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Tiny · Joined: 10 Apr 2006 Posts: 502 Location: London, UK

Evening everyone,

Here's the latest...

I've fitted a brand new big bosch s5 battery fairly recently, but it doesn't seem to be getting charged properly when the car is in use and I'm having to manually charge the battery. I am not an electrical genius but here is my situation.

I am getting consistent 14.7v at idle.

Idiot light functions fine, and is not lit while at idle.

I checked with a voltmeter for battery drain, and there is no significant draw when the car is switched off.

Even if my driving is all on motorway/highway (as opposed to short distance stop n' starts), after a couple of weeks, I plug this thing onto the battery and it reads only 50% full or less:
http://s7g1.scene7.com/is/image/BandQ/5035048210550_001c_v001_zp?$173x173_generic$=

If I do nothing, eventually I won't have enough juice to start the car.

I refreshed the part of the loom from alternator to starter recently...and double wrapped some juicy heavy gauge wire in heat protective shielding from NASA...and routed it all up nicely away form the exhaust. I did this not too long ago, so am thinking it should still be OK.

This leaves the fat wire that goes from the starter to the + positive battery terminal. This is the probably the same wire that came from manufacture and I am wondering if this is could be the problem where current is not getting back to the battery.

So my question to the electrical wizards out there is, is there any way to test it?

If I just go ahead and replace that fat wire, is there any particular type of wire that is best to buy?

thanks all,
Tiny
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*****1981 - 924 - Auto - London, UK******

Grenadiers · Posted: Thu May 24, 2012 8:16 am Post subject:

Had a similar problem with my '85 911, just ran a 'sister' wire thru the car to the starter. Works great, some day, well, maybe never, I'll remove the OEM wire. The alternator was charging, as in, 13 volts or so, but wasn't making it's way to the battery very efficiently.
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'83 944 Track car.
'88 924S Track car.
'89 944 Turbo
2004 Winnebago Vectra monster RV
2012 Jeep Wrangler
2014 Kia Soul
2001 Ford F350 powerstroke

Ozzie · Posted: Thu May 24, 2012 8:18 am Post subject:

The fat wire to the starter only provides a big chunk of power when cranking. It should not be draining the battery or preventing charging.

Tha failure in most things charging is the earth on the alternator and the power cable from the alternator.
These either are broken or undersize. I fitted 16mm cable to both. The earth I connected to the body of the car and the +ve directly to the battery.
Not had any problems since.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

Tiny · Joined: 10 Apr 2006 Posts: 502 Location: London, UK

Ah thanks for your help guys.
Yes...The alternator's earth! Mine is a verrrrry likely culprit.
I think you hit the nail on the head there once again Ozzie, and I can also route the +ve direct to the battery like you did too. What a good idea. Bring out the 16mm!!
Will sleep well now I gotta plan.
Thank you very much!
Tiny
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*****1981 - 924 - Auto - London, UK******

daniel · Joined: 18 Jun 2009 Posts: 687 Location: Sydney, Australia

I recently had to run a earth strap from the inlet manifold to the battery as the stock one that goes to the coil bracket was getting warm.

When you measure battery voltage I assume you are measuring across both battery terminals? Not positive terminal and chassis earth? If so your problem is power drain. Switch everything in the car off and set your multimeter to measure current. Dissconnect the positive battery terminal and remake the circuit with your multi meter. Report what it reads, it should be less than 100mA

If you are using a chassis earth, you could have a bad earth or the problem described above or both

Daniel
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Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet

!tom · Posted: Thu May 24, 2012 5:24 pm Post subject:

Ozzie · Posted: Fri May 25, 2012 10:26 am Post subject:

You are right but have you seen the size of the cable between the alternator and starter.
On most it is only 2x 2.5mm. for '80 on it gets up to 1x 10mm.

10mm barely manages 65amps at 70degC
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance