New 12V: car won't start after 2 days of sitting

There's a possibility something is draining the battery. Anything plugged all the time? In particular something new? Any recent accidents, mods, service?

Getting it checked with an electronic load tester will give more insight. You can also check for phantom loads.

 

probably wasn't fully charged upon installation, happens too often.

where did you buy it?

who installed it?

what brand?

 

A fresh/new battery from dealership should be ok without charging up. It doesn't hurt to charge it (I typically do), but it shouldn't be necessary. If you just bought it phone them, maybe they'll waive the testing charge (no pun intended). They use electronic load testers, can tell in a few seconds, the battery's Cold Cranking Amps, a good measurement of it's health.

If it turns out there is some phantom load drawing it down (and the old battery too, probably), the diagnostics for that could get pricey.

There's prosumer level testers (Solar BA9 for one) reasonably priced, that you can do the same test the dealership would do.

Is your dash cam possibly running when the car is off? If so, that's the smoking gun. Again, checking for phantom loads is the way to determine that, if you're not sure otherwise.

What I've done, to check phantom draw:

1. disconnect neg battery cable.
2. connect lead to negative post.
3. connect lead to ground.
4. run both leads out of hatch.
5. close hatch.
6. wait 30 minutes.
7. connect multimeter to leads, set to highest amperage scale.
8. drop down to lower amperage scale, then milliamps. (to avoid blowing meter if there's a large draw)
9. See what sort of milliamps are flowing. I saw around 15~20, with occasional spikes to around 40.

That's normal I think.

 

Note, when you're doing this you're actually powering the car through those skinny leads. When you reopen the hatch, the juice to do that is running through those leads. It's ok for that, but be cognisant of it, don't power up anything else, get the test leads off the proper cable reconnected ASAP.

Instead of flimsy test leads you could maybe use a jumper cable, as long as the hatch is capable of closing on it. That's the thing: you want the car all shut, asleep, so you can see what's still needing juice.

 

the only way to check it is with a volt meter or the mfd display. the only way to charge it is with a charger, or jump it and let it sit ready for 8 hours, or drive it at least 4, then recheck the voltage after it sits four 4 hours.

 

wow - we just had a Corolla roll in with this issue today. Anyways, typically these batteries have a charge on them at the time of purchase, but quite often the tech's need to put a small charge into it. So my guess is the battery didn't have a full charge. If your drive home was short it has probably just ran down enough that it can't do it's job. There is talk above about a "draw". Not a bad idea to check for that I suppose, but there will always be some juice being drawn out of it with modern cars.

If you or a friend have a 12v charger, I would go ahead and put it on charge for a few good hours or overnight.

Will add, if you typically don't drive this vehicle very often, it might be a good idea to get a battery maintainer. They are very cheap ($20 or less) Hook it up and then plug it in if you know you won't be using the car for awhile.

 

Ours spends most of it's time on a charger.

 

I was afraid he would skip the possibility that he blows the internal fuse of the meter (or worse, smokes the meter because it does not have this internal fuse...), but he was quite thorough...
Cars might/will have higher current draws when you unlock them, when lights or on, when they sence the presence of the remote-unlocking-key, etc. And this current draw might spike way beyond what the meter can handle. So I often start with the below setup:



Most current will go through the GREEN lead instead of through the meter. Once everything settles down, I remove the green led and all the current goes through the meter, showing the actual current draw.
The fuse in this meter is a very fast acting 20A fuse that was broken when I got the meter secondhand and set me back >10$ and a trip to a speciality store, so I do not intend on ever replacing that again...