no external power available – my thinking is that the battery power will last twice as long by using 2 batteries, and seeing I have a second battery, why not use it in this way.

I was also re-thinking the routing of the wiring – best is to go through the firewall I think, than through the floor (water leaks etc), but I will have to find a way to make a new hole in fire wall and use a suitable grommet – any ideas there?

I have another idea which I want to try as well - is to place a relay (one from my old winch control box) in the +pos line of the second battery. The re;ay will be activated by the starter motor relay and the winch relay - in this way the second battery will always be isolated unless starting the car or running the winch. Of course this set-up will only be used when I am in the boating mode....

If anyone thinks my ideas are suspect please advise or add your comments....

I have a suitable clamp meter that you can use come around tommorow and we can make a plan the winch realy will work 100.%

I ran mine thru the chassis- 25MM2 + in plastic pipe & earth free (no pipe)- works fine
Only need to protect the +ve against chafing
Don't run a winch, but calcs say you should not be pulling >125A sustained...think you will be ok