Effects of long-term disconnected car battery?

[MetroMPG]

Aside from losing obvious things like radio presets and clock, are there any other effects from disconnecting the battery in a modern car?

(I yanked the battery out of the Mirage back in July to use in my crapcan Miata.)

Feels like a dumb question and I feel bad for not knowing.

[Fummins]

I don't see any problems. I disconnect the batteries in my truck when I store it for the winter. It remembers how to start in the spring, I'd do the same for the Mirage without thinking twice.

[Macmoto]

I disconnected the battery on my Mirage when I left town for 5 months. Reconnected it when I got back, started immediately and ran fine right out of the gate.

[Loren]

Leaving the BATTERY disconnected with no occasional charging for a year might not be great for the battery. But, the car... it doesn't care if there's a battery connected. Probably good for it. No risk of battery terminal corrosion!

[Dodge Aries K]

Didn't notice any issues on the Mirage. However, older Chrysler's that I've owned before (especially Neons) lose their minds and have trouble idling and stalling out until the ECU relearns how to use the idle air control valve.

[Fummins]

I didn't even think of that, I recall having a similar issue with a few 2000-2004 gm trucks. Took a while til they'd idle on their own. Haven't seen that with a Mirage yet.

[MetroMPG]

Thanks for the replies.

What made me ask was exactly anecdotes like those.

You'd think that engine control related adjustments would be written in non-volatile memory, but it sounds like there are some that are lost when the lights go out.

[mpaton]

Thanks for the replies.

What made me ask was exactly anecdotes like those.

You'd think that engine control related adjustments would be written in non-volatile memory, but it sounds like there are some that are lost when the lights go out.

Non volatile memory does eventually "wear out" due to too much use. There's stories circulating now, about some non volatile memory in Teslas wearing out, and causing the HVAC systems and rear cameras to become inoperable. Until someone pays several thousand dollars for a new part containing a memory module.

The consequence of losing the contents of the KAM (or Keep Alive Memory) is that OBD2 readiness monitors become unset. Resetting these can take a few "drive cycles", and it's very helpful to know what these are rather than just driving. So if you thought your bring a vehicle back into service, and then need a state emissions inspection, that may take longer than you were hoping for.

I recently had to do a drive cycle that involved startup from cold, idling for 4 minutes, driving between 20mph and 30mph for 4 minutes, then between 40 and 60mph for 15 minutes, and then idling while stopped for 4 minutes, and then repeating the 2nd, 3rd and 4th segments immediately. In a somewhat urban area, the driving segments were the problem. 30mph is too slow to mix with other traffic, and the higher speed segment requires finding a 15 mile stretch that you can maintain speed on, or else a 10 mile stretch that you won't be a roadblock on. And you aren't told whether slowing below the segment minimum speed discards that segment, or the while drive cycle. You are told that going over 60mph or 3000rpm DOES discard the whole drive cycle, and you have to start again.

Which means parking till the engine cools off. Small wonder that people who thought they would just drive it have gone over 800 miles and still don't have readiness monitors set. :-(

[MetroMPG]

Now that you mention it, I experienced the same problem with a "non-ready" OBD system a few years ago when I took my Metro out of storage (batt had been disconnected) and it needed an emissions test.

[Wallythacker]

Lead acid batteries don't like going below 12v. Deep discharge types, RVs, boats, can tolerate it much better. Get a lead acid below 9.6v and it might not come back. I find most batteries are goners if allowed to drop below 11.5v AND stored in temps below 0C.

My 2017 battery is toast from being discharged deeply twice while I was in the hospital for extended times,

The problem with new cars is the fact they place a constant drain on the battery. My hobby car, an old 70's thing draws nothing while sitting and the battery can tolerate sitting for a winter w/o discharging below 12v.