New battery but car acts like it is dead
 

I am on my second car battery in a week. My car started acting like the old battery was dying (dash lights would dim just before car would crack)so I naturally replaced it with a new one and that is where my problems began. The new battery was dead or so I thought (zero power to the dash). I took the battery back to the store and it tested good so thinking I was dealing with a bigger problem I towed the car to the mechanic. They read the battery putting out only 4 volts instead of 12 so they jumped the car and all the readings showed normal. I had no problems for four days. Then I went to start the car and it acted like it had a dead battery (dash lights struggle to come on and car won't crack). I replaced the battery and I am still faced with the same problem.

I can jump start the car and it runs only to die when I turn it off and back on again.

Are the batteries the problem or my car?

It sounds like you aren't getting a charge to the battery. Did they test your alternator? What do you mean by "all readings show normal"? If you are talking about the OBD-II readings, they reset if there is no power and it takes a while for all the sensors to register any problems.

During the 4 days, did you run it at night where you needed headlights? Did they flicker or seem dim?

If the alternator tests good, you may have a loose connection/breaking wire from the alternator to the battery and during those four days the connection was good but then you may have gone over a bump and now the connection is bad.

regarding the alt. I pulled the battery cable while the car was running for a few minutes and it worked fine. Also, I had no signs of losing power with head lights on.

Would lose wiring between the alt and battery prevent the car engine from turning over or staying powered?

I'm not sure which test the mech ran. Only that he found the problem with the battery volts and once the car was jumped it ran with no visible signs of power failure for four days. On the fifth day I went out to start the car, the temp outside was around 55 and the car was dead except the dash lights would turn on with a little struggle but the car would not turn over.

You basically have one of two issues: you either aren't getting a good/adequate charge to the battery or you have something that is drawing off your battery when it shouldn't

I would have the alternator tested or check with the mechanic to see if he tested it. It may reveal fluctuations with the alternator itself that aren't revealed with the check you did. It's a free test a Advanced or AutoZone. It's a ruling out process right now.

The loose connection is a death by a thousand cuts kind of scenario. It could be a wire that is broken, but being held together via the sheath. Think of earphones that cut in and out because of a broken/loose wire. Sometimes you get charge to the battery, sometimes you don't.

I give at least 20-30 minutes to recharge a battery from a near-total discharge after a jump. However, I have turned off then restarted a jumped car within a few minutes (back in my youth) and the car did start, but it didn't have enough juice to do that a third time, so it's within the realm of possibility that for four days you were making just enough connections to charge the battery partly, just enough to keep you going, and on the fifth it was too far gone. Cold temps put an even bigger draw on the battery for start-ups, so that in itself may have been what put the battery over the edge, so to speak.

The other option you have is some kind of other electrical problem that is putting a small draw on the battery. I had that problem with a stripped light bulb wire. However, my problem showed up if I let the car sit for 2-3 days. When I would go to start, the battery would be drained/nearly drained. It started intermittently with a longer sit period (5-6 days) and increased until I finally tracked down the wire.

Check for other things that maybe drawing from the battery. Did you make any changes to the cigarette lighter power jacks? Are they charging something that is still plugged in even though the key is off? Check your doors when they close. Once my hatch wasn't closed all the way (looked it) and that drained the battery over night. No lights were on, but I think it was the security system. If I'm not mistaken, if there is a door that isn't closed all the way, it never goes to sleep as it is always waiting for that solid latch to be made, so it is drawing from the battery. You may have closed all of your doors but maybe one of your sensors is malfunctioning and sometimes it doesn't register properly with the security system.

Unhooking your battery while running is a major no no. You probably blew the regulator in the alternator. With car running your battery voltage should be 13.5 to 14 volts even with headlamps on. With car stopped (engine off) the battery should measure 12.5 volts. I kind of think the alternator is toast.

At this point your smart move might be taking it to the dealer for a diagnosis. Where in Virginia are you?