924Board.org

reilly525i

so I am moving the battery to the back. Ive got a box and some terminals but was wondering if I should use some sort of massive inline fuse in case the lead shorts somewhere on its long journey to the front. any insight?
_________________
"There are no kings inside the gates of Eden"

80 n/a 924.
76 alfa romeo spider
e34 525i.
xj6 vanden plas
vintage Sunbeam 6 speed bicycle

9XX Girl! · Posted: Wed Mar 31, 2010 9:05 am Post subject:

Do you intend driving the starter motor from this rear mounted battery?

Starters are shunt wound motors. If left to run with no mechanical load they will get faster and faster until they run to destruction... Another characteristic of the hunt wound motor is that they are a dead short when stationary and theoretically pull an infinite amount of current as they have a resistance of less than an ohm (until they start to turn).

This give, what size fuse could you use, 200A? 400A? Not cheap to replace if it blows which it will, eventually, and under normal starting conditions as it has to sustain a dead short each time you start the car.

I would just use 475/0.40wire (60mmCSA) 14.5mmDIA - 415A cable
and put a battery isolator close to the battery but use a cable to operate the battery switch and fit that, close to the driving position
_________________
924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY

!tom · Posted: Wed Mar 31, 2010 9:35 am Post subject:

9xx Girl!'s reply is a little extreme.

A resistance of less than 1 ohm does not imply infinite current. There isn't a whole lot of resistance in the windings, but there is some in the brushes, as well as the brush contact.

Also, electirc motors rarely self-destruct if run free, as the inductance of the windings limit the speed, as do brush and bearing drag and wind resistance (the armature acts as a fan and pumps air around). Also, the torque output is inversely proportional to the motor speed in this type of motor, so as the motor speeds up, its torque output drops, and these relatively small loads become significant.

The current does not rise instantly in the coils of the motor (they do have considerable inductance, being large coils and all). Using your logic, the transformer on the power pole outside your house would turn into a black hole and swallow the universe, or something like that.

Personally, I would consider using a fuse due to the long run of the power wires required from the alternator and to the starter, with different values for each, as well as location (the fuse should be near the source of the power, so starter fuse would be near the battery end, and the alternator fuse would be near the alternator end). This is due to the long wires having an increased chance of having an insulation breakdown in any sort of accident.

In order to be effective, the starter fuse would need to be lower than the CCA capacity of your battery. Other than that, it would need to be pretty big. Similarly for the alternator; it should be close to the rated current ability of your alternator. Due to the differences in what these numbers mean, the alternator fuse would be a little above the rated alternator output, and the starter fuse would be a little below the CCA rating of the battery.

Another option is a fusable link. Put a short length of small diameter wire in the circuit, then this will act as a fuse while still allowing most of the power to be used at the other end. If you're going to take this route, make sure you use wire with flame proof insulation.
_________________
78 924 NA
5-lug

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Mine is wired direct from the battery in the right gear cubby to the starter with one long run.
_________________
Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

mikebola · Joined: 07 Sep 2009 Posts: 361 Location: Parkside, PA

My lancer has a 100A fuse directly on the negative terminal of the battery. I haven't blown that out...
_________________
proud owner of a 1979 924 Sebring Edition with a 931 trans that looks horrible but somehow runs...

Rich H · Posted: Wed Mar 31, 2010 5:05 pm Post subject:

A fuse would be nice but if you fit a 100A fuse and get a 90A short you are still in a whole load of trouble.... I'd wire it direct but make sure the cable is regularly supported (Not just under the carpet!) with P clips riveted to the bodywork.

A modern car might get away with 100A fuse but I doubt a stone cold N/A will be the same... A 931 on the other hand spins easily...
_________________
1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

Joes924Racer · Posted: Wed Mar 31, 2010 5:11 pm Post subject:

I have no-fuse either only fuse I have is to the amp and radio.

_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

AFAIK, neither of the factory rear mounted installations on the 928 or 944 have inline fuses. However, I do know that the so-called IceShark battery cable kit for the 944/951 includes a fuse, but I don't know offhand what size it is or where it was intended to be wired in to the circuit. You might do well to search for "IceShark" over on Rennlist and see what info you can dig up.

One of the things on my to do list was to develop a couple of "IceShark" kits for the 924/931 in collaboration with Robby, who took over for the late IceShark. Robby and I both have a lot on our plate, so progress has been typically slow.
_________________
erstwhile owner of just about every 924 variant ever made

ic932 · Joined: 11 Feb 2005 Posts: 1104 Location: UK

Correct, all UK 944's from 85.5 onwards have the battery in the back with nothing other then a thick long cable. No fuses or special anything.

Rich H · Posted: Wed Mar 31, 2010 11:48 pm Post subject:

Are there any bits that could be transferred from a 944 to assist with this mod?
_________________
1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

ic932 · Joined: 11 Feb 2005 Posts: 1104 Location: UK

The battery tray and long cable would simplify such a conversion.

reilly525i · Posted: Thu Apr 01, 2010 12:52 am Post subject:

well I bought a batt box and picked up new terminals. but honestly in the afterthought the idea of shorting the battery out because of a worn grommet wearing the insulation out is making me think twice. . . hmmm
_________________
"There are no kings inside the gates of Eden"

80 n/a 924.
76 alfa romeo spider
e34 525i.
xj6 vanden plas
vintage Sunbeam 6 speed bicycle

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

With my exhaust off, I am in a pretty good position to post some pics of my battery install, so I'll try to remember the camera this weekend.
_________________
Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3