Dual Battery - Advice please on basic wiring - new Patrol

You need to install a Redarc in the engine bay, and run #8 B&S cable both black and red from front to Anderson plug.

I have this setup and it's excellent.

Hi Spliney

I'm presuming you have no control over battery types.

The simplest is still just to run two wires direct from your battery to the Anderson plug at the cars rear.
It will work well ,with wires no less than 8 sq mm or 8 guage.

Doing this will mean it is fuse protected by Patrols main 160A fuse.

Personnaly though I add another (> 80 amp) inline fuse as well.

Robin Miller

Hi Spliney, first off, take Mainey’s advice, 8B&S is way to small for long runs and will result in the trailer battery taking much MUCH longer to recharge and obviously, if you don’t drive long enough, it will never fully charge.

Next, never use fuses in high current circuits, circuit breakers go open circuit quicker that a fuse can, plus AC rules state that you can put higher loads on a circuit breaker protected circuit than you can for an equivalent sized fuse wire protected circuit and the principals are the same for either AC or DC circuits.

Last but not least, in the case of circuit protection, bigger is NOT better. Whether you use a fuse or a circuit breaker, both are designed to tolerate around 50% more current than the designated load for 1 hour or to put that in practical terms, if you fit an 80 amp fuse or circuit breaker, either device will allow 120 amp to flow through the circuit for about an hour before they go open circuit.

And the same device is designed to allow around 200 amps for about 5 minutes.

This is done deliberately to allow for the near dead short that occurs when you start a motor and some other high current devices. If they didn’t work this way, every time your fridge compressor started, you would blow a fuse.

Two 30 amp circuit breakers would give you all the protection and current handling you needed. Fit one next to each battery’s positive terminal.

Cheers

Spliney, you posted; "I won't be able to install a full-on dual battery set-up in the Patrol, but would like to rig up basic wiring to charge the CT battery. I envisage running a cable from the towbar (Anderson Plug), but do I run it back to the cranking battery or to the alternator? What would be the simplest solution?
(My fail-safe to protect the cranking battery will be to physically unplug it at the towbar when stopped.) "

-> The SIMPLE (and cost effective) solution is, because you only want to charge the CT battery while you are driving, to run a battery cable from the CT battery to the EXISTING battery, Eg, wire the two batteries in parallel, yes, separated by the Anderson plug at the tow bar, and with a 'fusible link' of about 20amp max in the cable near the Cranking battery, reason being, your fridge will presently have a 10a fuse in it and you only want to safeguard the fridge system and a few lights drawing probably 2amp, and most importantly you state you say you will disconnect the Aux CT battery from the Cranking battery when you are stopped as a 'safe-guard' to stop the Cranking battery being fully drained by your accessories.
(which won't happen overnight anyway - IF you use a decent battery in the Camper Trailer to run the fridge)

Cable size should be the same or thicker than that presently used by your existing Alternator / battery cable, reason being it will now have to run about ten times the length as the original battery cable and still with-out getting any voltage drop.

I use 2B&S (32mm sq) but that's considered extreme by some, however it's my choice.
Do it once - and do it right.

Spliney, I would recommend using a Redarc solenoid. They are fairly small and you will never have to worry about connecting or disconnecting the CT, the redarc will do it for you, every time. The redarc "clicks in" when your starting battery is over 13.5v and "clicks out" when your starting battery gets below 12.5v

I would imagine having to disconnect the CT every time you stop and remembering to plug it back in would be a big pain in the arse.