quick help with fusing for dual battery

[RanJ Avid PP Poster! Find all posts View Profile Close RanJ]

Between the 50A breaker and the main battery in the engine bay, where is the potential for overload? The only risk I see is that of the insulation being rubbed/cut through and causing a short. What am I missing?

[Mozz Member Find all posts View Profile Close Mozz]

If it is to charge a second battery you need cable capable of carrying the maximum amount of current you alternator can generate and if you want to protect it from overload and short circuit you need a breaker at each end as close as possible to each battery also capable of carrying the max change current but less than the cable. If you don't protect your cable at both ends an have a short circuit there is still potential from either battery to to ground. You need to alow for voltage drop over the length of the cable. You can't overkill when it comes to cable size.

[joel231 Find all posts View Profile Close joel231]

Also if the aux battery gets low and your alternator starts charging it, and you have a 50amp breaker it will blow if you alternator starts pumping more then 50amp down the cable to the Aux battery

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Generally you select a fuse to protect the cable, if the cable is rated at 60A continuos
then having a 150A fuse at the source and then saying I will protect the cable from
overloading by having a 50A fuse at the other end is a fool hardy arrangement,
why not put a correctly rated fuse at the source and you then know the cable is protected?

Assuming in the event of a fault condition that the cable will get a solid earth and
blow the fuse is also risky, for instance the cable might be abraided by a thin sharp
piece of metal that allows only a 100 amps to flow, unfortunately it is not a perfect
world and you just never know.

One could also argue that I have run the cable down a safe route, encased it in
corrigated tubing so why put a fuse in at all at the source? I prefer to know that if
I'm in a seroius accident or should some unexpected source of overload occur in my
wiring that my wiring practices are not going to result in a fire, how you wire
your car is up to you

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi again RanJ, as has been posted above, the 150 amp circuit breaker is just way to large to give adequate safety, when used with 6B&S cable.

As I posted earlier, 6B&S cable has a maximum constant current rating of 100 amps.

The problem with using a 150 amp circuit breaker is that if it is an AUTOMOTIVE GRADE circuit breaker ( or fuse ), there are specific standards set out governing the operation of AUTOMOTIVE GRADE circuit breakers and fuses.

An AUTOMOTIVE GRADE circuit breaker or fuse must be able to carry the marked current for 4 hours. But to achieve this, they will have to be able to carry higher currents for shorter periods.

As was pointed out earlier in this thread, this can be very high currents for a few seconds, but the standards state an AUTOMOTIVE GRADE circuit breaker or fuse must be able to carry 135% of their marked rate for around 30 minutes.

So RanJ, that 150 amp CB will allow 200 amps to flow through a 100 amp rated cable to 30 minutes without tripping.

As posted, the maximum current rating for a circuit breaker or fuse, used in a 6B&S cable circuit is 60 amps.

For an additional safety margin, I personally use 50 amp auto resetting circuit breakers and have don so for over 25 years and never had a problem, even when a total dead short has occurred and remained in place.

Once again, a 150 amp circuit breaker does not give the correct safety protection for 6B&S cable, and it would not matter if 50mm2 cable was run from the battery to the isolator, because there is 6B&S cable in the circuit, this governs the maximum current rating of the safety device.

BTW, I have on many occasions, jump started vehicle via a set of jumper leads with a 50 amp Anderson plug and a 50 amp auto resetting circuit breaker, and the CB has not tripped during the short period it took to start the disabled vehicle's motor.

[mullerwh Member Find all posts View Profile Close mullerwh]

Just jumping on the bandwagon with an interesting scenario that got me baffled on my recent trip to Fraser. I have Project DBC150 isolator with Optima D34 in front and Redarc BCDC1225 in the rear charging a Fullriver 120ah battery all connected via 4B&S cable (the Redarc is connected to Optima post the isolator and not directly to the starter battery). After not driving for 2 days both batteries dropped to 12.2 volt and whenever I started the vehicle(150) the circuit breaker would trip. To my knowledge there is no way the Prado alternator outputs more than 100A. I isolated the Redarc and rear battery and charged the Optima up front to about 12.7V. Then only was I able to connect the BCDC1225 and start charging the rear battery. Still not sure how to explain this - I don't have the problem when the battery charge levels are above 70%.

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi mullerwh, and I am taking a guess here, not being able to measure your currents, but after starting the motor, if the Optima is down, then the combination of the DC/DC device and the current the Optima is drawing, they are probably pulling everything available from your alternator and additional current from your starting battery.

You can get an idea if this is the case, by measuring the voltage at the starting battery, once the motor is running. Just see how low it drops once the isolator cuts in.

Again, this is just a guess.

[RanJ Avid PP Poster! Find all posts View Profile Close RanJ]

You know what... I just don't really give a damn! As mentioned before... The first point that no-one picked up on which is what should propably be pointed out to the OP is that his installation is incorrect.

The 50A breaker that comes with the Thumper is also a battery isolator and they want the unit mounted in the engine bay. There is also a breaker located within the Thumper enclosure. Making both the 150A isolator and breaker redundant.

So reality, we can all go backwards and forwards about it as much as we like, we are just going further and further off topic and it's fairly safe to assume that all of us are to damn stubborn to concede to others. So carry on gentlemen! I leave you all to it! lol

And remember... Dermis and feline can be divorced by manifold methods. Does that make them incorrect by default?

[shmick82 Junior Member Find all posts View Profile Close shmick82]

Thanks again for all the input. Great info. I ended up ditching the fuse box that came with the kit and got another breaker. I now have a 50A circuit breaker at the crancking battery and a 40A circuit breaker (cos supercrapauto didnt have bigger) at the thumper (well about 1m from the terminal).
Such a pain in the anus to install as i didnt have enough lugs of the right size and had to do some serious soldering to keep it all together.
By some miracle of nature it all works. So wrapped. Only issue (so far) was upon intial crancking the car got rough (like it was about to turn off) for a few seconds and all good subsequently.
Now i can have 60l of beer in the boot and a few cans in the cooler up front. QLD here i come.

[RanJ Avid PP Poster! Find all posts View Profile Close RanJ]

You've just removed the battery isolator.

[shmick82 Junior Member Find all posts View Profile Close shmick82]

What do you mean RanJ? Its power light is on and it says its charging when the car is on (under load, not idle). Did I make some big mistake?

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi shmick, don’t worry about it, you now have a perfectly safe setup and the 40 amp circuit breaker is not going to be a problem in any way.

Your job is done, it’s done safely, so just enjoy your holiday.

[RanJ Avid PP Poster! Find all posts View Profile Close RanJ]

Hi shmick, don’t worry about it, you now have a perfectly safe setup and the 40 amp circuit breaker is not going to be a problem in any way.

Your job is done, it’s done safely, so just enjoy your holiday.

The dual battery isolator that comes provided with the Thumper setup has just been removed.

You know the one... the one that isolates it from the crank battery when the ignition is off. The same one that prevents the auxillary appliances from draining the starter battery while camping.

[Chillihilli Lurker Find all posts View Profile Close Chillihilli]

Hi Schmick - I just thought I'd chime in to help here. I do a lot of dual battery systems.

Twin core 6B&S that I use in all my installs (Tycab) is rated to 84 amps. If you have used 50 Amp or less at either battery you are well protected. Plus your andersons are only 50 amp too (many people forget this) so you;re all good.

The "fuse box" you refer to - is that a fuse box or the battery isolator itself? If you've removed the isolator you have created a risk of flattening both batteries.

I'm in Melbourne too, happy to look over your install if you like.

cheers Pete.