How can I tell if the battery is bad?
 

I just got my wagon's ignition tumbler fixed and the car had been sitting for about a month....Well it seems the battery isn't holding a charge anymore. After about 16hours its "almost" dead again.

I don't see any interior lights on or anything. The battery should still be under warranty as I bought it in 2007....but how can I be sure its the battery? I had no problems prior to it sitting for a month.

A quick easy way to start your check is to check the voltage (with a voltmeter) at the battery before starting (should be around 12.00-12.90) and then after it is started (it should be well above 13 probably).

Bad voltage regulator? Alternator?

Charge it, run/drive it a while, park it, and check the battery voltage after it sits for like 8-10 hours....if it is below 12.56 volts, then a cell (one or more) in the battery is bad, and it should be replaced.

First, did you buy a good battery or a crap battery? If you bought a crap battery and you continue to be cheap, then take it back to where you got it and ask them to test it. If bad, get a new one. If you want to buy a good one, find an Interstate dealer, and have them test the battery.

I tend to use OEM MB batteries or the Duralast Autozone batteries....both seem to be excellent...

take it to AutoZone they will charge it, if needed and test it afterwards. If it is bad I'd only replace it with an BM or Interstate.

Are you sure any bettery below 12.56 should be replaced? My step mom has a battery less than a year old around 12.45. My own is around 12.40. My dads brand new one is 12.8

If the battery is down on voltage that means a cell has gone bad or is in the process of going bad....then it just gets worse. 12.4 is still pretty good. Mainly if its showing like 12.2 or less then its time to replace. One of the other cars in my sig had a 9 year old energizer battery that worked great up until one day, when it randomly died and wouldn't even crank at all. Voltage prior to that had been about 12.25 or so...and it was for quite a long time before the failure.

OK thanks for clarification. Your original post had me a bit worried. :)

I charged it and it dropped down to 12.26 overnight. I guess one of the cells has gone then?

Its a Kirkland battery. Still under warranty so I'll take it back to costco and get new one.

Just seems weird it all started after not running the car for a month. When I first started the car, the antenna was spinning which was really weird too. After we charged the battery it stopped though. I unplugged it just in case that was what was draining it, but still looks like the voltage slowly drops after charging.

I'm not an expert but I was under the impression that 12.26 was plenty of power to start the car. If Costco will swap it out for free then it makes sense to do it. You may have a different issue than the battery though. My battery is typically from 12.3 to 12.4.

Is it dropping like that just sitting disconnected, or while connected to the car? If its doing it disconnected, its for sure a dying battery/cell in the battery, if its connected, there's a possibility something is drawing current.

A more reliable way to test a battery is when it is under load (drawing a large current). You can drive to an auto parts store and they can load test the battery.

Two ways
 

1.Load Test with a fully charged Battery.
2.Fully charge the Battery and then check the specific gravity of each cell.

Both of the above assume Battery is "Topped Off" to correct fluid level with
DISTILLED WATER ONLY! Prior to Charging.

'For the purposes of this tutorial a fully charged Battery is one that has been
on a 2 Amp trickle charge for AT LEAST 36 Hrs.(SLOWER is always better).