linderpat

That's a good idea Jim. Also, for others following along, it is critical that the battery posts do not touch the underside of the battery box lid. Not a good result.

dr bob

For those playing along at home...

Single-exhaust cars use the longer battery box, made for the Group 49. The later cars with dual exhaust have the right-side pipe passing between the battery box and the differential cover, so not enough room for the longer battery. They get a Group 48.

The correct batteries have the terminal posts "recessed" so the top of the post is just below the level of the top of the case. They also come with a plastic cover over at least the positive post. A little razor-knife surgery on that cover makes room for the cable to escape. Then the cover can protect the terminal from contact with the battery box lid or accidental/careless tool contact.

Ed brings a great point to the discussion, and that's related to the hold-downs for the batteries. The lid for the battery box is steel, and it's mounted with a metal hinge so it's at chassis-ground potential. If the battery isn't secured properly, or you decide to try and fit the wrong battery in the well, there's a serious risk of shorting the positive post via the cover. There's a lot of available short-circuit current from the battery, plus even more from the alternator if the engine is running. The post will will try to weld itself to the lid. The sudden extreme current flow is known to cause a battery to expand and explode internally. Modern battery cases and the battery box will generally contain the explosion. It's still a mess though, especially if the engine was running and the top of the battery and the box are filled with the hydrogen and oxygen offgasses from charging. The correct batteries have a vent port for a hose to route those gasses and corrosive vapors out of the battery box, which may help some.

The hold-down system in my car has a retainer in front that captures a tab molded into the bottom end face of the battery case. So the battery drops into the rear of the well, then slides forward so the tab tucks under the retainer at the front. Once in place, there's a metal wedge piece at the rear that drops between the back of the battery case and the rear of the battery box, bolts in place from the top. It holds the battery forward, and the bottom of that wedge pushes down against the tab molded into the battery case.

While it may be tempting to fit a more common 24/74 or 27/77 size battery in the 928 battery box, you'll find that the factory hold-down system doesn't work, and there's almost no clearance between the posts and the lid. At least add an aftermarket/generic hold-down with J-bolts through holes you'll drill (!!!) in the box. and for sure add an insulating pad to the lid above the positive terminal area. Since the correct battery sizes are readily available though (think 24-hour WalMart SuperCenters), it's really hard to justify using the wrong battery.

Kiwi'79

Great info dr bob thank you.

Both my cars have the the original intact "un-butchered" battery boxes complete with "lugs" to affix battery too, however, both have a "normal market" car battery, way too tall and too small, I have temporarily and strategically placed a block of rubber over the whole battery, eliminating post exposure, I also leave the battery box lid open.

It bugs me greatly, hence the desire to find the correct battery as is described.

Look forward to your pic when you can Ed.

The Deputy

This shows the red cover for the positive post and how the post is lower than the top of battery housing, and yes, you have to make some adjustments like Bob says to the red cover terminal entrance.



I also cut a piece of rubber to lay on top of the battery. A battery explosion, right next to the gas tank doesn't interest me in the least...lol...so a little extra precaution goes a long way.



This is a 78, so it comes with an extra lid.



Brian.

linderpat

That's my set up too, altho the rubber piece is something I may add as well. Thanks for that tip Brian. Also, is yours a group 49 battery?

AirtekHVAC

I just picked up a refurbished group 49 for my 79 project car for $55.00 with a 6 month warranty

They had brand new batteries, made in the US, brand is Superlex. 3 year warranty. Guy told me $110 for it.

Kiwi'79

Cheers for the pics Brian, I assume we can still get the rubber trim for the battery box?

Had a quick gander for "superflex" batteries here, not looking likely at this stage.

Jadz928

Group 48 & 49 are the same height at approx. 7 5/8" high.
Only difference is group 49 is longer, fitting well in the longer '78 box.

Battery box on the '78 is lower/shorter, underneath the chassis well cover.
Important aspect is the cover has factory rubber insulators on the backside, and the box itself has no factory hold-down bracket.

So here's my question. Either battery at 7 5/8" is an interference fit on the '78 with the cover closed. Interferes by about a quarter-inch.
Is it possible this was intentional, where Porsche used box/battery/cover compression to hold the group 49 size battery in place?

Note: if this was their intention, then the shorter group 48 battery would not work safely without proper provisions.

Thoughts?

linderpat

Turns out the group 49 did not fit in my box - too tall. I went to a group 41, which is the size that was in there, and stuffed in some tennis ***** on either end to keep it in place. The box measures about 7" high, but the size 49 battery was much taller, per Jim above, 7 and 5/8". I did not want a gap between the lid and the box, since lots of wetness can get in there and foul the connections.

The Deputy

Apologies guys, missed the two questions.

Ed,...yep, used a group 41 also. Once l added the rubber pieces (1/4 thick) below and above, the battery lid squeezed everything together nicely...with no gap around outer edge.

Kiwi'79...l'm not sure if the rubber trim piece is still available...I just cleaned mine.

Brian.

Rob Edwards

A while back I got curious and bought 1 each of the battery terminal covers listed in PET- 78-89, and 90-95. I know the positive terminal 'distributing piece' changed in '90 (picked up an additional terminal) so I _assume_ the cover changed to accommodate that.

Begs the question of which batteries have the two post holes for the locating pins on these covers.


78-89 on the left, 90+ on the right:



Underside:

dr bob

The cover Rob shows came attached to the batteries I've purchased over the years. No clue that Porsche also shows a factory number for it.

AirtekHVAC

My refurbished group 49 has the holes