Earthing: back to battery or to chassis?

Too true.
Even with the heavy cable, you'll have to drive a long time to charge the battery, AND if you try to run a fridge off the same circuit- forget it- You'll effectively get little or no charge into your battery.

If you wish to run a fridge AND charge a battery, you need a totally separate circuit for each, (all the way from the alternator/battery) as otherwise there is just too much voltage drop to have any chance of battery charging.
Basically, unless you can see about 14v at your caravan battery, then you'll do little more than trickle charge at best.
Cheers

Hi David, although I do agree with your idea of running separate cables for the battery and the fridge, both the cost and work involved would rule this out but, and this does in part confirm your idea that the fridge will, while running, will pull the voltage down but by using the thickest cable that is realistically viable, which will also work out cheaper than running tow separate runs, as all fridges cycle, while the fridge is off, the battery will be getting a slightly higher charge voltage.

BTW, I’m too lazy to sit down and workout voltage drops based on cable thickness x the length of the cable x amps and so on so I just go to the site I have posted a link to below.

Have a look and you will probably want to bookmark it.

Two points when going to this site, all cables sizes are based on AWG and distances are in FEET so for every metre you need to multiply by 3, that's not accurate but it’s close enough.

As for AWG, B&S and G are all the same as AWG.

Cheers.

VOLTAGE DROP CALCULATOR

BTW, while your in that site, have a look around, there’s lots interesting info pages including a Battery Size Selection calculator.

This may be of some interest to you if your just starting to plan your dual battery system.

Cheers and here’s the link.

BATTERY SIZERt

Yes, as you say, a compressor fridge will cycle and give some hope of charging.....
but I've often seen people try to charge AND run a 3-way (11 amps or so) and they have NO hope of battery charging doing that on the same circuit. (Apart from high current draw, 3 ways don't cycle on 12v)

I have very rarely seen a caravan battery charging set up which works effectively,
however on a mates I hooked his heavy 12v leads to his inverter in the van, isolated the batteries via a marine switch and ran his C-tek smart charger off the inverter output.
Beeeeautiful, 25 amps into the battery when he drives! Works a treat.

(He used to complain that his batteries were the same voltage after two or three hours drive as when he started!)
Cheers

This ludicrous idea of running around with live 240 AC in a tow vehicle and/or caravan or camper trailer has been discussed here and on many other sites and as there is no way to make this set-up 100%, in an accident could not only be a danger to the vehicle’s operator but would be potentially lethal to an rescuers and the fact that it is a totally unnecessary operation in the first place, why bother.

You can easily charge a battery and run a fridge, even if the fridge is drawing 11 amps continually.

I carried out numerous tests while I was developing my gear and I had no problem charging a 200 A/H wet cell deep cycle battery from 11 volts to fully charged power off my RR’s alternator running at 14.3 at the front battery.

This took a little over 5 hours to get the bulk of the charge back into the rear battery ( around 85 to 90% charged ).

The battery was not fully charged but would only need probably 3 hours more driving to achieve a 95%+ charge state.

As stated above, the test were being carried out to test some electronics and there responses to different charge regimes not to test the battery’s ability to charge but these were a spinoff the intended tests.

The initial charge current was a little over 31 amps and this was down 7.5 metres of twin 10mm2 cable through an isolator ( the reason for the tests ), two sets of 50 amp Anderson plugs, one set between the tow vehicle and trailer and the other set on the rear battery.

This set-up would not only charge any battery array in a camper trailer or caravan, to would cost a whole lot less that buying a 25 amp charger and an inverter big enough to power the battery charger.

For a lot less than a large battery charger and a larger inverter you could simply add an additional battery or two and be in a position to stop longer between charges than you could using your charger/inverter set-up and your alternator would still be able to charge all the batteries at once while you drive.

Cheers.

Phil,
many years ago I too used to think you might get 20-30 amps into van batteries at times, but alas, no.
And yes I would run BOTH the chassis connection AND separate negative all the way, all in the interest of reducing resistance.

The combination of long cable run, resistance from connectors, deep cycle battery (which is very slow to absorb charge) and insufficient voltage from your alternator to start with, you'll almost never see anything like 20-30 amps unless you have Welding Cable!

I've often tested people's setups for them with an ammeter to prove the point- they are always surprised!
Cheers and regards

That's to put on the doorknob to stop the kids coming in, isn't it?
LOL!!
Gerry