Alan

Just for grins here are the specs of the Porsche GTS battery (made by Douglass) - basically they are the same as an Exide equivalent (and many others for this group size):

Value Battery Specification
======================
650A Cold Cranking Amps (CCA)
120min Reserve Capacity (RC)
75 Ah DIN: Rating – Amp Hours (Ah)
370 A DIN: Cold Test Current (A)

These Optima specs have much lower total capacity see the Ah/RC specs.

These are standardized measures for total charge.

Alan

JPTL

..didn't make me grin. I thought I was getting the Smack-Daddy of batteries.
So does a battery with a lower Ah rating; and a lower RC mean that it holds less of a charge and runs down 30 minutes quicker under identical conditions?
I do like the idea of more CCA than just enough, however.

dr bob

J.P.: How well does the Optima fit in the hold-down? Just curious.

----

For my S4, the battery seems to provide power for starting just fine. The SoCal city driving cycle seems to have a lot of idling and waiting for lights to change, traffic to move, make-up to be installed and text messages thumbed. So while most batteries will start the car OK, system voltage/charge rate varies a lot based on the time of day and destination. Bottom line is that total AH when new is important, total AH capacity a year from now is just as important. And that's where the Duralasts I've been using seem to break down.

In the last month, I've done a lot of sleuthing, connection-cleaning, alternator-refurbishing, and it seems to be OK. But if the Duralasts keep wimping out in less than a year, it will be time to look at other options again.

JPTL

Bob,
Haven't gotten it yet. I'll post fitment details when it comes.
If the Yellow Top had higher CCA, I would have opted for that one, since it's RC and Ah ratings are a tad better than the Red Top's.
As a reference, although my Duralast 38D lasted 7 years, it's been teetering on demise for the last 3 years. If I missed a maintainer hookup for a week, I'd need a charge before starting in cold weather. As of last year, a week w/out the maintainer would leave me in a no-start predicament - regardless of temperature. It did take a charge yesterday, but I don't plan to venture far with this battery, and will be parking on hills where a bump start is an option if need be...until my new battery gets here.

Alan

Might be time to reduce the amount of make-up and get more driving in....

Some battery specs explained:

Generally the physical size of a battery is a fair indication of its total charge storage potential in Coulombs. This is more commonly expressed as Amp Hours (Ah) a more convenient numerical quantity than Coulombs for a large battery. A battery can also be considered like a very large capacitor with Coulombs expressed as Volts x Farads. The 928 GTS battery is rated at 75Ah = 270,000 Coulombs = 22,500 Farads (at nominal ~12v).

Battery charge is also often referred to via an auto industry measure called ‘Reserve Capacity’ (RC). Technically RC is defined as the number of minutes a fully charged battery takes to discharge to the point its voltage falls below 10.5v while it is supplying a constant 25A at a temperature of 80˚F. So an RC of 60 is equivalent to 25Amp hours under those conditions.

Thus RC is proportional to the length of time the battery can run the vehicle’s systems if the alternator fails. Obviously the higher the rating the better, but the actual run time will vary depending on equipment usage, so if this happens to you - turn off all non-essentials.

Batteries are also specified by their CCA rating (Cold Cranking Amps). This is a measure of the peak current the battery can supply to the starter under cold (worst case) conditions. This is a useful indication of relative ability to start a vehicle; bigger engines generally need more CCA. Battery performance varies somewhat with temperature & the engine will be much harder to turn over when cold due to the higher viscosity of the oil.

Technically CCA is the current that can be supplied continuously for 30 seconds at 0˚F at a voltage not below 7.2V.

If you see a specification noted as ‘CA’ or ‘HCA’ these are NOT equivalents:

The Cold Test Current in Amps is a DIN (Euro) spec for the current that can be supplied continuously at -18˚C (approx 0˚F) at a voltage not below 8.4V for 30 seconds and is specified AND for 180 seconds at a voltage not below 6.0V. (both conditions must be met) It provides similar information to CCA. It is not as commonly referenced in USA battery specs.

Alan

Franks928s

Alan,

Thanks for the "Education!"

Like J.P., I'll post regarding my fitment of the yellow top once I receive it. I, too, would have opted for the Red Top with higher CCA/CA but, in my case, I run a lot of accessories. I run a stereo/video deck, after market amplifier, rear view mirror video screen, GPS and we can have two cell phones plugged in on a trip. Hence, my thoughts of the benefits of the deep cell. Additionally, my thoughts are I can recoup my $115 investment for a $200 battery via ebay if it should appear to struggle.

ShawnSmith

For the record, I just slid an Interstate 91 MPT into my 88 S4's battery box. $105 out the door. Fit very nice - locks down cleanly with factory hold-down bar. Cranks the motor vastly better than the half-size odyssey former inhabitant.

Folks with earlier cars and the larger battery box can probably take the longer 93 MPT battery if you want.

RED SHARK 1990

I just recently replaced mine with the higher group interstate mp91-with more cranking power than the group 48-i was told by an interstate facility in San Diego,from what i learned there are several batteries manufacture-Johnson co makes -interstate batteries,Optima, walmart,Die hard,and others- but Walmart have the group 48 for the 928 with more cranking power and the same as the Optima 91 for about 65.00 and a lengthy warranty.

Franks928s

I heard there was a more pwerful version of the interstate MPt, MPT-93.

ShawnSmith

As far as I can tell, the Interstate MPT-91 is the correct fit for the later-year 928's. The MPT-93 is indeed more powerful, but I think it's several inches longer and won't fit in the stock S4 battery box. The older cars may have more room and can take the MPT-93...

JPTL

That's the real question.
Here's what I found with the 35 Red Top:

• The post location configuration is fine with the 35, as the positive is offset to the right a bit. The positive lead reaches, no problem.
• The width of the battery is fine
• The height of the battery (to the top of the posts) is fine (see below)
• The dimensions of the length of the battery is less than the hold-downs are designed for (see below)

..so the real issue is the length of this battery, as well as the base 'footprint', in that there aren't any tabs like with the Duralast. My box has a fixed raised 'tab' hold down @ the front of the box; and a screw-down bracket @ the back. These are spaced properly for the bigger Duralast, but have no effect on holding down the Optima without modifications.
I made a steel hold down using 1/8" stock (see pic). This straddles the base side tabs of the Optima, and fits in the front tab opening and under the screw-in rear bracket.
After bolting it down nice and tight, I used the red plastic top cover that came with the Optima 35 and trimmed down the post areas (see pic). The cover fits over the battery with the neg & pos leads connected. (My guess is that this cover doesn't come with the 75/25).
Most importantly, this plastic cover serves as an insulator between the posts and the metal box cover. Since it raises the overall height of the battery, the box cover closes snuggly over the battery and by adding the plastic top, the metal box cover actually holds down the battery - as an added measure if the hold-down method isn't as effective as it should be.
There is now no way that this battery can move - in any direction..no matter how hard the area's jarred. This battery is locked in.
So was it worth the effort of making this hold down? I'd say no. Hopefully this battery holds up well, because I can replace it with another 35 in a snap, now.

dr bob

JP--

Thanks for the update. I guess that moves the Optima onto the candidate list.

JPTL

I left my hood open while I was messing with my heater valve. The friggin' light stayed on for several days (switched LED hood light - next project).
This drained my new Optima to a "deeply discharged" state (under 10.5v). Since Optima's are very low internal resistance, many battery chargers without an AGM setting will either recognize the battery as fully charged, or not capable of taking a charge. This was the case with my charger. I couldn't for the life of me get it to put a charge on the battery. It ran a diagnostic looking for resistance and defaulted to no charge, indicating a bad battery.
Optima says to recover from a deeply discharged state without a late model charger - or a professional commercial charger:

• You need to first remove the battery from the car and put it in a well ventilated spot.
• Connect a good 12v battery w/a healthy charge in parallel using jumper cables.
• Hook the charger to either battery and set for 10 amp charge.
• Periodically check for excessive heat or gassing of the Optima, and if neither exist, keep it charging for 2 hours.
• After 2 hours of this rapid charge, remove the charge/jumper from the Optima and check the voltage. If it's >10.5v, then you can put the charger directly on the battery and charge at a low 2 amp charge.

I did all of this, and my Optima did take a 2 amp charge overnight and seemed to have recovered fully (at least as fully as a non-deep cycle battery will recover).

AO

Great information. I think I'll try one of these when my battery dies.

As for the hood light switch mod. BTDT. Read this.

JPTL

I thought I remembered a neat inline switch install in someone's car...
...you've jostled my memory. I saw yours at last year's Frenzy.
Thanks for the link.