Battery Relocation

[jackherer 543]

It's not necessary and adds a lot of weight. All four cars on my drive currently (BMW, Alfa and 2x 205s) have the batteries in the back and use the body shell to earth with no problems.

[unariciflocos 49 1 Cars]

You seem very sure of yourself. I've learnt to leave some room for doubt over the years.

 

Mind you all the signals to the ECU will be affected by noise, and filtering, both hardware and software, will be far better on more modern cars. Having the battery electrically closer (path of lowest resistance) to the alternator will help. Anyway, my recommendation stays the same, but I may be wrong after only studiying industrial automation for 5 years and 10 years in the business.

But you are right, it will work with using the body as an earth. However, considering not everyone is a a certified electrician, without the proper terminations, crimping tools or the start washers to pierce the paint, it's just a matter of time until topics like "engine cuts out at <insert driving condition here>" pop up.

Edited November 17, 2014 by unariciflocos

[jackherer 543]

If anyone seems sure of themselves it's you! You're arguing that all OEMS are doing it wrongly as well as pretty much anyone that has relocated a battery in a (e.g.) race car.

[unariciflocos 49 1 Cars]

I don't want to drag this discussion out forever. Everyone has eyes to read and minds of their own to make up. I've presented my arguments, you have yours, now each to his own.

[Tom Fenton 1,542]

My honest opinion is that in practise you would not be able to tell the difference between a battery earthed in the boot and one earthed at the front of the car via a long cable.

[dobboy 231 2 Cars]

I'm an electrical engineer, and if i was doing it i'd be running a negative core.

 

All sorts of strange phenonmena can happen with (high) current flow/microscopic air gaps etc.

 

Without looking up the net, i wouldn't be surprised if a bare piece of metal (ferrous/containing iron) would be more prone to rust if it was negatively charged.

[jackherer 543]

As I understand it older cars with positive earth were slightly prone to corrosion but negative earth actually prevents it.

There is a good analysis here - http://www.w8ji.com/negative_lead_to_battery.htm - of why the bodyshell is better than a long negative cable.

The break-over point where the chassis becomes superior to an number 1 AWG copper cable is usually about five to eight feet.

Admittedly that is quite close to the length needed in a 205 but at best equal performance with a cost and weight penalty doesn't make sense.

[td_w 2]

Thanks guys, I haven't done it yet,, still trying to find reasonably priced cable

[dobboy 231 2 Cars]

Had a squint at the link Jack.

 

Imo, after reading your link i'm more inclined to use an extended negative wire than the chasis.

 

In extending the wire your keeping the manufactures earthing design intent relevant. You just have to make sure the wires are sized correctly.

 

Fyi, 2x 3m lengths of 35mm tri rated weighs around 2.2kg

[jackherer 543]

After reading it you disagree with it? Why?

 

The manufacturers intent is to keep the ground cable as short as possible for the reasons outlined in that link.

[dobboy 231 2 Cars]

I thought the manufacturers were strategic in there positions of earthing of the various systems etc.

 

They are saying the alternator and battery leads need to be as short as possible because these draw/produce high current, therefore highest voltage drop on cabling.

 

You want to keep the voltage high as current is proportional to it, therefore power is proportional to it.

 

Volt drop is calculated from cable data from the mV/amp/meter, therefore if you select/calculate the optimal conductor size and cable type from cable data it doesnt matter a hoot the length of cable.

 

Basically Ohms Law on a series circuit.

 

You could have the battery sitting in the back of a trailer if you wanted, but it might mean you have a 240mm cable going to your starter motor, obviously that example is not practical, but you get the drift.

 

But using cables keeps the eathing arrangement the same as pug wanted it.

 

Not saying it wouldnt work using the chassis though!

[td_w 2]

Well, this leads me on to a question. How do I easily work out the thickness of cable I require, like I know the distance and capacity of my battery so should be able to work out cable required, but not sure what the maths is

[welshpug 1,657]

go to a breakers yard and liberate a BMW, the cables are sized for generally quite high cranking loads and large batteries, as well as lots of electrical load from all the toys.

 

will do the trick just fine in a little pug with everything but an engine taken out of it (not that there's much to start with)

[unariciflocos 49 1 Cars]

Well, this leads me on to a question. How do I easily work out the thickness of cable I require, like I know the distance and capacity of my battery so should be able to work out cable required, but not sure what the maths is

 

Well two electrical engineers recommended 35mmsq, but if you don't want to go into the formulas yourself there are plenty of calculators online. The size is calculated with the aim of having a voltage drop within tolerance in the conditions the cable will be used in. By running a negative to the orginal earthing point of the car, which is under the bonnet, you'll be replicating exactly the OEM design.

 

Edited November 18, 2014 by unariciflocos

[welshpug 1,657]

yeah but are you automotive electrical engineers?...

[unariciflocos 49 1 Cars]

I swear this is my last intervention on the topic. I've just expressed an opinion, nothing more, come to think of it, I just shared how I have done it and how it works on my car, not even a recommendation. I don't claim to hold the absolute truth as others do, I will hapily accept to be proven wrong with solid arguments, calculations and/or measurements. Maybe someday I'll get bored enough and take the oscilloscope to the garage and find out if I was wrong or not.

 

No, I am not an automotive electrical engineer, don't even know if there is such a thing. Are you one perhaps? Electricity is electricity, and please don't get me wrong, I trully respect your vast knowledge of everything Peugeot related, your information has probably helped at one point or another more than half the people on this forum, but when i step out of my field, I ask arund with people that are experts in those fields.

It's not as though both dobby and myself haven't addmited repeatedly that it would give perfectly acceptable results earthing through the body.

Edited November 18, 2014 by unariciflocos

[dobboy 231 2 Cars]

td, as you probably know i'm no mechanic, and i don't deal with DC/batteries at all at work. If you have access to a tong tester you could measure the peak starting current on what you have just now. To put it perspective the pug cabling from battery to starter to Alt looks to be no bigger than 16mm.

 

But working backwards from the 35mm i thought would be required, here's a few assumptions/calcs, and i'm working on the side of caution so it works for you.

 

So,

 

I imagine a starter motor would behave similar to a 3ph (ac) motor getting started "direct on line". These motors require initial heavy current for the first (up to) 0.5 second or so, and on an AC motor their starting current is around 6 times their normal running current.

 

I've got a Bosch S5 battery in my car which i think is the recommended one for GTI6 and this has a tabulated Cold Cranking current of 830A, this gives me an idea of the Pugs peak current demand of the initial turning of a starter motor.

 

So, 830CCA / 6 could deliver 138.5Amps fine if it behaved like the AC motor i assumed. (it's a 95Ah battery, so it could deliver the 138.5Amps for 40mins or so in theory at ideal conditions)

 

If you look at the cable data for a 25mm and 35mm, the current carrying capacity is 136A and 167A respectively.

http://www.batt.co.uk/upload/files/tri-ratedcablepanelwiring105c6001000vcurrentratingstablebec100_1219689537.pdf

 

As i'm working on the side of caution i'll pick the 35mm with the 167A current carrying capacity, as it's above the 138.5Amps.

 

The resistance of a 35mm from the table is 0.554Ohms per 1000m

 

So, if you were to use 2x3m lengths your resistance for the + and - will be (6m x 0.554)/1000 = 0.0033 Ohms

 

So the voltage drop on the cable can be calculated 138.5A x 0.0033 = 0.46V

 

Which i assume would be ok, (and you will also have a bit of Volt drop on pugs wiring that you joint to.)

There is a bit more to sizing cable including method of installation, cable grouping factors etc, but i'm sure 35mm will be adequate.

The link Jack put up shows a requirement for a 1AWG, which is bigger than 35mm, but i think 35mm will be ok as i know someone who has done his in 25mm and it seems to work fine, but i think he's got a big FO battery.

 

If manufacturers were running 1AWG cables in their cars the gypos would nick them for the scrap!

[td_w 2]

Thanks guys, I will report back, asap! Really appreciated

[dobboy 231 2 Cars]

td, found this and thought it might interest you.... (the battery location)

 

http://psa-sportive.forumactif.com/t33-aide-205-gti-gutmann

[td_w 2]

Interesting... doesn't show where the cables went though

[dobboy 231 2 Cars]

yeah, if you've no spare wheel underneath maybe you could put it on the other side, run the cables in the back quarter then out and along the inside at the sill, to keep the cables as short as possible.

 

It's a great looking car from the front?

[scbond 15 1 Cars]

Not looking to make anything worse for argument's sake but I see no reason why a short negative from the boot to the chassis for its earth is an issue?!

 

td, as for cable, I'd say 37/0.90 is perfectly fine. This company is very well-priced and often the cheapest I can find. Been down to them and they're very helpful and know their stuff...http://www.autosparks.co.uk/index.php?cPath=85_116

Edited November 22, 2014 by scbond

[petert 586]

I've been following this on/off for a while. If it helps any, I use 38x7x7x0.12 (4 gauge) cable on my car, which I bought from the local electrical parts store (Jaycar). On my previous car I bought similar from a welding supply place. I've connected the earth via the body and also direct. It makes no difference. I like the weight saving of conncting to the body. Also my 535 has been running perfectly for 17 years. I'd definitely liberate a dead beamer if time isn't an issue.

 

And buy a lithium battery.

Edited November 22, 2014 by petert

[dobboy 231 2 Cars]

I've been following this on/off for a while. If it helps any, I use 38x7x7x0.12 (4 gauge) cable on my car, which I bought from the local electrical parts store (Jaycar). On my previous car I bought similar from a welding supply place. I've connected the earth via the body and also direct. It makes no difference. I like the weight saving of conncting to the body. Also my 535 has been running perfectly for 17 years. I'd definitely liberate a dead beamer if time isn't an issue.

 

And buy a lithium battery.

 

Hi petert, you may get away with "38x7x7x0.12" earthing via the body (short length), but i'm guessing wouldn't on full lengths due to volt drop.

 

I think you'd probably see the insulation smoking/getting hot if you couldn't get the car started and kept turning it over, out of interest have you ever measured the starters current?

[td_w 2]

Thanks for the cable link @scbond

 

So Dobboy... do you mind having a look and letting me know if this is sufficient - current max is 170 amps so looks about ok based on your post above. http://www.autosparks.co.uk/product_info.php?products_id=1784

 

I am guessing that its 37 strands of 0.90 mm - which is 33mm not 35, but that should be ok based on the maths above, right?