Camper trailer cable issues and VSR

Submitted: Wednesday, Mar 19, 2014 at 15:13
ThreadID: 106797 Views:3295 Replies:4 FollowUps:6
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Hi all

We recently purchased a used CT and have some concern regarding the wiring.

Firstly it would appear the cable is 5mm twin core which is 2.9mm squared and rated at approx 25-29amps.

The trailer has a 105ah deep cycle battery in the rear and a 37 litre Waeco compressor fridge at the front.

It would appear there is only one positive lead from the CT battery so I would assume however not yet tested that the fridge could run directly from the tow car power supply if the trailer battery were isolated.

One concern I have is whether I am going to keep blowing fuses should the battery become quite discharged or will the cable just limit the amps to what it can handle (a 30amp blade fuse is currently fiitted CT battery end)

With regard to powering the CT from tow vehicle I currently have cable from the start battery to rear with an Anderson plug however do not have an isolator.

I have been investigating VSR's and are aware how they work with cut in/out voltages however in most cases it takes a while for the starter battery voltage to drop to the cut out voltage.

The concern I hold is that if the vehicle is restarted before the VSR has isolated the CT battery wont I be drawing current through this thin wire and blowing fuses all the time?

I really want to know if I can use everything as is with the simple addition of a VSR in my vehicle or do I need to re wire the trailer?


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Reply By: Member - Rosco from way back - Wednesday, Mar 19, 2014 at 15:50

Wednesday, Mar 19, 2014 at 15:50
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.
AnswerID: 528678

Follow Up By: Peter T9 - Wednesday, Mar 19, 2014 at 16:31

Wednesday, Mar 19, 2014 at 16:31
Sorry Rosco I shouldn't use acronyms, VSR is a voltage sensitive relay.

FollowupID: 811288

Reply By: Ross M - Wednesday, Mar 19, 2014 at 16:39

Wednesday, Mar 19, 2014 at 16:39
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.
AnswerID: 528681

Follow Up By: Peter T9 - Wednesday, Mar 19, 2014 at 16:54

Wednesday, Mar 19, 2014 at 16:54

The small wire is only in the trailer. I have 10mm sq and a circuit breaker in the car to Anderson plug just not an isolator.

I have thought of a dc to dc charger (have actually got one in back of car to charge agm battery for another fridge) however not sure if it will work because of the way the fridge is wired in ie if running from CT battery only the charger would be between battery and fridge.
FollowupID: 811290

Reply By: Member - Rob D (NSW) - Wednesday, Mar 19, 2014 at 18:32

Wednesday, Mar 19, 2014 at 18:32
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.

If you relax at a faster pace you can get more relaxation in for a given time.
Regards Rob

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AnswerID: 528689

Follow Up By: Peter T9 - Wednesday, Mar 19, 2014 at 19:11

Wednesday, Mar 19, 2014 at 19:11
Ok thanks Rob

As I don't know the rating of the breaker already fitted in the car if I fitted a new 30amp one at that end and stick with the 30 amp blade fuse at CT end would that suffice or should I go lower?

FollowupID: 811300

Follow Up By: Member - Rob D (NSW) - Wednesday, Mar 19, 2014 at 20:24

Wednesday, Mar 19, 2014 at 20:24
If your cable is rated at 30A then a 30A current limiting device at both ends is all that is needed.
If you relax at a faster pace you can get more relaxation in for a given time.
Regards Rob

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FollowupID: 811306

Reply By: The Bantam - Wednesday, Mar 19, 2014 at 21:06

Wednesday, Mar 19, 2014 at 21:06
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 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.

AnswerID: 528700

Follow Up By: Peter T9 - Friday, Mar 21, 2014 at 08:23

Friday, Mar 21, 2014 at 08:23
Thanks Bantam for your detailed reply. Will look into a heavier charge lead for the trailer.

Would it be an issue if the trailer cable were the next size up than what is on the car?

FollowupID: 811386

Follow Up By: The Bantam - Friday, Mar 21, 2014 at 22:15

Friday, Mar 21, 2014 at 22:15
If anything an advantage rather than a problem.

FollowupID: 811442

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