Best Battery for old 911

[steve meltzer]

I just replaced the dual batteries on my '69E. Got the Mazda Miata units from The Battery Mart. They are about two months old and won't hold a charge for more than a few days. I have several questions here: 1) If one battery is moderately discharged, will the car fail to start? I would think so. B) The tech at the vendor said that if the battery is on the charger (Schumacher ?? Spelling) and it shows the green light of a fully charged unit, a reading of 12.6 is not good enough, and that it should be 13 or 13.2. He questioned the condition of my charger, tho' it's a good one and seems to have always worked fine, as far as I can tell. iii) what is the best battery for these dual battery 911 cars? thanx steve

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[HarryD]

Sounds like a bad battery. Get them checked at a local Auto Parts Store. What is the voltage on the charger when it is charging your batteries?

[Hi_Fi_Guy]

I’m a fan of odyssey batteries. Run them in all my Porsche’s and air cooled cars for years. Use battery tender or Ctek brand chargers. No issues so far.

[steve meltzer]

I just borrowed a CTEK "smart charger" from a friend...gives no readings like a traditional charger. However, after being charged until completed, one battery read about 12.48 (a double martini doesn't help my recall!), the other battery was still charging when I came in from the garage for the evening. suspect both are bad. Any recs for a replacement? thanx. s

[dreichman]

I’ve had great luck with Interstate

[NYNick]

I recently tried to start my 72T that had been sitting for some time. 3 year old batteries were dead.

Trying to charge them one at a time, they wouldn't come up to speed. I disconnected the drivers side and charged that...bingo! Didn't take long. I disconnected the passenger side and did the same. Took a little longer but it charged right up.

I suspect the passenger side is weak, but also that trying to charge both by hooking up to one creates some kind problem.

Meanwhile when they die, I'm going to Antigravity Li-Pho. 2.8 lbs. Have one in my BMW 2002 and it works fine.

[steve meltzer]

I charged both batteries individually and grounded to chassis, not the negative battery terminal. neither battery can charge to better than about 12.6 thanx s

[Mr. Merk]

I run the little PC680 in my '88. If I didn't drive it enough the clock would run the battery down. Initially I just unplugged the clock and set the time to 9:11. More recently I deleted the clock all together and added a Rennline phone mount.

My wife runs a PC925 in her 997, now that she drives it more often we haven't had to charge the battery in quite some time.

[steve meltzer]

thanx for the info. i think those cars run a single battery. no? s

[Mr. Merk]

Originally Posted by steve meltzer

thanx for the info. i think those cars run a single battery. no? s

https://patrickmotorsports.com/collections/battery-mount-packages/products/pmp-911dbk

[HarryD]

Originally Posted by steve meltzer

thanx for the info. i think those cars run a single battery. no? s

Steve, I have a single optima for my 1973. Is was done many years ago. No issues. Search my handle to read the whole story. Bottom line, battery on side and connection to second battery removed from battery terminal.

[cmcfaul]

Been down this road many a time. Conclusion is, drive the car to a place that sells lots of batteries (Batteries Plus). find the two that that fit the best. Done

Chris

73 E

[gregpark]

I put a ride on lawn mower battery in my '67 4 years ago. Probably half the size and weight. Starts and runs like a top, sometimes a few weeks between starts with no tender. I think batteries are a crap shoot. I've spent big money on fancy ones that lasted 2 years and less than a hundred on some that lasted 8

[darrin]

Steve:

One think don't see addressed here is whether you've confirmed that your 911's alternator is charging -- with a properly-functioning alternator/voltage regulator, you should see 13.3+ volts at the battery when the engine's running. Seems strange that both batteries would simultaneously fail so soon after purchase and wouldn't want to see you at the same place again 2 months from now. Also wonder whether you should pursue a replacement from the original battery vendor first?

[T6andT]

Agree with Harry D. Have Optima on side in 71’ Works great, now 3 years.

[NYNick]

You don't need two, 12 volt batteries in an early 911. Porsche only put two in front for weight balance on each side.

One 2.8 pound, 12 volt Li-Pho eliminates the issue entirely.

[Quickstep192]

Originally Posted by Hi_Fi_Guy

I’m a fan of odyssey batteries. Run them in all my Porsche’s and air cooled cars for years. Use battery tender or Ctek brand chargers. No issues so far.

This is what I do. It’s been working great for years. (I hope I didn’t just jinx myself

[cmcfaul]

You do need two batteries. Why do you think Porsche designed it that way? Need the weight in the corners. Previous years has lead weight added. The further you stray from stock the harder to go back. Find two batteries that fit the boxes best and call it a day. I bought the uber expensive, glass matt, Susconsuian batteries. They lasted two years. bought batteries that fit the boxes best (From retail store), still going 10 years later.

Chris
73 E

[florio]

Bosch S4 fit well, and it's Deutche engineering (but probably made who knows where...)

Dario

[NYNick]

Originally Posted by cmcfaul

You do need two batteries. Why do you think Porsche designed it that way? Need the weight in the corners. Previous years has lead weight added. The further you stray from stock the harder to go back. Find two batteries that fit the boxes best and call it a day. I bought the uber expensive, glass matt, Susconsuian batteries. They lasted two years. bought batteries that fit the boxes best (From retail store), still going 10 years later.

Chris
73 E

They did it because they had to. They didn't have any other practical way to offset the front/rear imbalance.

Batteries and technology has changed. Everything the factory did in 1973 doesn't necessarily hold true as the best way in the 21st century. Old cars can change for the better if you want them to. Losing weight behind the engine is a much more efficient way of balancing the car than adding batteries at the other end as ballast.

These cars were never perfect.