wire size for dual battery

[boxy]

hey guys
have searched heaps but the word dual battery comes up a hell of alot and nothing about what im after ,im not great with electrical stuff but im still happy to give it a go in hope that i get more knowledable with it
so i just have couple of quick questions:

1st question,my battery dual is in the front of my tub approx 3 meters from cranking battery will 26mm outside diameter(215amp) battery cable be suitable to not allow voltage drop over the distance

2nd question i have a mall panel next to my gear stick that just lifts off id like to put a fuse bank in ther so i have bower close by instead of pulling the dash aprt every time i want to wire something up, what size wire would i need to run to a fuse bank from my cranking battery?

cheers boxy

[drivesafe]

Hi Boxy and unless you are planning on doing some winching and want to join the batteries while winching, that cable is a bit of an overkill.

For a dual battery set ups, with both batteries in the engine bay, you could easily use 8B&S ( 7.9mm2 ) and it will be fine. I supply 10mm2 with engine bay kits and this is heaps big enough for all but winching operations.

As for voltage drop, there is a huge misunderstanding about this and it’s nowhere as important in short run set ups as people think.

Even with cable as thin as 8B&S, the amount of voltage drop is only significant when your auxiliary battery is in a very low state.

At that point, there can be a fair bit of voltage drop but this will only slow the charging down and as the battery charges, the amount of voltage drop reduces.

The very same think is going to happen with thicker cable but the amount of voltage drop be less.

If you drive long enough, the amount of voltage drop becomes irrelevant because your battery will still reach a fully charged state, it just takes a bit longer if you have thin cable.

If you already have the large cable, use it but if you are yet to buy it, you will gain very little for the extra cost of buying much thicker cable than is needed.

[TheOtherLeft]

Hi Drivesafe,
With using thinner cable is there a risk of the cable heating up too much? I assume with increased voltage drop (compared to thicker cable, eg 6 AWG) and the same current there is more heat generated in the cable???

[drivesafe]

Hi TheOtherHand, and yes you can have a problem but it depends on the actual set up.

The single biggest potential problem with dual battery set ups is NOT short circuit events but current overloads.

If the set up is a single auxiliary battery under the bonnet then the problem is not that great but I still supply a single 30 amp auto resetting circuit breakers with 8B&S, 40 amp with 10mm2 and 50 amp with 6B&S.

These sizes protect in the event of a current overload but it is usually not a requirement to protect against short circuits for wiring under the bonnet.

But when the auxiliary battery is housed anywhere else, then two circuit breakers are required, one at each end of the positive cable running between the two batteries.

These size circuit breakers then give both short circuit and current overload protection.

While the size of the circuit breaker for each cable size may seem small there is a lot more to matching safety devices to cable sizes.

For instance, the maximum CONTINUOS current rate for 8B&S cable is around 65 amps and while a circuit breaker may be marked as a 30 amp current rating, if it’s an AUTOMOTIVE grade circuit breaker, it is specifically designed to carry a constant load of 30 amp for 4 hour without tripping.

But here is the problem, that same 30 amp circuit breaker must be able to carry around 135% of its marked rating for up to 30 minutes and much MUCH higher currents for shorter periods.

So even using a 30 amp circuit breaker with 65 amp rated cable, you can still get situations where the circuit breaker could carry that 65 amps for 5 or more minutes before tripping.

I had a situation where I had a near dead shorted alternator and it was pulling 155 amps out of the cranking battery.

At the time, while I had an obvious problem but at first was unaware of what the exact problem was.

As the cranking battery had gone flat, I disconnected the cranking battery and connected an optima Yellow Top through a 60 amp circuit breaker and some 20mm2 cable.

The 155 amp short still took about nearly 2 minutes to trip the 60 amp circuit break.

You can have similar problems with a dual battery set up if, for instance, the auxiliary battery drops a cell and the battery then pulls high currents.

This is not a short circuit situation and if the circuit breaker is to large, you can actually melt the cable before the circuit breaker trips, if it trips at all.

So it is imperative to match the safety device to the cable size to give the optimum protection.

NOTE, automotive fuses are designed to work in the same manor.

[-Scott-]

Tim's given an excellent explanation, and some good tips.

Ultimately, the it's the cable insulation that is damaged by sustained overloads, due to the temperature. Cable gets hot, plastic insulation gets soft / deforms / burns.

The temperature of the insulation depends not only on the current, but also the ambient temperature. Under bonnet, working hard on a hot day, temperatures can easily reach 50 degrees. Most cables are rated for a 25 degree ambient, and PVC is often only rated to 75 degrees. So a cable that is borderline at 25 degrees ambient may exceed its rating under the bonnet of a 4wd.

Some cables are available with higher temperature insulation, so they can be operated at higher temperatures (from memory, some silicon insulated cables are rated to 200 degrees). This may look like you can get away with a smaller cable for the same current (and you literally can), but remember that it simply means that the cable is operating hotter.

If in doubt, go up a size. The cable will run cooler, and you'll be losing less precious power to voltage drop / cable heat.