Parallel batteries

I run a 30amp blade fuze and holder in + line before the anderson plug, I have two 100amp battries in the camper and all seems ok to date.

After 3 days in one place, running fridge and lights, battery charge never fell below 12.8 volts. The fridge is a 90ltr Reefer DTV.

The function of a fuse or circuit breaker in this instance is to protect the cable against excess current in the event of a short circuit to earth.

The rating should thus be of the weakest link in that cable (in terms of current carrying capacity). 6 B&S can safely carry about 80 amps, so here the weak link is the 50-amp rated connector.

The fuse or circuit breaker should thus rated at about 50 amps and located as close as feasible to the battery.

Regarding fuses versus circuit breakers: historical US insurance records show that the former can be a fire hazard - and that circuit breakers are safer. This may have changed however since the widespread adoption of blade type fuses.

Fuses and cheap circuit breakers are temperature sensitive. They need locating in as cool a place as feasible (certainly not over an exhaust pipe etc). My own choice is a really good marine-type circuit breaker (Electric Boat Parts in Sydney is a good source). A good one will set you back about $50. Avoid the $15 cheapies - they work but are very temperature dependent.
Trust this helps
Collyn Rivers

No expert, but from everything I have read, you should not let your batt get below 11.5 V. When the experts put my system together they put a (Black box ??) that won't let me get to 11 V.

Vince

(QUOTE)Will run 6b&s (13.52mm2) to the trailer with a 50amp anderson plug.
Please keep in mind that the trailer BATTERY may be at 10 to 11v when connected to the car aux bat.(end quote)

IMO the 6 B&S lead is possibly too thin, remembering you are going back to the CT and you will be getting some voltage drop at the rear battery, I use 2 B&S cable with nil voltage drop back to my two 80A/h DC batteries.

Why would your CT battery be 10v to11v..?
Remember at 10 volt your battery is basically flat and soon will be on its way to the big battery graveyard. It would be advisable to use an electronic disconnection switch so the battery is automatically disconnected from the fridge etc, to save your batteries life, nothing worse than having a dead battery in the bush.

Re fusible links - agree they are just fine. I did not mention them as they tend to get confused with normal fuses.

6 B%S cable should be about right - assuming an 8 metre conductor run and 20 amp charge, volts drop is only 0.2 volt.

All in all though, if that trailer battery is run down regularly to '10-11 volts' the advice in this thread is irrelevant as the battery will have a half-life of only a month or two!
Collyn Rivers

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Luc

All is fine.

The 40 amp C/Bs are much larger than needed for the load current, but it is possible that charge current could reach that momentarily if the trailer battery is very low. The C/Bs are in any case fine for the cable capacity (6 B&S [about 16 sq mm] cable will carry close to 80 amps continuous current).
Collyn Rivers

Luc

Bear in mind there will be zero voltage drop until you apply a load.

The purpose of the C/B is to protect the cable against overloading. The cable you are using will handle a continuous 70-80 amps and 150 plus amps on a cycling basis (if used for a starter motor cable).

If the trailer battery is low there is a real possibility of the alternator pumping well over 30 amps for a few minutes.

There is no point in using a C/B that is only just big enough to handle the proposed load where you have such a big cable safety margin anyway.

Go for the 40 amp unit - and if hard to find, pick a 50 amp unit. Do however buy a really good one (about $50). The cheap ones are simply too temperature dependent and may drop out at lower current.
Trust this helps
Collyn Rivers

Collyn,

Thanks once again. Just recently heard of a lead earthing on a vehicle chassi but failed to trip the 30amp c/b. I'm unsure on the details because I've heard it 3rd hand, but could this be due to the aux battery on the trailer being earthed to the chassi rather than directly to the parallel battery?

Cheers,
Luc