Earth Wire or Chassis

The old idea of "possible voltage drops caused by panels not quite contacting each other due to vibration" has not applied for 50 or so years. Since vehicles were constructed by spot welding the chassis bits and body panels together there has not been problems with poor body or chassis earthing. Any modern vehicle with body earthing either has connector problems (bad earthing connectors) or the body is so corroded that it is unroadworthy.

PeterD

Hi Waynepd

The body panels and chassis on most (not all) vehicles are made of steel while electrical wiring and bus bars for automotive use is generally copper. I think the relative conductivity of the two materials has something to do with it

Cheers Pop

Hi Sand Man,

" "10 mm square" may be the total diameter of both the conductor (wire) and the insulation together and may not be of suitable size to eliminate voltage drop." I think you are thinking of the strange habit in automotive wiring to refer to overall wire diameters such as 4mm, 6mm etc. I've never heard a notation like 10 sqmm or 10mm square ever refering to anything other than the cross-sectional area of the conductor.

Ian

Yep,

My intention is 10 mm square conductor.

Rosscoe

Hi again,

"and with the leads extend with light wire,"

should read

"and with the MULTIMETER leads extendED with light wire"

As I stated in my original reply, the BEST way to run and connect a circuit used for fridges and other heavier duty use, is to run both wires back to the battery, in my opinion.

The habit of connecting the negative circuit wire to the body (or chassis) of a vehicle appears to be a solution favored by installers, for a little extra expediency and cost, as you only need to run a single wire to the back of the vehicle.

When one looks at the extra cost of two wires vs one, it hardly matters.

Installing a twin cable is no more time consuming than installing one, unless you are really tight for room, in which case you may need to revert to the single wire solution.

With the dual wire method, there is absolutely no chance of a high resistance joint or connection causing problems down the track.

Bill.

So if I had a a piece of steel 5m long and 25mm thick and I connected one end to the battery and the other end to what I was powering, would it work as well as connecting a battery cable that was 12mm thick?

If not why.

As in a chassis on a 4x4, that was one piece of steel, it would work as good as or better than wire that was 12mm thick.......IF the connection on both ends was onto clean bare steel and was coated with something to keep it a good clean connection..

But do people who use the chassis as an earth check to make sure its one piece of steel from back to front and do they also run a cable from the chassis up to the battery ? The original earth cable is there to run other things, not to be overloaded with extra current .

All depends whether you want to do a good job or just an adequate one.

I would run an extra cable just to be sure ..

If you are not going to use the chassis earth then you should double the gauge of your +ve cable to maintain the same low voltage drop. If you are using an earth cable you are doubling the length of cable in circuit. If you can get a good conductance meter and measure the earth return you will find a 10 mm squared wire has a lot more resistance than a chassis return.

PeterD

Peter if you haven't got a suitable earth it doesn't matter what size cable you use as the supply.

You can not decrease voltage drop by going to a larger cable if you have a bad earth.

As a rule of thumb the earth should be the same or greater in carrying capacity as the supply.

We always try and use the chassis rail if we can and it is nothing to do with time or money or using more consumables.

If it done right it can be more efficient and more durable then running a separate earth wire.



Regards Richard

Mate,

I believe you are confusing what you use to earth the auxiliary battery to the vehicle (chassis rail) and what is used for the circuit to the rear of the vehicle.

Two different situations.

I still maintain a dual wire circuit between the (auxiliary) battery and the accessory (fridge) socket is still the best solution.

Bill.

Richard

I am a little perplexed by your answer to me. I think basically you are in agreement with me. That is a well engineered body earth return is better than a separate earth cable. Your suitable earth can be found within reasonable reach if you are down the back of the vehicle. If you are working under bonnet it is possibly just as short to go directly to the battery.

An unsuitable earth return can include a bad connection to the battery with a separate earth return cable. It is just as easy to bodgy up the connection there as when bonding to the chassis.

If you can get a good chassis earth then you can consider the earth return to be zero resistance. When calculating the voltage drop in the cable you only have to consider the drop in the +ve lead (and possibly the earth jumper to the chassis.) What I was getting at was that if you are going to use an earth cable back to the battery (either because you can't find a suitable chassis earth or your own prejudice) you have to use twice the diameter of copper in each leg that you would use in the +ve leg with a body return.

PeterD

You have lost me Peter, I think what you mean and what I read are two different things.....I don't understand what you are on about.

Bill when we run a power feed down the back we always use a large cable, usually 3 B&S and terminate it to a 4 way fuse holder in the back via a maxi fuse at the feed point.

The main reason we do this is because a customer will come in and say they are just going to run a fridge only.....six months down the track they come back and want to supply power to there caravan and also charge a battery up in the back and later on they want to put an inverter in.

I suppose the smart business thing would be to do what the customer want and charge them $200.00 and then when they come back in six months and want more things wired in to tell them what they paid six months before is not going to work and we have to do it again and it will cost them $300.00 this time......so instead of them paying $300.00 only to do it properly now it's cost them $500.00

It cost about $100.00 more at the original installation time to make the circuit handle 150amps instead of 20 to 30 amps and give the customer the option to run higher current devices.

As we are all aware 4x4ers love adding more and more thing to there vehicles.

Regards Richard

You've totally changed the question now.
May as well start a new thread.

Hi Phil,

Not really. The question still stands and the answers, although a little divided, have firmed things up in my mind.
My last reply was in response to Serg (Vic) who made reference to the intended application at the back of the vehicle.

However, you may have a point, because the Ranox unit, especial the latest version, is very new on the market.

Usual disclaimer. I have no connection whatsoever with Ranox.

This product, if it does what the manufacture says it will do, is just what I have been looking for my particular car caravan set up.

Rosscoe

Rosscoe,
Now you have REDUCED the size of the power cables from 10mm² to only ... 6mm²

The Anderson plug is "designed" to take 13mm² cable, so my question is why would you not take full advantage of the benefits thicker power cables do produce.

Mainey . . .

Hi Ircon, to your first question, no professional installer would use the chassis as the earth return as there is no way of knowing in advance if this is going to be a suitable earth or if there is going to be some form of resistance and the more modern the vehicle the greater the chances become of having a poor earth return via the chassis.

By running both a positive and a negative cable you then have a know resistance and can calculate the correct size cable to be used based on the expected current load and the distance of the total cable run.

In reality you don’t need to do any math at all, just speak to someone who does dual battery installations and they can usually give you an idea of the size of cable you need.

In your case, as you want to charge two 100 A/H batteries, the thicker the better and I would not fit anything less than twin 16mm2 and this is the largest cable that will fit standard 50 amp Anderson plugs so you won’t need to trim the copper down to fit the terminals.

Next, why on earth are you going to fit a 25 amp inverter to charge two 100 A/H batteries. Even 100 A/H flooded wet cell batteries ( which are the slowest to charge ) in a low state of charge can pull 30 amps each so by using a 25 amp inverter you are actually going to take nearly twice as long to recharge the batteries if they are low than if you just used decent sized cable in the first place.

If you had a single 70 to 100 amp ( max ) battery then that device would be of some advantage but to charge two 100 A/H batteries at the same time you really need something like a Sterling 50 amp battery to battery charger or you are just wasting your time and money.

Mainey,
I am still thinking of using at least 10 sq mm cable to the Anderson plug at the tow bar. This is for charging the batteries in the van.
6 mm is only for the Engel fridge which lives in the car and to the OEM accessory outlet so that I can use it on the odd occasion for a small inverter or 12V fluoro etc. 6 mm because that comes standard with the Engel receptacle and is suitable for reasonable size connectors for the accessory switch.

Drivesafe
I am not intending to use an inverter as such. The Ranox unit is a voltage booster/smart 3 stage battery charger. The manufactures claim features such as motor running detection, with outputs to drive an optional fridge isolation relay, optional load change over relay relay and battery load relay for low voltage cut-off to protect the batteries from excessive discharge. These features are in the soon to be released new version.

http://www.ranox.com.au/

Rosscoe

Rosscoe
Your post asks ""Should I run a 10 mm square "earth" cable back to the battery negative or just run a short wire to a suitable position on the chassis""

If you read your thread in full you will see it clearly gives you the information you have asked for.

In short, using a large or thick brass cable is more *efficient* than using an earth connected to the chassis, for the many and various reasons stated by the relevant posters, who I believe have the technical knowledge to know the relevant information they are posting is correct.
(The Inverter should connect direct to the Aux battery, on it's own circuit, and not with skinny 6mm cable)

Mainey . . .

Thanks Mainey,

Yes the thread does, I think, give me the answer to my original question.
I am considering doing something different to what I understand is the norm and perhaps it should be a new topic as one of the has suggested.
Because the Ranox can detect motor running but must have no other loads on the input to do this and because I only want the van batteries connected to the car when the alternator is operating, I am looking at using heavy gauge (10 sq mm or greater) straight from the CRANKING battery via a 40 A fuse and Anderson plug, which in turn connects to the Ranox.

Then I only use the Engel and rear accessory receptacle (with low voltage protection) off the auxiliary battery. The auxiliary is separated from the cranking battery in the normal so it can keep the Engel running when the car is stopped..

What do you think of this idea

Rosscoe

Hi Ircon, this is still an inverter and is way too small for what you can achieve by simply running decent sized cable in the first place.

There are a number of these devices on the market and in the right situation, will be an advantage but with the size of the total battery A/H you may need to replace, most of these units are just too small.

Some over simplified math may explain why. If your two batteries are flooded wet cell batteries and are down to 11.5 volts ( around 20% SoC ) when you start your days driving.

I referred to flooded wet cell batteries as these are the slowest to charge but if you have AGMs then the thicker cable set up will allow you to charge these even quicker.

With your batteries at 11.5, each battery can absorb a charging current of around 30 amps at a charging voltage of 14.2 volts.

In reality, with two batteries trying to pull 60 amps, this sort of current over that length of cable, you would more likely be only getting around 40 to 45 amps to the two batteries because of the voltage drop caused by the high currents.

This is still nearly double what a 25 what inverter will do.

So after 3 hours of driving using nothing more than thick cables, you will be able to replace the bulk of your battery capacity and have both batteries up to about 75 to 80% while using a 25 amp inverter, even with the higher charging voltage, you will need to drive for at least another 2 hours at achieve the same level of charge.

The final 20% of charge will be quicker with the inverter but the cable set up already has a two to three hour advantage.

Just doesn't make sense when you have a total battery capacity greater than 100A/H to use a 25 amp inverter over just using correctly sized cable.

My company researched this type of device many years ago and came to the conclusion that something this small had only a small market potential because of the small current limitation.

There are already much larger devices on the market and as posted, one of the best is the 50 amp Sterling Battery to Battery and no I do not sell them just know of there ability to do a good job, particularly in the motor home industry.

Cheers.

Drivesafe,

Does the vehicle alternator output 14.2 volts or there abouts all the time or is the output varied by the alternator/engine compartment temperature.

I am going to run as heavy cable as possible, regardless.

Do you know if the Sterling battery to battery out put is effectively a smart charger or just a voltage booster similar to the Arrid unit.

Rosscoe

Hi Rosscoe, I’ve posted a link below to the sterling site.

Have a good look around there as there is some excellent info on battery to battery charging plus there is a fair bit of info about their gear.

You will also find a link on the sterling site for the Australian distributor.

Cheers and hope it helps you.

Sterling Power Products

Rosscoe, BTW, I posted 14.2 v as this is the average constant voltage most alternators put out but some will be higher and some will be lower.

drivesafe,

Thanks for the info.

Just when you think you are winning the rat race the rats get faster!

Ranox $350.00 with nice features but max 25 A

Sterling $695.00 not so nice features but 50 A

The van doesn't arrive until next month so I have some time to ponder even more.........

Rosscoe

Hi again Rosscoe, I was unaware of the price for the Sterling but I actually thought it would be a bit higher than that, still it’s a lot to pay just to top up your batteries.

The other option, as posted, which would be far cheaper than getting either inverter, is to fit the thicker cable and you will just about achieve the same results.

Anyway keeps us informed, it will be interesting to see what you decide on.

Cheers

Drivesafe

I have had some friends check the output of their alternators. Sure they output well over 14 V when they first start their engines. However, after they have run down the road for 20-30 minutes the temperature compensation in their alternators regulators has reduced the charge voltage to less than 14 V, generally to around 13.8 V.

With your suggested heavy cable method you will certainly get more charge current into the battery for a few minutes but after that the RanOx will give a far better charge with its 25 A output.

PeterD

Hi Peter, over the years, I have checked the voltage of probably over a thousand vehicles while working dual battery set ups and the average voltage is around 14.2v.

With newer vehicles it can vary from 13.2 to 15v but the average is still between 14 and 14.2 and even at 13.8v the potential current draw of two low charged 100 A/H is going to be way, WAY over over 25 amps and not just for a few minutes but for a few hours and as posted, this is based on the batteries being flooded wet cell batteries.

If the batteries were two 100 A/H AGM batteries then the combined current draw would in most cases will exceed the maximum current available from the vehicle, not just far exceed a 25 amp inverter.

I have carried out extensive research into battery charging over the years while developing new equipment of different forms and base my finding on how batteries are used in day o day situations and while a 25 amp inverter charger would be an advantage for flooded wet cell batteries up to 70 A/H, by the time you get to a 100 A/H flooded wet cell batteries there is a break even point and with AGM batteries, depending on the brand, the break even point, where just having the battery charged straight of the alternator via decent cable will be the same time as using a 25 amp inverter, is below 80 A/H.

Remember I am talking about replacing the bulk of the charge in fair low charged, large capacity batteries so that they can be used as soon as possible, as is the need when people are travelling on trips, not just topping of fairly well charged batteries.

If you need one of these device just to top off batteries, fine they will do the job but why then do you need such large capacity batteries in the first place.

Cheers

Drivesafe,
One issue that you haven't mentioned is the maximum current for charging AGM batteries. For most 100Ah AGMs, a limit of 30amps is in the specs.

I've recently bought a single 100Ah AGM, which is installed in my rear canopy. When wiring it up, I decided against going with a hugely thick cable, because I didn't want it to potentially get say 60amps in the first hour. I limited the cable to 8Ga, with fusible links at each end, which will handle the 60 amps, so theres no safety issue, but used 5 metres of it, so in theory I'll limit the high current. I haven't had the opportunity to measure current when the battery is discharged yet. I also could have installed a 25amp inverter charger, but am trying to "KISS" and limit the number of things that can go wrong on a trip!

Hi Phil, thats a very good point and one that most people, including many so called experts, overlook.

The problem is that the maximum tolerable charging current for AGMs varies greatly from one brand of AGM to another and varies from 20% of total capacity for something like a Fullriver ( 20 amp for a 100 A/H Fullriver battery ) to the likes of an Optima which can take full inrush current.

Another point people miss is that AGMs, although they can charge quicker, are as effected by the charging voltage as any other type of battery.

So where there is a long cable run, you can still use thick cable because the higher the current requirement of the battery(s), the greater the voltage drop, so the whole set up becomes self regulating and as such self protecting.

I am also an avid follower of the “KISS” principal and nothing could be safer or simpler than just running decent size cable in the first place.

Cheers and good point raised.

In a lot of new vehicles the manufacture does not like people putting in extra earth wires, if you have to connect an earth you have to connect it to one of there specified earth point.

Most new vehicles are built very well today and earthing is not a problem.

Most of the problems we see are from people not using the right cable, terminals / crimp lugs or they crimp terminal with a pair of pliers and battery lugs in a vise causing the clamping area to distort not making proper or full contact.



Regards Richard

First line reply. I did

Cheers

Then of course the electrolysis that can take place between the non-alike metals. Best to have decent copper all the way. Have had all sort of things not start because the earthing through the body earthing inadequacy.

The battery-to-body connection in my 8 months-old vehicle has copper cable connected to brass lugs connected to steel bolts and bodywork - as have my vehicles for the last thirty years - I've NEVER had a probem with connections to the bodywork.

"While you guys are all arguing about cable size, nobody apparently bothered to look up the conductivity of steel versus copper."

I know the resistivity of steel is ten times that of copper. That would be relevant if I were considering putting in the same size copper or steel wire.

The reality is that there is already a huge sheet of steel connecting the front of the car to the back of the car.

The reality is that the resistance of this existing conductor is equivalent to running a 4 Ga (21 sq mm wire).

The manufacturer of the vehicle has found it adequate for all earth returns. I've found it adequate for all earth returns include an HF radio that _Affordable_Storage_Drawers.aspx 20 amps and very sensitive to voltage drop.