Not long after purchasing my Gen 1 Mazda 3 (2.0 sport), the battery started to go dead every couple of days, and it has been the same ever since...If I start the car every day then It seems fine, but if I leave it for 2/3 days there isn't enough power to turn the engine over, and leaving it any longer than that makes the battery go completely flat to the point where not even the key fob works to open the doors. I originally thought that maybe something was draining the battery, but I stuck a multimeter on it today with the car running, just to rule the alternator out, and it's sitting at around 15.3v! Which is obviously too high. But what could be causing this? The car is currently off the road so I have plenty of time to sort the problem, but it is soon to be my daily driver, and I would like to pin point the problem before I start throwing money at it. Is the problem really obvious to any of you? Are there any tests that I can do to find the exact problem?

My theory is that the alternator is overcharging the battery, which has caused the battery to die over time, and that's why it no longer holds it's charge. But I could also be completely wrong.

 

Alternator is controlled by the PCM. Remove the alternator, get it tested. Then get the battery tested.

I've had to replace faulty PCMs before for charging issues.

 

Is the PCM the engine control module? Or am I getting things mixed up? The battery is being replaced either way, as it's too small for the battery box, and not the recommended "specs" for my car. As for the alternator, how would I go about getting that tested? Can I test it myself, or does it need to be sent off?

 

PCM & ECU & ECM are the same things.

Testing the alternator is specialised task.