[flash]

ok - so i'm going to move the battery to the trunk - we all know that porsche did the screwy "multiple ground" thing



i've looked at the battery grounding of the US car - we have what looks to be a 2 ga cable that goes from the bell housing to the negative terminal, and a smaller lead that goes from the negative terminal to the battery box sheet metal



i've looked at the way the brit cars are set up - they have the battery in the left rear "cubby" with the positive lead strung to the rear ang a ground strap at the rear - no negative cable to the rear



i've already figured out what cable to run, what terminal blocks to use, and all that - just one thing left



here is the question:



if i take the negative cable from the bell housing and ground to the chassis, and do the same battery grounding they did on the brit cars, and forego runnng a negative cable all the way to the back of the car, can anyone think of any specific issues?

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[sasilverbullet]

I'm afraid to tell you yes just by electrical knowledge - theoretically, yes. But you know how Porsche's are, they just never seem to be theoretical!



I'd wait for some confirmation - you can blow the ECU if you goof up.

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[flash]

ok - yes, you can think of some issues - what are you thinking?



i'm just not particularly comfortable with the grounding potential - the galvanized body seems at odds wiht that - that being said, it is the way the brit cars are done - seems shaky to me though



i may just run it anyway to be safe, but the extra cable, cost and weight (though it's only 50 bucks and 10 lbs) are not desireable things

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[orphanowner]

Flash, when I built my 944T racecar, I relocated the battery to the area right behind where the front passenger seat was. I installed a gel cell and used the right side seat belt anchor nutsert for the ground cable. This worked great and I had no issues. For the positive, I ran a cable up to the original battery box and made a connection to the old positive battery cable with some electrical hardware I got at Home Depot. I think if you can find a solid factory nutsert in the back and use some star washers, it should be fine.



I'll look for some pictures of the installation and post them if I can find them.

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[Greimann]

That grounding scheme should work. That is how the wiring diagram shows it for the RHD cars. . You can't get a larger conductor than the unitbody.



The one thing that I would keep an eye on is charging voltage. Now that the battery is that much further away from the alternator, any voltage drop in your new cable will not be read by the voltage regulator. It senses voltage at the starter terminal.

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[968gene]

Dave, with all of the small movement and flex in the unibody (and I know Flash has solved most of this with the roll bar) is there a possibility that using the entire chassis as a conductor could introduce corrosion at any or all of the unibody joints? It sounds like it will work, but long term will it cause issues?

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[flash]

if this were not a galvanized car, i would not be worrying about it - just wondering if anybody had any real info on how that affected conductivity - i know i'll have to break the galvanizing at the contact point, but wondered if there was any other issue with ground potential across the rest of the car - on the US cars, they sure went to great lengths to ground the bell housing to the battery, and not the chassis



no worries on loss dave - this was a major concern too, since i have the power pulleys, and already have deficit spending at idle - however, i'm going to the 4200 strand 1/0 guage - should be zero loss (at least according to the manufacturer, and any charts i can find on cable requirements)

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[Greimann]

Corrosion won't be an issue unless the area is kept wet for a long period of time when voltage is flowing. If I know flash he doesn't even drive in heavy fog!



Acutally, all the cars have this situation right now as the body is the major conductor for the ground loop, but there is really no concern.



Electrolytic corrosion needs three things: an anode, a cathode and an electrolyte. If one leg of this trangle is missing, then the corrosion process can't complete. In a vehicle body, it is difficult to keep this triangle intact as the paint defends against moisture and the zinc coating is the sacrifical anode to keep the steel from being the donor.

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[flash]

which brings me to the concern - it may just mean that i need to keep a closer eye on my ground connections - 105 amps is a lot of current, presenting the opportunity for spikes (read "smoked DME")



i may be worried about nothing - i've never had any problems requiring a negative cable all the way back in any other car - it's really just the galvanizing that worries me - i've never played with that



and you're right - i don't really even drive in a heavy fog - i don't enjoy rubbing the car out that much

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[sparky]

Flash



Speaking as a qualified electrical engineer (ala 'sparky' <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" /> ) while not claiming to be an auto-electician, but owner of an Australian 968CS with battery in left rear corner, I believe you would be adding a couple of unneeded kikogramsby running two battery leads to back of the car. After all the huge amount of metal in the body offers very low electrical resistance and you will have no issues provided you create low resistance the connections (as you rightly discern)



In my car the negative lead runs for 6 inches from battery to a (factory) 13mm hex bolt into the bodywork. (Your car may even have same unused connection point - not sure if US cars have the battery tray behind LH read wheel?) I haven't looked closely at starter connection wiring but I could take a peek or even a photo if its useful to you (in next week or so I'm planning to refurb my starter motor). But I don't expect to find anything special, just another solid cable connected to bodywork.



So just make sure every connection point is solid with very low resistance. Preferably bare metal-to-metal with some corrosion-resistant conductive goop smeared on it (your Auto electrician should have some). I wouldn't fret over the galvanising, its only Zinc coating after all.

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[flash]

thanks - i had hoped for that answer - the extra 10lbs of cable was not something i was looking forward to - neither was routing a second large cable like that back there



woohoo! looks like i will finally get that thing moved



no, we don't have that plate there - besides, i am planning on having it on the opposite side of yours - US cabs have a 42lb boat anchor in the right rear - i have removed it with much improved results, but now that i have removed an additional 60 lbs from the rear, i am looking to put the battery where the boat anchor used to be



that should make for a very nice balance correction



i'll post pics when i get it done

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[968gene]

auto electric advice from a guy named "sparky" from down under...ya gotta love 968 forums!



Sparky, Do you regularly track your car? What wheels are you running? Thanks,

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[flash]

hmmm - do electrons spin the other way there? kidding

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[sparky]

968gene

yep, she's a track car alright. Wheels are 2003 Boxster S 17x7 17x8.5 (offsets 55F 48R) With additional 17mm spacer on front they fit perfectly, and they're a few kgs (oops, I mean lbs) lighter than stock CupII wheels. (and IMHO they really suit the car). Here we are in our big debut in Classic Adelaide this year ....



PS: flash, I'm sure electons do spin the 'right' way down here <img src="/forum/images/smilies/968/dry.gif" class="smilie" alt="" /> but I'l go check on a few !

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[Cloud9...68]

Flash,



Just a thought, but as an alternative to the large amount of work involved in moving the battery to the trunk, have you thought about replacing your battery with an Odyssey dry cell, and leaving it where it is? Living in SoCal, where it never gets cold, you could probably get away with their PC925 (925 cranking amps, 26 lbs), and maybe even with the PC680 (680 cranking amps, 15.4 lbs), provided your starter is healthy, and you don't have any current leaks. I know the latter tends to be a problem with these cars, but, knock on wood, I can keep my car parked for over two weeks, and it starts right up, so it isn't universal. It would probably be a good idea to keep a solar plug-in charger on hand, though, especially given your pulleys.



A lighter battery won't correct your car's balance as much as moving the existing battery to the trunk, but it would be a lot easier, and knock a significant amount of weight out of your car to boot. Just my 2 cents.

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[flash]

actually, the primary reason is for balance, so moving it to the rear is what i am really looking to do - i've yanked a lot of weight back there, and need to rebalance the car, and there is little more i can remove from the front



also, because i have the power pulleys, i need the larger capacity battery - i am in deficit mode at idle now - any prolonged sitting in traffic, especially with any accessories on, and it gets a little nerve wracking



i am contemplating going to the odessey anyway, but likely the big one

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[sparky]

[quote name='Cloud9...68' date='Jan 1 2006, 02:35 AM']Flash,



Just a thought, but as an alternative .. replacing your battery with an Odyssey dry cell[right][post="13961"]<{POST_SNAPBACK}>[/post][/right][/quote]



Or .... you could do both!



In fact I have the Odyssey PC680 in back corner of my car. Weighs just 15lbs ( saving around 22lbs over stock battery). It starts car just as well as stock but it does have less storage capacity so essential to have zero overnight drain, and give it reasonable drive / charge time vs usage with motor off. For example mine struggled one day at a wet motorkhana event, lots of short runs, engine on/off, using wipers without motor running etc But it was fine after the drive home. Also keep in mind things like security systems. I have removed all such luxuries.



Also they are a much safer battery, completely sealed = no holes = cannot leak (e.g. they're allowed as freight on commercial aircraft unlike most batteries) and can be mounted at any angle.



Here's their official comparo table. PC 625, 680, 925 all candidates: Odyssey Battery Specs

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[flash]

the leak thing is why i am really considering it - unfortunately, i am pretty sure i am not going to be able to run the small battery - i just don't have enough current at idle

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[S_Cal968]

It appears you have your answer...

I just wanted to add that battery trunk [boot] kit for classic domestics has been around for a long time. And the kit typically comes with a 1ft. negative cable and 20ft. [001g] positive cable. So what you're planning is a common mod.



Battery Kit

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[Cloud9...68]

Sparky,



Thanks for sharing your experience with the PC680. It sounds like a viable option for those of us in warm climates, and who are lucky enough to not have any overnight drains in our systems. I'll probably go with one (along with a solar charger) when it's time to replace my battery.



Flash, yes, with the power pulleys, the small battery would probably be pushing it. Good luck with the conversion!

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