Ok I'm going to chime in here, for each of the past 40 yrs I've been storing my 1979 Fiat Spider convertible for about 6 months thru Wisconsin winters, all I've done with the battery was simply disconnect the battery cables and let it sit in the car in the garage. Come Spring, I put my battery charger on to top it off, and away she goes without a hitch. No battery issues or anything else for that matter. If it sat for more than a year, it could probably develop a few issues like oil seals becoming brittle, sticking brake calipers, compressed suspension bushings and etc., but for the most part 6 months of storage never caused any issues. Btw, I also have been storing my 1990 Jaguar for 6 months and again no major issues related to storage. Shell Premium (w/nitrogen and no ethanol) is my fuel of choice, my 2 LS400's are never stored but are still rock solid with 275k and 330k respectively. I guess you can say all 4 machines are for the most part still "happy", and so am I if they stay that way

 

I have experience regarding this regarding with my 85 MR2, which has the subject auto headlight feature. I noticed the parasitic drain was an issue after a few instances of a dead battery after the car sat for weeks and was cured when I started using the manual switch for the headlights. I didn't analyze the circuit for a definitive explanation, but I assume the auto circuit is energized to keep some circuits/components ready for the photo-cell to activate them. I don't agree with the EE's who are designing the circuits, but because lights are a safety feature, it likely comes down to some sort of liability issue. I think Lexus came up with improved lighting circuits in later vehicles, but even with my 2008 SC and 2011 ES, I'm still perplexed at times by the actions of some of the electronics, including the lights. Rather than fight it, I've conceded to manually turning off the lights when leaving a vehicle parked for any length of time. It sucks because the "auto lights" feature is great and would be even better if one could set it and forget it!

Is this another candidate for "newer isn't always better?" The more electronics they pack into vehicles, the higher the parasitic drain. At some point, EE's will need to rethink this, or come up with better battery technology to counter it.

 

Having the auto headlight switch set to auto doesnt cause any parasitic drain when the car is off, theres no way that it can. Experience with a 35 year old car notwithstanding.



They have already begun the process of doing that, eventually modern cars will all be on 48V electrical systems. On my Pacifica there are two batteries, one that powers the accessories and one that starts the car.

 

The two-week figure for letting a computer-equipped vehicle sit wasn't really my figure, as I never let my vehicles sit that long, even after my two operations. Pat Goss, Motorweek's Lead Technician, used to advise people on that in his auto-talk-sessions.

 

W/r/t "modern cars will all be on 48V electrical systems"
Why not?
1. It'll kill you. The current in an automotive system is enough to do serious damage, but the 12 volt differential cannot transfer enough of it to be fatal. The same current @ 48 volts will leap a small gap and fry your brain. That small spot where a wire under the seat has rubbed away its insulation will now burn up the car. Everything hot in the car (wire terminals) must be more widely separated from everything else.
2. Greatly increased inductance. This means current is induced to flow in other (completely unrelated and unconnected) wires in the harness. As if it's not that bad now, where anything affects everything...

 

Can the lights come on when the car is off? Yes, as I said, it is a safety feature. It stands to reason that all of the circuitry associated with the lights is subject to being energized, intentionally by the EE's design, or not, as an unintentional ghost circuit. I've owned my 85 MR2 for 35 years and I figured this out over that time. If your hunch is more accurate than my real life testing and conclusion, then you are more brilliant than I thought!

 

Started the toy up, was sitting 4 months and was happy. Hand turned the crank over 10 times before starting like I usually do so the seals aren't shocked, took it out and hit the end of the speedo and reparked it.

Other stuff gets driven enough I'm not worried