michael7810

Battery on my 87 944T died while out of town. It has an Interstate MT-41 that is 7 years old. Interstate's website shows a size 91/T-6 and Batteries Plus recommends a size 40R. The MT-41 fits good and has lasted a long time so I'm leaning towards that as a replacement but wanted to ask if anyone knows the "correct" size for this car. I'm in a small town so no Porsche dealer and I only have a couple days before I have to drive 150 miles home. Thanks for any info you can provide.

Arominus

I run an interstate in my car, the proper battery for the car is the MTP-91 though. The fit is perfect.

michael7810

Thanks. MTP-91 is what is listed on Interstates website.

7B944

I put in a T-5, same as in my C6 Vette. It's not as long as the proper one, but works great so far. Walmart has them in Everstart Maxx @ $120.

KevinGross

You can use a somewhat smaller battery without issues. I have had a Group 47 battery (a Sears Diehard) in my own car since '12, it works fine. Just make sure the terminals are located properly and the battery is clamped in place.

John_AZ

I use the Bosch group 41 in both cars. PEP Boys. About $120 -less web coupon for new = about $100 and tax.
The Bosch battery in AZ heat will last 4 to 5 years if properly maintained. My battery lasted 5 years.
Replacement pro rated after 3 year free replacement period cost me about $70 for another new Bosch battery. Cannot use web coupon on battery warranty replacement.

Sams Club does have the group 40 in stock and is listed for the 944/924S Porsche. About $90 and tax.
I try to get a battery that is a new manufacture. Some stores may hand you a "new old stock" battery that is 12 months or more old.

J_AZ

michael7810

Batteries Plus has a Duracell for $100 including web coupon. Anyone have experience with Duracell car battery?

SloMo228

Heck, I run a lawn tractor battery in mine. I had to bend some aluminum bar stock for a tie-down strap but the little battery starts the car just fine. It doesn't have a lot of reserve capacity for repeated attempts, but it hasn't gone flat on me yet and shaved about 18lb off the front end. I'd like to go even further with a lithium battery but haven't done enough research on them yet to feel comfortable picking one.

marc abrams

I have plenty of experience with lithium polymer battery's with R/C helicopters. Those batteries required specialized charging equipment. They catch on fire too! Not something you would want to use in a automoivte application.

michael7810

So I got a Duracell 40R/T-6 at Sams Club for $86. The fit is spot on including my battery cover. Car starts better than I can remember; I guess the old battery has been weak for a while.
The Duracell has provisions for a vent tube on both front and back ends. I plugged the front and left the back open with no hose but thought of adding a hose later and run it under the car to vent. The old battery didn't have a vent hose and there is no corrosion around the battery so I guess it's not necessary. Curious to see if others added a vent hose.

SloMo228

Yeah, I have experience with the Li-Po hobby batteries and have a specialized charger for those, which is what makes me cautious about lithium batteries in a car.

That said, the chemistry used in the batteries you'd potentially put in a car isn't the same as the RC packs. They're typically LiFePO4 batteries, which are less susceptible to catching fire and exploding (at the tradeoff of somewhat lower energy density). People use these batteries in cars regularly, some even building DIY packs out of individual cells, but most of the commercially available batteries don't have any kind of balancing circuits, overcharge protection, or low-voltage cutoffs. Those that do tend to cost a grand or more.

Tremelune

The way you treat the battery has a profound effect on its life; much more so than brand/type, in my opinion. I suspect most are cut short by suboptimal charging systems, long periods of sitting, and outdoor parking through winter. Every vehicle I have has an on it unless I expect to drive it in the next day or two. The battery I pulled out of my last car (always garaged) was a run-of-the-mill lead acid battery that lasted 9 years.

Most batteries are well over-specced by 2-4x, depending on what temperature you expect to start the car in. If you never start the car at 0°F, you can cut the cold-cranking amps (and therefore size and weight) in half. You can cut them in half again if you never start the car at 32°F.

I would go AGM to avoid the hassles of lead acid batteries, but really...anything that will fit will work.

Most auto batteries are made by just three manufacturers in the US: Exide, Johnson Controls, and East Penn. I think this is the current breakdown, but I have no great source for the data:

Exide - Champion, Exide, Napa

Johnson Controls - ACDelco, Diehard, EverStart, Duralast, Interstate, Kirkland, Motorcraft

East Penn - Deka, Braille, Odyssey, Duracell, Lifeline