Should I jump start my car immediately? Or can I leave it till tomorrow?
August 29, 2014 10:17 PM   Subscribe

Just went out to the family car to find it has a flat battery. Such a lovely day outside, I really don't feel like jump starting it, and I don't need it today. However, I was speaking to my father (who's overseas, I'm house sitting) and he'd prefer I do it today as car batteries shouldn't be left in an uncharged state as doing so does damage. Is this true?

Additional details: the battery has probably been flat for about four days now -- I just haven't used the car. Its also a brand new battery.

It's going to be a little difficult to jump start the car as it will need to be pushed out of the narrow driveway (a two person job). Tomorrow I can get a couple of friends to help.
posted by tomargue to Home & Garden (12 answers total) 2 users marked this as a favorite
Best answer: What if you hadn't discovered it until tomorrow? I am not a mechanic, but I have dealt with my share of dead batteries. I would wait until tomorrow if you don't need the car today and don't want to deal with it. If it is a new battery, check its warranty. Maybe you can get a new one rather than having to jump it.
posted by 724A at 10:32 PM on August 29, 2014

Best answer: It's true that sooner is better, but I doubt one more day will make much difference in this case. Also, unless you know why the battery ran down, you've either got a bad one or there's something wrong with your charging system, so you'll need to deal with that.
posted by in278s at 10:43 PM on August 29, 2014

Best answer: Are you going to need to replace the battery, or is it dead because you left the lights on?

If you're going to need to replace the battery (ie, the battery died because it's old), then you shouldn't jumpstart it now, because there's no point. It's not going to be able to recharge even if you drive it around for a while after the jump, so best case scenario you're going to have to jump it again before taking it to the mechanic or replacing the battery, and worst case scenario, it poops out on you while you're still driving around.

If you drained the battery through a mistake, like because you left the lights on, then sure, go ahead and jump start it if you want. Or don't. The amount of time the car has been left with a drained battery (granted, I'm thinking in terms of hours or possibly a few days, not weeks on end) never has seemed to make a difference in terms of the amount of effort/time it takes to jump start it, at least in my experience. And once you jump the drained battery, you have to drive the car around for a while to recharge it, but once it's recharged, it's recharged and you don't have to do anything (you don't have to take the car for a repair or anything). For what it's worth, I drained my battery a couple times by leaving the lights on and it didn't seem to affect the life of my battery one way or another, either.
posted by rue72 at 10:46 PM on August 29, 2014

Response by poster: Thanks guys, I'm going to spend the afternoon in my hammock :). I love ask mefi!
posted by tomargue at 10:46 PM on August 29, 2014 [1 favorite]

Jump it when you have time. Dead battery is a dead battery. You'll need to run the car for a while to get the battery recharged, so do that when you have time, all before you need it. Easy-peasy.
posted by Sunburnt at 10:48 PM on August 29, 2014 [3 favorites]

Maybe you can get a new one rather than having to jump it.
Warranties are for manufacturing defects. Flattening a battery is a user or application issue.

What your father said is correct - lead acid batteries don't like being left flat and you will have significantly reduced its life by leaving it that way. If it's already been 4 days, then another day will be 20% of the time left flat. I would charge it as soon as possible, but it's up to you.

What happens with batteries is that they have an electrolyte that is (assuming a wet cell battery, fully charged) dilute sulphuric acid. As the battery gets discharged, the sulphur combines with the lead plates; as it charges, the sulphur is kicked off the plates and back into the water. Each time you do this (a cycle) a little bit of sulphur stays on the plates. If you leave it flat, the sulphur is harder to get off the plates because it tends to crystallise.
posted by mewsic at 10:53 PM on August 29, 2014 [5 favorites]

Make sure the new battery that was put in recently is the right one for your car before you try to jump it because the battery may have power, it just doesn't fit right to the engine connection.

I had this problem recently and it turned out that AAA had installed the wrong sized battery for my Mini Cooper. The car started fine for a few days and then the cables disconnected because it was too small.

I thought the battery was dead; it wasn't, it was just the wrong battery.

Also, if you're in an area where there's AAA, I would call and have them do it. You can become a member when you make the call. Just make sure they put in the right battery.
posted by kinetic at 2:44 AM on August 30, 2014

If the battery has had a cell go completely bad you may not be able to jump start it today or tomorrow.
posted by COD at 5:08 AM on August 30, 2014

This being a Saturday, I'd probably want to jump start it today, and take it to a shop that might be open and do a charging system check. You might be in need of a new alternator.
posted by Thorzdad at 6:31 AM on August 30, 2014

Dead battery is a dead battery.

No, this is not true. A lead-acid car battery that is in "deep discharge", below 11.5V or so, for a significant length of time, will build up sulfate corrosion on the plates and be unable to recharge.

That is what OP's father is alluding to, and it's why lead-acid batteries should always be stored fully-charged and disconnected from anything that could discharge them.

When I have been in this situation, I have just hooked the battery up to a cheap 12V battery charger (normally $30, $20 on sale from KMart or similar) and left it there overnight. It's easier than jump-starting and doesn't involve driving around in circles to recharge the battery from the car's alternator.

And then you can go sit in the hammock.
posted by Kadin2048 at 9:15 AM on August 30, 2014 [4 favorites]

If the battery has a dead cell, you can jump start it and it will run just fine. You just won't be able to start it again once you shut it off. The spark for a running engine is provided by the alternator.
posted by humboldt32 at 10:17 AM on August 30, 2014

Probably not relevant, since it's August, but if a battery is discharged it can freeze in cold weather. It can be jump-started and all seems well. But, in about 3 months it will be totally shorted, a jump-start won't work, and it will be time for a new battery. Maybe somebody here can explain the mechanism.
posted by H21 at 12:33 PM on August 30, 2014

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