When I looked into it, I found it was cheaper to buy a reasonable pair of heavy duty Jump leads, and use the wire from them, than buy the cable by the meter.
They do tend to "Over rate" jump leads, e.g. saying they are 800amp, but when you check the cable size you find it is only 100amp continuous use, or saying it has 8mm cable, then you find they are talking about the overall diameter, inc insulation, rather than the cross sectional area, so in the end, I got a pair I could look at in Tesco's of all places. Then saw a pair at a boot fair the following week, for a quarter of the price:doh
You will also find that while Jump leads tend to be over rated, welding wire is the other way round. In the end, I read up an a few websites that take into account duty cycle, voltage, lack of bundling.. blah blah and went with 16mm2 cable, 140amp relay and with 125amp fuses at each end.
Going by the websites, 16mm2 cable should be good for about 300amps, so working on the idea that it should be over engineered, and there should be very little volt drop over that size cable I went with the heavy duty 4 metre jump leads, giving me 8 metres of cable. The main problem is, to do the job safely you need so many joins in the wire, all of which will have a slight effect on the overall resistance, and add up.
Mine goes
- from battery clamp to 1st fuse holder (4 joins) bat term to clamp, clamp to cable, cable to soldered on terminal ring, Terminal ring to bolt on fuse holder
- through fuse (2 joins), fuses bolt onto cable terminals
- from fuse holder to relay (4 Joins) Terminal bolt on fuse holder, terminal ring soldered on to Cable, cable to soldered on terminal ring, bolt to relay
- from relay to terminal distribution block (3 joins) relay bolt to terminal ring, terminal ring soldered onto cable, cable into distribution block.
- Distribution block to second fuse (3 joins) Distribution block to cable, cable soldered to terminal ring, Terminal ring to bolt on fuse holder
- through fuse (2 joins), fuses bolt onto cable terminals
- from fuse holder to second battery clamp. (4 joins) bolt on fuse holder to Terminal ring, terminal ring soldered onto cable, cable to clamp, clamp to battery.
From a wiring point of view that is a scary 22 connections in the cct adding resistance that can stop the second battery charging properly, which is why it is so it important to go with bigger rather than smaller cable, and make sure all of your joins are properly crimped, or soldered.
The main problem is if you run one of the batteries flat, when your relay initially connects it to the other charged battery, you can get huge currents flowing for a short period of time, which is exactly what jump leads or designed for, which is another reason I went down that route.
