docmirror

Here's the deal. IF the battery is fully charge, then yes - letting it sit for 3-4 days is no problem. If the car sits for 3-4 days, then is driven for 6 min to WalMart, stopped, then started again, and drive 6 min from WalMart to home, the battery is NOT at float full charge voltage. If this is the common duty cycle, and the driver is using the AC on full, radio, maybe the lights, then the battery is never able to get to a full charge state. So, pay attention to what I write, because you didn't get this far:

"Go for a drive, and do not shut the engine off during the drive. We want to fully charge the battery so that there is no question your battery is fully charged. You could do this with an external charger, but going for a moderately long drive will get everything warm, and allow the alternator to get its workout charging the battery fully." Bolded just for you pedants.

Horsepwr

LMAO, I payed attention, I just think you are incorrect....

808Bill

Dave's not home man...

hatchetf15

One of the classic comedy albums of all time! For those of you who remember “The Big Bamboo” from Cheech and Chong.

mrdeezy

All 996’s are hard on batteries. Get an agm battery, and one of those sweet novo genius battery tenders. Really solved a lot of problems for me!

c didy

Does it start fine at home, then won't start when you come out from shopping? And your battery is new or checks out ok? Only has problems after driving? Check your Y cable connecting starter and alternator. $75 or so. Does that sound like what happens to your car?

Robrall

I had the same problem in my 2003 cab Replacing the Y-cable fixed it.

wildbilly32

"Who is it? It's Dave, man. Hurry up I got the stuff! Who? It's Dave! Dave? Dave? Dave's not here!"

Funny thing is I can quote entire routines but sometimes have trouble remembering my own phone number! "Seen the damage done..." True! True! It was fun none the less...I think...!

dporto

I've told this story before, but I'll do it one more time for posterity and for Davey the 75 year old first time Porsche owner! That's keeping the dream alive!!! Anyway... when I bought my car (in Chicago) the battery was for all intents and purposes dead - the car started with a jump, but the battery wouldn't hold charge (Interstate proper large spec. battery - don't know the model off hand) even though the volt meter showed it was charging. I took the car around the corner to the local "AutoZone" (I don't remember the actual store, but it wasn't O'Reily's - though I tried them too), they put it on their tester and they claimed they could charge it - "come back in two hrs"... When we got back, they said the battery wouldn't hold a charge, was shot and that they'd be glad to sell me another. Well, the battery was less than a year old and was still under warranty so I wasn't ready to give up on it yet - unfortunately, they couldn't honor the warranty because they didn't carry Interstate. Fortunately for me, I was planning on being in town for another two days so I had a little time. It just so happened Interstate had a factory distributor in a industrial park in the next town over, about 25 minutes away. I brought the battery directly to them to see if they'd honor the warranty. When I got there and showed them the battery, they said they would absolutely honor the warranty, BUT!... They'd have to try to "deep charge" the battery first to determine if it would actually hold a charge - he said it would take 8 to 12 hours on their special computerized system to fully discharge then charge and test the battery to see if it met factory specs. At this point (it was about 2:00 in the afternoon) it meant I'd have to leave the battery overnight and come back the following morning. If the battery turned out to be no good, they replace it free of charge. So, I called the following morning (Sat) around 10 am and they said they were just doing final testing, but everything looked good and I should come in around 11:00 to pick it up. I've got to admit, at this point I was sort of hoping that the battery was bad, and they'd just give me a new one... No such luck! The original was fine, took the charge and passed all cranking amp tests with flying colors! Oh boy, I've got to drive this thing almost a thousand miles tomorrow, and I'm hoping the battery doesn't die on me Fast forward 4.5 years, and I'm still using the same battery! It's never died on me, and it always cranks strong. The car sometimes sits for over a week and has never let me down. If I know it's going to sit for more than a week or two I'll bring it up to my garage (off site from my home) and put it on the battery tender. All this being said, it's really important that you lock it up when parked so it "goes to sleep" and reduces the power usage (alarm/system monitors) while the car isn't running.

Cuda911

Never had any problem leaving my car for weeks, unlocked. Always started right up.

docmirror

Oh yeah, forgot the Y cable fiasco. That is on the 'to-do' list for all 996s. Good catch.

docmirror

I swear, I can write stuff on this forum, and people just don't care. They 'know' what they know, and no one can tell them otherwise(not referring to you, of course - you did it right). Even people with letters behind their name, doesn't mean a thing. Sigh.

Here's what Interstate knows:
https://www.upsbatterycenter.com/med...stic_curve.jpg

Take a look at where charge voltage(constant) crosses charge volume. At about 6 hours, of continuous charge, the battery is now at ~85% of it's rated volume(ampacity), but most would consider it 'fully charged'. It - just - is - not! Almost all vintage car batteries live in this 60-85% charge state, and will provide enough current for long enough to barely get the car started, and running. That's why the DD cars batteries last much longer than seldom driven, short duty cycle batteries. Also why I recommended a battery hydrometer. The chemistry of the battery never lies. If you had checked the battery when it wasn't starting the car, you would have found it to be almost dead reading on the hydrometer(red, maybe yellow ball floating, others not). People with trickle chargers rarely have this problem, but to expect a modern car battery with all the parasitic draws to maintain starting current, is madness, I say - utter madness!

dporto

^^^ I hear ya Doc ^^^ ! Had I not experienced it first hand, I have to admit I'd probably be skeptical my self. In fact, the guy I spoke to, did a pretty good job of explaining that chart and their digitally controlled "deep cycle" charging system. In hindsight it's pretty obvious that he was confident that the proper charging of the battery would work - otherwise he would have just given me a new battery on the spot. And, yes - While I'm not as old as Davey, I remember the "old days" before "sealed batteries" when we could buy new electrolyte (battery/sulfuric acid) and just replenish our own batteries...I think the Insurance companies had something to do with the end of that practice...

Splitting Atoms

Assuming battery, alternator, and Y cable are good, I would hunt for parasitic draws that exist while it sits.

Does it have an aftermarket stereo system? If so, this may be the source of your problem.

I did some experimenting with mine, and it appears there is a small, constant, draw from the immobilizer/central locking system. I installed a switch on the battery that I open if I plan on letting it sit for more than three days.

There are some great YouTube videos on identifying parasitic current draw in the many electrical circuits by methodically narrowing them down with a volt meter until you find them.

docmirror

One can buy battery acid right on ebay or amazon:

I've used it before to rejuvenate a battery. It's not rocket surgery. BTW, a lead/acid battery is never sealed. Although the top may be flush, there is always a vent somewhere. Usually the flat center flap can be peeled off what appears to be a 'sealed' lead/acid battery. They cannot be sealed because charging generates vapors that must be discharged or the battery will asplode. I've seen batteries asplode when the vent gets blocked. It is not fun. As a boy of 14, I worked at a tire/battery shop. We rejuvenated hundreds of used batteries. We also sold used tires, and recaps. My dad had a shop on Garnet Ave in Pacific Beach. I could surf in the morning, and change tires, and batts in the afternoon in summer. Remember recaps? Jeez, I'm old.