My car won't start. I used a multi meter and checked the battery while it was running and it was reading over 14 so I know the alternator is working. I check the battery after I shut the car off and it was reading 12.51 and then I go in the car, unlock and lock the car a few times then try to start it but it won't start. After trying to start I check the battery again and it is 11.77. I have been only driving short distances to and from work the last few months (15 minutes). When I used to drive 45 minutes to work sometimes my battery light would come on and then shut off. Haven't seen the battery light lately because I haven't driven a long distance in the car. I believe my drive belt may be loose too because it makes a chirping noise in idle. I have been meaning to get under the car and tighten it a little because I don't have an automatic adjuster. Don't know if this has anything to do with the battery. I just bought a new battery 3 months ago because I was having this same problem. I haven't had any problem until now. Normally I would think it is just a bad battery not holding a charge but I haven't had it very long. So I am a little puzzled. Don't really want to spend the money to buy a new battery I guess I should have kept the receipt for the warranty!
Does sound like the battery has a dead cell. All batteries have a sticker on them showing the date of manufacture. Any reputable store can use that for warranty purposes.
Also, when testing charging voltage, go to a lower scale with one place after the decimal point for more accuracy. 13.75 to 14.75 volts is acceptable. 13.6 is low but would be rounded off to 14 volts.
You can also put a charger on the battery on the lowest setting for an hour, then see how long it holds up. If there's no change, suspect the battery.
The alternator should also be load tested with a tester than displays "ripple". Most Nippendenso alternators are rated at 90 amps. If you can only get one third of that and the ripple is high, suspect a defective diode. 30 amps is barely enough to run the fuel pump and computers with little left over to recharge the battery even though the charging voltage might appear to be okay.