vehicle to charge 12volt camper system

Thanks for the advice. A question about the solar panel is it just a cheaper type you buy at the auto shops, because that seems top be what I have in mind not relying on the panel to do all the recharging just to check it topped up. Do you have any cut off switches between the panel, the bateery and the vehicle power cables?

Roy,
The solar pannel that I use is a 5 watt unit from Jaycar. I run it through a voltage doubler and a regulator. That way even on a cloudy day it puts something into the battery. The regulator has some smart electronics built in to protect it from reverse polarity and backfeeding. If you don't use a regulator you could flatten your battery at night because the solar pannel acts like a load on the battery and in full sunlight you will over charge the battery, like putting 20 volts on a charged battery will cook it.
This system is good for around 200 milliamps which is good enough to keep a battery trickling over nicely. I keep the pannel on always, even when the camper is parked in the shed.
Just to recap on some comments from other forumites, when I say I use 4mm sq. cable, I'm talking about electrical cable, not automotive cable. The difference is electrical cable is measured in the area of the conductor only, whereas automotive cable is measured including the insulation, which is deceptive. 4mm V150 stranded cable is good for 35 amps at 15 volts. I have 10 amp fuses at each side of the plug, and have never blown one. Trailer plugs will handle 10 amps easily without getting warm and the ONLY reason to use large cable from your auxillary battery to your trailer is to reduce voltage drop. If you can get the same voltage at the trailer battery, that you have at the cars battery, then your system is functioning efficently. Of course this scenario is only for charging the trailer battery and would be different if you were going to run, say a fridge in the trailer as you were driving.

Chaz

Hi Chaz, you posted the cable is 4mm, I take that to mean 4mm2 and the 35 amp rating is at any voltage.

The voltage rating of cable relates to the insulation and 35 amps is the amount of current the cable can handle over 1 metre. The amount of current the cable will handle over the distance from the front of the tow vehicle to the trailer would be no more than about 10 amps if your lucky and this is also why your trailer plug does not get hot and also why you don’t blow fuses. The cable itself is limiting the amount of current that can ever go through to the trailer.

You can charge your trailer battery using thin wire like speaker wire, the drawback with using thin wire is that it will take a much longer time to charge the rear battery and might not ever fully charge the battery.

Cheers

Drivesafe,
I have to agree with most of what you have said, but consider this. The resistance of my 5 meter long 4mm2 cable is around half an ohm, and even if the plug gives me another .5 ohms of resistance, then my cable is still good for 14amps at 14 volts. This is more than enough to keep a battery topped up considering that wer'e only running a couple of CFL's for a few hours a night. In reality though, it never needs that much, because the battery never dropps down below 11 volts and the most I've seen it charge is around 4 amps. This is the beauty of charging from a car with a regulator, because the battery will only take what it needs, unlike a battery charger that tries to pump out 10 amps by running at 16 volts.
All I'm trying to suggest here is that there is no need for overkill, and if you need 60 or 80 amp supplies to go to your trailer, then heaven help you if you get a short circuit. The reason car manufacturers use the smallest cable size possible isn't just to cut costs. Personally I hate using large cables, because I've seen too many valiants burn to the ground because of their ampmeters and associated cables.
Interesting topic though.
Chaz

Hi again Chaz, unfortunately you have thing a bit out of plum.

If you had a battery down to 11 volts and you were only getting a charge current draw of 4 amps at the rear battery then either the rear battery is VERY small or the same reason for not blowing fuses is controlling the amount of current getting to your battery in the first place and that is that the cable is to small.

A normal wet cell cranking or deep cycle battery being charged from 11 volts will pull up to 15 amps and an AGM can pull even more so if you are only getting a 4 amp draw then you have a problem.

This problem may not be of any disadvantage if you are spending a long time driving between uses.

If you are using 4mm2 cable then that is rated at about 40 amps, but this is only over 1 metre.

You go on about using 60 or 80 amp cable as over kill. Quite the opposite is the case. To get any where near 15 to 20 amps at the trailer, you would need to use 100 amp cable.

One more point, car makers use thin wire to save money. It makes no difference how thick or thin the cable is as to whether it will burn. The type and quality of the protective device on the cable is the ruling factor where burning wire is concerned.

Cheers

Drivesafe,
Well I guess being an electrician for the last 30 years has taught me nothing.
We are getting off the point here, I’m only running two 11 watt fluro’s, not a winch.
When I said I was getting 4 amps into the battery, it was at 14 volts, so the battery had already reached it’s float voltage. I have never checked it at 11 volts, but I do know that it only takes around 30 minutes of driving to come up.
The other point I should make is that my trailer battery is in parallel with the auxiliary battery in the vehicle, so they are sharing the load, and the charge levels out when the vehicle is not running.
The point I’m trying to make is that it works ok if your only running a few lights, and I agree that it wouldn’t be up to the task of heavier loads.
My trailer battery is a CAT cranking battery. It’s huge, and although not ideal, I get them from work for free, so I’m not too worried how long it will last.

Chaz

Drivesafe,
I have to ask you one more question. Where do you get the 1 meter thing from?
As far as I am aware, length has nothing to do with current. If you have a 35 amp cable, it doesn’t matter how long it is, it can still carry 35 amps.
The only thing affected by the length of a wire is its resistance which will increase the voltage drop across the cable. You only drop its current rating so it doesn’t get hot over that distance.
My point is that if I can get a full 14 volts at my trailer, then the voltage dropped across the 5 meter length of cable is negligible. That’s why it works.

Chaz

A number of the auto cable makers list their cable this way and you will find comparable charts with most of the AUTO cable manufactures.

A point that I should clear up, those cable makers that list the current over a 1 metre length of cable, do so as an indication not as a mandatory calculation, it is simply a reference point but a handy one.

As an electrician, working with 240 vac, you can have a volt or two drop and it will have no effect and as you said, the 4mm2 ( and thats obviously AC cable ) will carry about 35 amps. What it won’t do is charge your battery in 30 minutes. To charge a battery from 11 volts to anywhere near fully charged in 30 minutes you would need an electric welder putting out high current at about 20 volts DC and quite obviously the battery just could not handle that sort of voltage.

With your 4mm2, which by the way is a tad smaller than 6mm automotive cable ( around 4.6mm2 ) you could charge your battery but it would take at least 5 hours and probably closer to about 8 hours to fully charge it.

If you have a driving light globe handy, try connecting it in place of your battery and start your motor. A 130 watt globe will pull around 10 amps and a 100 watt globe will pull around 7.5 amps.

Now with the motor running and the globe attached, measure the voltage at the front battery and then measure it at the globe near the rear battery.

If you have 14 volts at the front battery, you will be pushing up hill to get 13 volts at the globe, and thats down your 35 amp cable.

Also by the way, I have had 30 years of designing and manufacturing automotive and domestic electronics and specifically, nearly 20 years of designing, manufacturing and installing dual battery control electronics and systems.

Each unto his own. Cheers.

Thanks for you help It looks like I am on the right track!

Thanks for your advise I have a solinoid between the two vehicle batteries. What size fuses are you taking?

Hi royboy for both convenience and safety, you would by better off fitting a circuit breaker at either end of the cable running between the two batteries and try to place them as close to each battery’s positive terminals as is possible.

This will not only protect the both batteries from overload but will give you protection incase of an accident.

Cheers

Thanks for the advice. I love the photos where is the river crossing?

Glad you like the photos. I can't work out how to title them, so I'm not surprised you need to ask where things are!! The river crossing is at the Pentecost River on the Gibb River Road, just before you get to El Questro. HWMBO wanted me to take a photo of the rig crossing the river, but there was a large sign about the salt water crocs and I'm not silly!!!
Cheers
Anni