Earthing: back to battery or to chassis?

Submitted: Saturday, Jun 02, 2007 at 00:22
ThreadID: 46169 Views:14551 Replies:8 FollowUps:7
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I will be installing an Anderson plug on my towbar to charge a battery in a caravan. Is it best to run the earth cable all the way back to the battery or to a bolt on the chassis rail?
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Reply By: Wayne-o (Pilbara WA) - Saturday, Jun 02, 2007 at 00:38

Saturday, Jun 02, 2007 at 00:38
No problem earthing it it chasis, just ensure it wont be in such a place that will be exposed to mud and bleep e, if not after a while you may experience a bad earth.
Good luck
Wayne-o
AnswerID: 244110

Reply By: Richard Kovac - Saturday, Jun 02, 2007 at 00:40

Saturday, Jun 02, 2007 at 00:40
chassis rail

There's more metal in the chassis rail than a 6mm squared wire cable

As long as the chassis rail has a good sized strap to the battery (if it didn't it wouldn't start)

Regards

Richard
AnswerID: 244111

Reply By: Pomgonewalkabout - Saturday, Jun 02, 2007 at 07:21

Saturday, Jun 02, 2007 at 07:21
I was told to run it back to the battery as there will be less voltage drop. But who's to say what I was told is correct. However that is what I have done for my camper battery

cheers
AnswerID: 244120

Reply By: drivesafe - Saturday, Jun 02, 2007 at 07:32

Saturday, Jun 02, 2007 at 07:32
Hi Gone Bush, it’s far safer to run both positive and negative cables from the battery under the bonnet to the battery in your caravan.

Theoretically, you can get a good earth if you use the chassis, the problem is that there is no way to know or test to see for sure that the chassis is a good earth return to the battery before you carry out an installation. In some cases the chassis does not make a good earth and can result in poor charging of the battery and the newer the vehicle the greater the problem becomes and if you have only run a positive cable, the only remedy is to go back and run a negative cable.

It’s heaps easier and safer to run both in the first place and you then know for sure that you have a good earth reurn and as you are spending the time to run the cable, make sure you install thick enough cable in the first place as well.

The thinnest cable to use for running all the way to a caravan is 10mm2 if you want to be able to charge the battery in a reasonable sort of time and 6B&S ( 13.5mm2 ) would be an even better choice and 16mm2 is the thickest cable you can fit into standard 50 amp Anderson plug terminals.

Using anything thinner than 10mm2 will probably mean your caravan battery will never get fully charged, no matter how long you drive.

Cheers.
AnswerID: 244122

Follow Up By: David N. - Saturday, Jun 02, 2007 at 08:07

Saturday, Jun 02, 2007 at 08:07
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
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FollowupID: 505067

Follow Up By: drivesafe - Saturday, Jun 02, 2007 at 10:25

Saturday, Jun 02, 2007 at 10:25
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

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FollowupID: 505099

Follow Up By: drivesafe - Saturday, Jun 02, 2007 at 10:39

Saturday, Jun 02, 2007 at 10:39
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

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FollowupID: 505101

Follow Up By: David N. - Sunday, Jun 03, 2007 at 09:26

Sunday, Jun 03, 2007 at 09:26
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
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FollowupID: 505266

Follow Up By: drivesafe - Sunday, Jun 03, 2007 at 11:10

Sunday, Jun 03, 2007 at 11:10
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.
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FollowupID: 505285

Reply By: Bonz (Vic) - Saturday, Jun 02, 2007 at 08:10

Saturday, Jun 02, 2007 at 08:10
Hey there Gone Bush, drivesafe is on the money in my book, from a reliability perspective, as you are sure then that the connection to the Anderson Plug is continuous and also the least resistance. Also, if you earth to the chassis rails you will need to bare some steel around the bolt hole and I reckon that asking for corrosion and things to start.

I would run the 16mm2 cables for the 50amp Anderson too, bigger is better.

The argument against the chassis rails is that you dont know what form the rails connected to the crossmember connected to the thighbone connected to the battery earth, so why take the chance?
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AnswerID: 244124

Reply By: Member - Phil G (SA) - Saturday, Jun 02, 2007 at 09:33

Saturday, Jun 02, 2007 at 09:33
I'd be following Drivesafe's advice, and run it back to the cranking battery. It will be carrying a lot of current (hopefully 20-30amps at times) and a chassis connection is also likely to corrode with time. You can do both for not much effort to be sure, to be sure. If you were to do a chassis bolt, its best to run a tap through the thread, remove the paint under the bolt head and smear everything with Vaseline.
AnswerID: 244136

Follow Up By: David N. - Sunday, Jun 03, 2007 at 09:35

Sunday, Jun 03, 2007 at 09:35
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
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FollowupID: 505268

Reply By: Gone Bush (WA) - Saturday, Jun 02, 2007 at 10:15

Saturday, Jun 02, 2007 at 10:15
Thanks everyone.

Now I know i've got some Vaseline in my bedside table.......
I'm glad I ain't too scared to be lazy
- Augustus McCrae (Lonesome Dove)

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AnswerID: 244144

Follow Up By: Member - joc45 (WA) - Saturday, Jun 02, 2007 at 12:51

Saturday, Jun 02, 2007 at 12:51
That's to put on the doorknob to stop the kids coming in, isn't it?
LOL!!
Gerry
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FollowupID: 505123

Reply By: Member - Roachie (SA) - Sunday, Jun 03, 2007 at 10:38

Sunday, Jun 03, 2007 at 10:38
Or..............you could just use one of these....................... www.arrid.net.au/twin.html

Works for me.
AnswerID: 244309

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