Camper trailer cable issues and VSR

G'day Peter

I have to admit I'm not familiar with the term "VSR", however be that as it may, with your current arrangement, a simple solution would be to have a relay installed such that the CT battery is isolated from the starting battery when you hit the starter (in much the same way as your radio is automatically turned off momentarily). Then your CT battery would not be connected and as such there would be no chance of it trying to supply starting current, which as you suggest would likely exceed the cable capacity.

With that configuration in place I believe the insitu cable would be adequate to run your fridge and/or charge the CT battery from the alternator.

Of course a battery management system would be the duck's nuts but the above would certainly do the job.

The 2.9mmsq copper wire twin core?
I would imagine that current rating is really a rating for when the wire has resisted the current flow and that rating is the, getting too hot, likely to fry rating of the wire.

If from front to tow bar and Anderson and then to battery in back of a camper, that sized wire will be acting as a resistor and not allowing the full charge of the trailer battery to ever happen.

A VSR, Voltage sensitive Relay will drop out when a starter is operated as the voltage will go down.
You should be using resettable circuit breakers, possibly 90amp rated and positioned in a cool spot in engine bay and trailer ie not near a radiant heat source in the engine bay or bolted to the side of the black trailer which gets HOT in the sun. One breaker near the battery at both ends will help in cases of a serious short circuit.

A large capacity cable will allow the maximum current to charge the trailer battery with minimal resistance. Too small at the moment. Some like a DC DC unit, to be at the rear battery so it will definitely charge the trailer battery to full charge.

If a DC DC used then there is no flow back to vehicle possible.
If required an additional wiring and manual battery switch is probably required to directly connect the front and rear batteries.
Possibly another battery switch at the VSR so the rear can be used to assist the front if required.

Hi Peter,

Be careful of the advice given above, lest you start a fire.

You must have a current limiting device (fuse or circuit breaker) rated to the current carrying capacity of the conductors as close to the batteries as possible on both ends of the cable connecting two batteries (a short circuit could occur at any point along the wiring). The reason for this is that the current limiting device is there to protect the wiring so that it does not overheat and start a fire. Excessive current can flow from either battery.

If you have an appropriate current limiting device at both ends as required then just give it a try, you will not damage anything other than blowing a fuse. I doubt that the short transient current surge from starting would cause a fuse to blow or a circuit breaker to trip on the camper trailer battery.

The ideal solution is to have a DC/DC charger if you can afford it.

Peter mate yu have a clear problem.

The cable in the trailer is way too small.....nothing unusual.

In 12 volt electrics, unless the cable runs are very short, current rating does not enter into the matter....appart from the fact that there are some...um...variations...on how cabling capacity is rated for automotive use.

Resistance and voltage drop are the over riding considerations.

The 10mm2 cable you have running from your battery to the anderson plug on the rear of your vehicle is barely sufficient.
The cabling from the anderson plug on the trailer to the battery should be at least 10mm2.

If you expect expedient charging of your battery over the cable length in question, 15mm2 or larger would be my recommendation.


As for the fuse blowing...Hmm yeh well.


If you have heavy enough cable, sufficient alternator capacity and a healthy battery in the trailer. It is reasonable to expect initial charge current in the 20 to 50 amps.....if the trailer battery is deeply discharged and the vehicle battery is welll charged at the time of connection charge currents may be considerably higher.

I would recommend a 60 amp circuit breaker at both ends of your charging cable as near to the batteries as practical........depending on specification that 60 amp breaker should hold at 120 amps for at least 60 seconds.....sufficient for the connection peak to pass.

While you can manage your second battery manually, by plugging & unplugging, the use of a voltage controlled realy is by far preferable

In tne real electrical world, we would rate 10mm2 cable at around 100 amps...10amps per mm2, less thermal considerations.
In the automotive market, it may be pushed considerably harder.

The bater copper fuse pint of 10mm2 copper will be in the couple of hundred amps range, and the 75C rated insulation will probably remain in tact, in a cool environment at arround the 150 amps continuous.

A 60 amp breaker is a pretty safe choice.

If you rewire the trailer in 10mm2 cable it should be functional, but may be not as lickety split as it could be in the charging department.

Best bang for bucks 10mm2 cable is 10mm2 solar twinn

The 60 amp manual rest automotive circuit breakers are fairly commonly available...jaycar, altronics, various solar shops and on line.

If this is the standard of the main battery wiring I would check the rest of the trailer.

cheers