BrandonH

OK, it's hard to do a true controlled experiment, and what are web forums for if not wisdom-via-anecdote?

Therefore, I'm ready to call it, if you use a Battery Tender on your 993tt, lock the doors/arm alarm with the remote.

I have multiple Battery-Tender brand 4-pack and individual units, and use them for years on 10-15 cars at any given time. The only time in decades that they've failed me is with this car. This morning, for the third time, checking the tender on the tt, the light is red and I believe at quick glance that the battery case is swollen.

Sure enough, the car was unlocked. Here are the facts:

Battery is standard Porsche OEM Bosch, purchased from Pep Boys

timeframe: replaced ~6 mos ago, after prior battery slowly declined. That battery was only a year old, and failed the pep boys tester and had a swollen case, was replaced under warranty. THAT battery had been replaced about 2 yrs earlier, when the charger itself also seemed damaged.

historically the car has sat for long periods between use, say 4-6 mos, but the charger behaves fine and battery seems fine.



if you google threads in this forum, you'll see a wide range of reported experiences. But based on some comments made, i suspect there is something about the cars electronics that messes with either the charger electronics, or the car's internal power management circuitry. Whatever it is, it seems not to be a problem if the car is locked and alarmed. So from now on that's what i'll do!

LexVan

Is your vent tube working properly?

BrandonH

i don't know how'd you'd test for 'working' properly, but it is installed correctly, and installation hasn't changed between periods of being unlocked and locked.

LexVan

Can you get air through it? Careful of the acidity, obviously.

911PERVY

Mine has been on a trickle charger for 6 years

BrandonH

Do you lock/alarm doors?

Basal Skull

FWIW, I find my lithium battery will turn itself off (low voltage protection) quicker if car is left unlocked, and lasts longer if locked, so something is energized more when unlocked.

OverBoosted28

First interstate lasted about 10 years, constant trickle, and primarily locked. Still doing same ritual, with new battery.

PiB993

I use a Braille lithium battery and and disconnect it when the car is not in use to prevent parasitic drain which this car does have a fair amount even when working correctly. My apartment in the city does not afford me the ability to easily keep it on a tender. I should ask my contact at Braille but does anybody know if the lithium Braille batteries have a low voltage shut off?

Basal Skull

Afaik, Braille doesn’t have a low voltage cut off, and one of the reasons they can stay charged for a long time (disconnected) as it doesn’t have much power use on its own, cf voltphreaks that uses some juice even disconnected (for the low voltage cutoff) and will drain over time. Benefit is the cutoff before it goes completely dead, so you can turn it back on by pushing a switch with still enough juice to start the car. (At least that was the case with the Braille a few years ago when I bought the voltphreaks)

911PERVY

Yep

mtgadbois

Mine is a 1997 993tt, sitting in the garage hooked to a ctek charger connected directly to the battery in the trunk. Trunk is on the first catch and the bulb or fuse is pulled (don't remember which). Car is unlocked, battery is a common Porsche battery and is about 4 years old. FYI, I've never had a charging problem.

Quadcammer

my car has generally sat unlocked on a ctek for the past 6 years with no damage to my interstate lead acid battery.

Campy

​​​​​​​Ditto for me except I have an optima ...

LexVan

On a 993 series 911, this is really a non-issue, but on the newer 911s like the 991 series, there is a ton of electronics running in the background. So, it's best to lock the doors so those eventually shut down, go into hibernation, and draw less power away from a battery and stored car.