Battery not keeping a charge

[joseturner88]

Hello, I'm new to this forum and i been having some trouble with this 2006 m35x. My car stop driving one day while i was driving went and got it towed back to my house and jump started and it died short after i jumped it. So i went too get the battery charge and auto zone said the battery was bad so it is still under a warranty and with interstate battery. So i got a new battery and i thought i was good. But my voltage keep going down and not going to a safe zone around 14v it sitting around 12v after the car running. So i go to whats next the alternator and went to get it tasted and auto zone said it was good, so i didn't believe them so i took to another auto zone and they said it was good too. so i started searching anything that can help searching. not finding nothing. I checked the wire and it looks ok i don't see nothing exposed. please somebody help i don't see what i missed or I'm doing wrong?

[frapjap]

Welcome to NICO!
Bummer that you're having trouble with your M.
I'm not all that familiar with the car, but here at NICO, we have Factory Service Manuals available free to members. Take a peek at the link in my signature and find the FSM for your car and you'll have all the info you'll ever need to make the repair and try to diagnose it for yourself while help is on the way.

In the meantime, I'd check the grounds and see if any are broken or disconnected.

[Rogue One]

[Rogue One]

Q: So is it true I should have a 12.6V reading on a fully charged battery anything less it needs to be charged? And it shouldn't dip under 12.6 in less then 24 hours?

A: You misunderstand the basics... it isn't the voltage that matters as much as the available amperage. To use a classic analogy of water to electricity, voltage is like the water pressure in a pipe, while amperage is like the size of the pipe. All other things being equal, the work that a stream of water can do has more to do with the size of the pipe.

That's why a simple voltage check of a multicell wet battery doesn't tell you much, unless one of the cells is bad, which a voltage check can turn up. What really matters, in terms of cranking power in a car, is the total current flow available, aka the amperage, which can only be checked under load.

Here's a quick thought experiment that may help you to understand... if you stacked up 8 AAA batteries at 1 1/2 volts each, that would total 12 volts. Add one more and you'd have 13 1/2 volts, way above what you think you need... but try to start your car with it and you'd get nothing but a stack of warm batteries as they discharge in vain, because they aren't anywhere close to being able to produce enough current for the job.

In contrast, if you had a couple of old auto batteries lying around that only measured 11.5 volts each and couldn't quite crank a car by themselves, wiring them together in parallel to double the available current could probably do the trick.

[PapaSmurf2k3]

Yeah I was going to say, it sounds like what the OP is talking about is normal operation. With the car on/running, you should see about 14v or so. Once you turn it off, you no longer have alternator voltage, you JUST have battery voltage. If anything is on (dome lights, ecu still active, door open, security system), you're probably only going to see about 11.9-12.1 volts.

Basically what I'm saying it, it sounds like your system is operating normally now after the battery change. Its not dying anymore, right?

[lovemycarM35]

It's your alternator