Does your car battery drain if you leave your car unused for 3 weeks?

I had a little debate with someone over this question. She claimed that if you leave your car unused for an extended period of time (3 weeks?), when you try to start it the battery will be dead.

I said I thought that was baloney, since I had left my 85 honda civic unused for months and it always started.

So is this true? Does it vary by car? Or is it true of some cars but not others?

How about these two cars?

91’ ford Taurus wagon.
85’ Honda civic

She’s wrong. Assuming the battery is healthy & fully charged, and assuming there’s no power drain, the battery should be O.K. for many months.

I say “should” because there are some (rare) circumstances where this is not necessarily true.

Well, my '92 Civic has sat for the last 8 months (one of these days, I’ll get around to selling it.) I started it in November, and then just a week or so ago, and it started just fine both times. So, no…the battery won’t die if you leave it sit for too long. If it’s an old battery that can’t hold its charge any more, it may die over the course of a few weeks, but a healthy battery should not.

It depends less on the car and more on the battery. If the battery loses power after 3 weeks, it’s cheap, old, and/or needs new battery fluid. I’ve certainly left my truck sitting around for more than 3 weeks without any problems. Hell, before I had it it was sitting in my uncle’s driveway for several years unused, and it started up.

As mentioned it is a definte maybe. It shouldn’t, but if your battery is weak it could. Most cars do have a slow power drain (IE - Clock, radio, main ECM, etc) but it shouldn’t be enough to kill a battery in weeks. If a person were to keep their car parked for several weeks/month, I would recomend pulling the battery out of the vehicle. If it happens often enough that battery can go bad.

It depends on 4 factors:

  1. What kind of shape is the battery in?

  2. What kind of charge does it have when it’s parked?

  3. What kind of active drains are there on the battery?

  4. What’s the environment?

Your more modern cars may have a number of small battery drains going on, even when the car is parked. You got the clock, sometimes an alarm, various computers may maintain their memory through a small power drain, the keyless entry, etc.

A short in a low power system might not get noticed for a long time, if at all, yet drain the battery quickly once the car’s parked.

The weather, and the shape the battery’s in are going to matter, as well as the immediate environment of your car’s engine and battery compartment.

Very damp or wet weather and a dirty compartment may create a mild drain. Cold weather seems to kill batteries. What it’s resting on may be important. I don’t know why, but when I took the battery out of my tractor for the first time and put it on the concrete floor, it lost all its juice fast. If you put it on a block of wood it seems to do ok. So, what the battery is resting on matters too.

If you want to play it safe, don’t let your batteries get more than three years old, and if the car’s gonna be sitting for a while, there’s a little connector you can buy with a little wing nut on it that disconnects the battery if it’s going to be sitting for a while. Keep the battery clean and park it in a place where it won’t be sitting in water, and out of the day/night cycle so that dew is minimized.

Lead-acid batteries DO lose charge over time. I’m pretty sure the industry-standard spec is 1% of charge lost per day, but I’ve never heard anything to prove whether or not it’s true.

If the car has a digital radio (virtually anything made in the last twenty years), it is hooked up to a source of constant power, regardless of the position of the key, so that your pre-set pushbuttons won’t be lost. Similarly, the clock draws a little power at all times.

If the car has an alarm, that draws a small amount of power too…the last time I measured one it was an Audiovox PRO-9842A, and it drew .01 amps when armed.

I once owned a car with a defective underhood lamp; it stayed on all the time. If only parked overnight, the battery didn’t drain, but if I left the car unused for several days, it of course wouldn’t start.

Car batteries usually last around three to five years, and there’s often no sign of trouble, until you get the first cold night of winter and there just isnt enough juice to start the car. Many places, like Sears or AutoZone, will load-test your battery for free; it’s wise to do so each fall.

In Sleeper, Woody Allen’s VW Beetle started first time after being left idle for about 200 years.

If you know that the car won’t be driven for an extended period of time (over a couple few months to years) then it’s best to remove the battery. A trickle charge at 4 to 6 amps for 24 hours every few months will keep the battery happy. A battery that is discharged (“dead”) may freeze and burst. A fully charged lead acid battery will not freeze in extreme cold. As an aside, never attempt to charge a frozen battery. As a further aside, an auto battery thought to be long dead can be revived by a slow, careful charging over a couple days.

Modern cars have so many devices, locks and clocks and computers and such that it is entirely possible for the battery to drain over an extended period. Not in a matter of weeks, though. This indicates a short, or trouble elsewhere in the circuit, perhaps a marginal battery, marginal charging system or corroded terminals etc.

I’ve often wondered what the ‘record’ is for an old pickup or car sitting unused and starting up. 2 years? I dunno.

A car battery could power a small lightbulb (1.2W, like the interior light or the one in the trunk or motor compartment) for three weeks and then it would be dead so there must be a leak somewhere if the battery cannot start the car after only a few weeks.

Yeah, after only 3 weeks there must be a power drain somewhere, probably corroded connections. Be sure all the connections are free from rust.

It may be true that her battery went dead after 3 weeks of sitting. However, it’s not normal. This illustrates the potential pitfall of technical advice from laymen–they often believe their experience applies universally. They’re often wrong.
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See?:slight_smile:
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As others have mentioned, it’s not true for healthy cars. If it applies to a car you own, I suggest have the parasitic drain on the battery tested. If it’s OK, replace the battery.

(It’s fine to have the battery tested also. However, testing won’t prove a battery is good. If it fails, that proves it’s bad; if it passes, it’s probably OK.)