I just got back from a 3 week vacation and my car starts fine. but the next day my car bat. got weak. I got a jump from my neighbor and it works fine until the next day when it went weak again. My bat has a green eye indicatior and it is green. So what is going on?

 

check the alternator with a mulitmeter..

 

if the battery is fairly old, that 3 week rest may have been enough to pretty much kill it. take the battery to autozone or any car store and have them test it (most places should do it for free), I would think it's just the battery, cause letting sit for a long time without driving it will do that.

 

Bad battery or bad alternator. Maybe both...

Before you do any of this checking, make sure your battery cables & ground cables are OK. Clean & tight at both ends, & look for corrosion. Including the cable that grounds your engine/transmission to the chassis.

After you start it, let it idle & measure the voltage at the battery. It should be around 14v or so. Then leave it idling & turn on all the lights & fans & defoggers. It should still be around 14v. If it drops much, then your alternator is weak.

Now check the battery. After driving around for 15 minutes, it should be charged up. Measure voltage when it's idling, like above except don't turn on all that stuff. Then turn it off. Turn EVERYTHING off. Now the battery voltage should be something like 12.5v. Then wait a few hours or overnight. If you never turn anything on, it should still be at least 12v tomorrow morning. Turn on your headlights, without starting the engine. Now the voltage should drop maybe 1/2 volt, so it'll be 11.5v or 12v. If it drops too much then your battery is going bad.

 

while the car is running my meter shows 14.5 volt so it is not the alt. probably the battery because before my vacation, it was fine.

I went to kragen and they tested my bat and it was bad. The guy said that the diag. tool could tell if my bat needs a charge or it has a bad cell. How does the tool know if my bat is bad or needs a charge?

Anyway, i bought an exide bat for $61. Is exide a good brand? I was desperate because if i didn't buy it, I would have to tow my car back

Why does not using the bat for a long period of time make it bad? I heard that you could bring it back to life by using a charger and charges it for a day or two. But i'm not sure.