I’m in the process of wiring in a 2nd 130ah agm battery in my old Arrow
What size leads should I use to link the 2 batteries?
Will only be 500mm positive and 300mm for the negative
Is 125amp cable thick enough?
Google: "To join batteries in parallel, use a jumper wire to connect both the positive terminals, and another jumper wire to connect both the negative terminals of both batteries to each other. Negative to negative and positive to positive. You CAN connect your load to ONE of the batteries, and it will drain both equally."
With any wiring job the cable cant be too big, but if its too small you will have voltage drop, your theory sounds OK to me. Bit of weight there, I have our Walden rewired with all LED & we have one battery, TV is the strongest user.
PS: Battery sellers advise against linking older & new batteries together.
Use what you have - I linked my 4 x 100 AGM's together with 16 square mm cable - good for around 50 amps. They only need to carry half the current you'll be using from the pair of batteries at any given time or their max charge current if they are flat - this could be 40 or 50 amps for a short time if they are very flat and your charger or alternator are capable of this. No need to go too big for this at all. In my Viscount, max current draw is around 30 amps from the batteries if I have the microwave on and start the diesel heater at the same time which uses a bit of power for the ignition on startup.