Good 12v battery losing charge?

First, what a great resource for Prius owners. Grateful to have a group of knowledgeable people willing to help out. Thank you in advance! Now onto my dilemma:

I just got back from O'Reilly's where I had my 12v battery tested. The test came back that the battery was good. The battery is under a year old with less than 10,000 miles on it. It is the SuperCharge battery that O'Reilly sells for the Prius.

I have had issues on a couple occasions now where when the car sits overnight, it is dead in the morning. Not every time, but has happened twice in under a week. I can jump the car fine and it is good until it sits overnight again. I am making sure no lights are left on and no doors are open.

The diagnostic test using the MFD showed 12.1v in ACC (press start once) and then 11.8v when it was under load (press start again without using brake). When I start the car it bumps up to 14.1v.

What is even more strange is that last night after a 30 minutes drive the car would not start after sitting for just a few minutes. It did respond to a jump and I was able to get it home; another 30 minute drive.

After taking to O'Reilly today for the test, the battery is at 12.3v when in ACC. I am starting to record the voltage before and after trips with the car to see if it just keeps dropping.

The car is driven 5-6 times per week. Typically the trips driven are 10-15 minutes long. Could this possibly be why the battery drains? What has me questioning things more is why the car would not start after a 30 minute drive, I would have thought the battery would have been recharged after that long of a drive.

The battery I have has a 3 year warranty on it but the test machine says the battery is fine. Is there something else I should be looking at/for?

Thanks so much.

 

Thanks for the replies. I have searched the forum and other sites regarding my issue. Which is what leads me to believe the battery may be bad.

I guess my question is can the battery be bad but a battery testing machine at the auto parts store show it as good? If so, is there another way they can determine it is bad? They tested it by putting it on the machine for about 5 minutes and told me it was a good battery.

I would prefer to not fork out another $200 to replace the battery as it would be covered under warranty given it is under a year old.

Britprius - thanks for the info. I am not sure I know what you are referring to but will see if I can figure it out and test it that way. I assume a DVM is a digital voltage meter?

Thanks again for all the help!

 

Thanks for the explanation John, much appreciated. I will try to pick up a DVM and get the reading. No downside, I keep losing presets anyway as I have to have it jumped frequently

One thing I forgot to note was that for about 2 weeks prior to the car not starting one morning, I noticed almost daily that the clock was off. The car started, but the clock was off several hours. Not sure if that helps in diagnosing the problem.

 

Thanks John. I see you commented on another post here about the ground wire. I did not run across this particular post until your comment about the ground wire led me to search differently. This sounds real similar to my situation, will check ground connection and make sure the ground wire is clean.

toddrav - I believe O'Reilly brought the battery to full charge when I had it tested today. I too read about charging it monthly and will likely make that part of a maint. routine.

 

Thanks John. I will keep that in mind when purchasing a charger.

 

Some good news. Yesterday, I removed the ground and positive on the 12v battery. I used some sandpaper to clean up the contacts of both batteries as well as the screw and body where the negative attaches. I noticed the ground clamp and screw were a bit loose so I took care to secure them well when I put them back on.

I took a reading using the built in diagnostics at around 4:30pm and 8:30 pm last night and again this morning around 7:30 am (around 45 degrees F). In ACC I got 12.1v and IG-ON 11.9v all three times. The car started this morning just fine and the clock was the right time.

I seem to find conflicting information regarding the reading from the MFD as to what is acceptable. I have yet to get a voltage meter or charger as am low on cash. My plan is to monitor the readings from the MFD and record them to see if I see any voltage loss.

I was also going to purchase a charger but not sure what to get. Any suggestions? I am looking at ones that specifically state for use with AGM batteries but they vary based on amps. Is there too high of amps to charge at? Should I stay with 2 amp or is 10amp okay?

I figure once I get a charger I can make sure the battery is fully charged while I monitor for voltage loss.

Thanks for the help. Praying cleaning the ground and re-securing the clamps solves the issue.

 

Just wanted to report back that it must have been a dirty or lose connection. Thanks for the pointer about the ground. All has been well since I cleaned up and tightened the connections.

Clock has been keeping time. Voltage readings have been good. Funny thing is the radio used to go up and down in volume once in a while. Now that has gone away too and I have consistent volume.

Thanks so much for the pointers and help.