Vehicle storage without a battery maintainer

I'll be relocating to Chicago where my 2004 911 will have to be parked in an underground parking facility that will not have be access to 120VAC for a battery maintainer. (I may however be able to run a long extension cord for periodic top ups.) There will be both winter storage and periodic drives the rest of the year. Any suggestions on how to prevent the battery from going flat? If I can periodically access an outlet to top up then how often should I do that? I've heard that repeatedly draining the battery will kill the DME? True? Will disconnecting the battery between drives do this?

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The self-discharge of a battery without an electrical load on it (factory alarm system and owner installed accessories) is pretty well nil. Especially true for an AGM vs a FLA battery. Why not measure the current draw in the "parked" condition. Or install a battery monitor on it (draws a couple of mA) and monitor the battery. My experience with batteries is I prefer to "spot charge" them over leaving them continuously connected to a battery maintainer. Of course if you have a high current draw, "spot charging" becomes impractical. My longest lasting FLA batteries have lasted 16 and 14 years and they were never on a charger/maintainer. Best thing is to remove the battery, store it at home during long term vehicle storage.

Thanks. There seems to be this need/desire to keep a Porsche on a trickle charger aka battery maintainer. (Porsche even sells one.) I've never experienced this before, having left cars to sit for weeks before starting it right up. Can you explain this? Something special about our cars?

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My 993 has a "parked" current of about 35 mA due to its factory immobilizer. Anything that draws current will deplete the battery and this is based on the current draw (factory + accessories) while locked/parked and battery capacity (80 Ah for example). The 991 shuts systems down after some time and puts the car to sleep. In fact the cigarette lighter shuts down 6 minutes after shutting down the car (I had my dash cam temporarily connected to it and it would shut down in 6 minutes). Same here, my 993 or 991.1 GTS are never on a charger/maintainer during the driving season unless I am away on vacation or during winter storage. There are times my cars just sit for a month. BTW, the original 991.1 Banner battery is now in its 10th year with no starting issues. Internal resistance is 3.30 milli-ohms. Even my BMW motorcycle was supplied with a BMW charger/maintainer and it is only used as mentioned above. I charge it up, disconnect it and then monitor the voltage every week. Starts at 12.9V and it takes 3 weeks to drop to 12.8V on a stock vehicle with no user accessories connected.

My car (same badge, different model...) has a similar ~~35mA parasitic drain current. For practical purposes, it was slow to crank if I left it a month while I was away on a project. Stored in a garage within hiking distance of an outlet, a good maintainer now keeps it alive nicely. But as many will tell you, having the battery 'cycling' on a cheap maintainer isn't good for the battery. Now that we live where winter storage is the new normal, I do disconnect the battery during the winter. I still keep the maintainer connected, but absent that low drain on the battery it does almost nothing. Porsche and M-B were supplying their own private-labelled versions of the CTEK 7002US with their cars last I looked. You can by that minus the private-label markup at exclusive suppliers like Amazon. There are several others that have popped up in the space, and I've test-driven a few that sell for a lot less, but sadly the performance and durability seems to be roughly proportional to the cost. One under-$25 unit received for a review was so poorly made that I hesitated to even plug it in. It's available for shipping cost should someone be braver than I am. The last battery in my 928 lasted over a dozen years, split between the SoCal never-winter and the current (sorry...) never-in-winter use patterns. I replaced it when it started showing a bit weak on the annual specific-gravity test, part of it's annual electrical maintenance service. Still cranked and ran fine, but I hate getting stranded by stoooopid stuff so a new one went in. That long-lifer was a WalMart gold, identical except for label and cost to the prior Advanced Auto versions. Currently (did it again, darnit...) an Interstate from Costco is in there. A few years in it's still doing fine. There are more than a few battery-disconnect switch options in the market. From simple knife-switches that clamp to the post, to better marine rated mid-cable twist switches (recommended), these are good alternatives to pulling a cable from the posts every time you want to drive it. Find a less-obvious spot to mount the switch and gain some very slight anti-theft value in the process. Near our previous home I tapped into an overhead-lighting junction box to install a 15A twist-lock outlet for a friend. Powered a maintainer safely, and the twist-lock made the cord less prone to accidentally falling out. It was a low-ceiling garage which helped a little. Anyway, there are some creative ways to borrow power in parking structures.

If you charge like I do where you charge the battery and then disconnect (provided you do not have a high current draw while parked), I call it spot charging, you might consider one of these from Battery Tender for under $20. If the charge cables attached to your battery end in a SAE connector (most do except CTEK and others like it), get one of these to monitor battery voltage. Press the button and the display stays on for 15 seconds and then switches off. Battery Tender Voltage Indicator With LCD Display.jpg (27.26 KiB) Viewed 2778 times

My cars are on battery tenders thru the winter AND if they are going to sit for more then a month or so. My understanding is they really don’t like being fully disconnected from battery power. I suppose if you are not close to a power outlet you have to do what you must. Take the battery and keep it on a charger in your basement.