911 SC - Another Battery Thread

[Locker537]

I’m replacing the conventional lead-acid battery currently in my 1979 911 SC. The car is a European coupe with a bit of a hot rod 3.0 with Webers. I have questions, but I do also hope that having this information in one place is helpful.

Odyssey PC925T - From reading other threads, the PC925 and PC680 are popular options. The PC925 has higher CCA and nominal capacity at the cost of higher weight. I linked the “T” model because it comes with the SAE terminals, which I believe to be correct for the 911 SC.

Group 48/H6 AGM Battery - There are many brands that make this size/specification battery. Interstate is a popular choice; the model numbers are H6 (MTP91) and H8 (MTP93). H6 is the smaller of the two, but either will fit the original battery location in a 911 SC.

I believe Odyssey makes a Group 48/H6 battery; it is the 48-720
I believe Odyssey makes a Group 49/H8 battery; it is the 49-950

Here are my questions:

1. Can anyone confirm the terminals on the PC925T are correct for a 911 SC? Is that the correct model for the 911 SC? There are multiple different PC925 models.
2. For the smaller PC680 and PC925 batteries, what are people using to securely mount them? Odyssey sells generic application mounting kits, Rennline sells one, and I’ve seen plenty of custom mounts. I don’t mind making my own, but the kits may have some nice features.
3. Has anyone used an Odyssey 48-720 in this application?
4. I leave my cars on battery tenders over the winter. I use Battery Tender Plus tenders. It's unclear to me if I need to a different tender for AGM batteries; any suggestions?

Thanks!

[SCadaddle]

2 years ago I searched and pondered and spent way too much time trying to decide on a battery for my 82SC. In the end I wound up buying the T6-DLG from Autozone. It's not an AGM battery but it was made in Germany and fresh on the shelf at the local store. It has a 3 year full replacement warranty and then another 2 years prorated. It fits the smaller of the 2 factory hold down spots like a glove. It spins my SC engine over with authority. I've got summer performance tires on the car so it doesn't get driven when the temps are 40 and below. I probably drive the car 60 (Sixty) miles at one time per month on average. Otherwise it sits in my garage. I've never used a battery tender/charger with it. The 38 year old clock in the dash still works.

1) It works
2) It fits great
3) 5 year warranty
4) Autozone store locations in the USA are a dime a dozen should I need to use the warranty.

[Locker537]

SCadaddle, makes sense to me!

I checked with my local auto parts store today. They carry Odyssey and had the 48-720 in stock. Given the availability, price, and OE size, I decided to go with it.

[Trackrash]

There was a thread about alternators, regulators and over-charging a while back. The theory was something about AGM batteries not being compatible with our ancient charging systems.

Does anyone have any experience with AGM batteries in 911s?

[Driven97]

I ran a PC680 for about 4 years before it died. I deep cycled it a few times on accident, eventually it didn't hold a charge anymore. I replaced it with a Group 48 or 49, IIRC both are compatible, I chose the smaller of the two. There's two holes you can bolt the OEM clamp down at.

For my PC680 I bought bolt-on SAE terminals from eBay and built my own mount from some steel bar stock and threaded rod. Not pretty but worked great and passed tech. There's $$$ options out there if you want them.

~30lb difference:

[darrin]

Originally Posted by Trackrash

There was a thread about alternators, regulators and over-charging a while back. The theory was something about AGM batteries not being compatible with our ancient charging systems.

Does anyone have any experience with AGM batteries in 911s?

I've been running a group 48 agm battery that I purchased at auto zone for around 6+ years without issue. Both the odyssey and optima batteries are also agm-type batteries (which use the same chemistry and have the same voltage cell-to-cell as traditional "wet" lead acid batteries)

Could you be thinking of lithium ion batteries (which can be super lightweight but can also be more sensitive to charging voltage, etc.)?

[ramonesfreak]

Can’t help you with odyssey but my autocraft (made in Germany) platinum agm group H6 from advance auto parts went into my SC no problem and has correct size terminals.

My car is always on a battery tender when I’m not driving it and I have watched it go from red to blinking red to green no problem so I’m assuming it will do the job

[Locker537]

Thank you all for the replies!

I'll confirm which common tenders work with AGM batteries and report back my findings.

[ramonesfreak]

When i said battery tender above, I’m referring to the brand “battery tender”. Mine is 20 years old and still going strong despite it being lopsided from my front tires hitting it when I pull in garage. Tough little bastard

[Locker537]

I use the brand Battery Tender as well. Specifically the Battery Tender Plus model.

I have two of them, one is quite old, and I've been happy. I double checked the specs on that model and it specifically says safe for AGM.

[Reiver]

I tried the Odyssey for awhile and it worked fine...until it rained one day...with the headlights on and the wipers going I noted the fluctuation in the volt gauge every wiper swipe...serious change. No AC in the car just the radio.
I figured my alternator was working a tad too hard with that battery so went back to a larger AGM. Under the same circumstances not an issue anymore.

[crgm]

Originally Posted by ramonesfreak

Can’t help you with odyssey but my autocraft (made in Germany) platinum agm group H6 from advance auto parts went into my SC no problem and has correct size terminals.

My car is always on a battery tender when I’m not driving it and I have watched it go from red to blinking red to green no problem so I’m assuming it will do the job

Hey ramonesfreak sorry for the thread response years later but you said you put an H6 AGM in your SC and I'm wondering if you connected the battery to ventilation tubes.

I just replaced the battery for my boss's 1978 Porsche 911 SC and I got an AGM H6 because of reading threads where people recommended it.

Problem is, I don't know which tubes are the ventilation tubes. The last (lead-acid) battery that was in the car wasn't connected to anything.

Are these the right tubes to connect? One of the tubes leads to the side of the front of the car, the other splits into two and leads towards the passenger cabin.

[ramonesfreak]

Maybe I’m a dummy but I’ve never heard of ventilation tubes or seen them. I don’t have them in my 83 911 as far I can see.

My battery is still runny fine btw

[pampadori]

AGM batteries dont have the vents on the top like the lead-acid do. They are spill proof and can't boil over.

[Marine Blue]

For those reading this thread, I wouldn’t recommend the PC925 for our cars, they don’t hold charge well and are not reliable. Odyssey won’t warranty them either as they say it’s not designed for the cars. The only way to keep them from dying is to keep them on a trickle charger consistently.

[Nditiz1]

I have a 925 sitting on the shelf and has been there for a year no problems. I have the pc680 in my 914-6 conversion. Has been doing fine for the past two years. I don't run a tender. If you do not drive your car st least once every few weeks, yeah I could see your battery going dead. My 914 has no radio and no clock, so pretty much no drain.