I don't drive much. How can I keep my 2013 Prius c 12v battery in good shape?

Update: For context, today is Sunday. I got the new battery installed last Monday, charged overnight, and removed the charger (Battery Tender Jr.) on Tuesday. The car has been sitting since then, so for 5 days, with only one short trip on Thursday. This afternoon, I tested the voltage of the battery at the terminals with my multimeter, and it read 12.38. I also tested by putting the red lead from the multimeter into the positive part of the quick connect harness, and touched the black lead to the metal door latch (the body of the car didn't work, likely because of paint?), and it, too, showed 12.38, so now I know that I can, indeed, successfully test voltage via the harness without having to pull out the battery. Yay!

I also tested the voltage again in the cigarette lighter with the key turned to Aux, and it again showed 12.2. So, from this, I'm guessing I can surmise that when I test in the cigarette lighter, I can add .2 to the reading to get the real voltage, which is even more convenient.

I'm guessing/hoping that a voltage of 12.38 on a new battery that has effectively been sitting for 5 days is okay?

I also want to take this opportunity to thank everyone who offered help with this. I really appreciate it!

 

Well, the weird thing is that the first reading I took, at the terminals, showed, I believe, 12.52. Then I shifted the battery a little, checking the vent tube and ground connections, and then I took more readings. It was 12.39, and then after that 12.38 a few times. This all in a matter of a few minutes. Very strange.

I'm assuming it will be fully charged tomorrow, and I'll check voltage again. And I guess I'll check once a day for a few days to see if it's draining for some reason, though I can't imagine how/why, since nothing is on, or plugged in, and doors are closed and locked, and no inside lights are on.

How long should a battery hold a full 12.6 or 12.7 charge when sitting in 35-55 degree (F) weather?

 

No, it was after sitting for 5 days, being driven once to the store and back in the middle of that, about 10 min. each way. After its first charge, it showed 12.7. But I don't know if I can trust that 12.52 reading, since the subsequent ones were lower. But maybe the 12.52 was the real voltage. I need to do more testing this week...

 

Update: Okay, so I left the battery charging overnight. This morning the light was solid green. I disconnected the Battery Tender Jr., and when I tested with my multimeter at the harness, it showed 12.9 (which was a relief). Then I opened the front door, and the car made that usual whirring sound (which surely uses a bit of power from the 12V battery), and I turned the key to Aux (maybe a little more power?), and took a reading with the cigarette lighter gizmo (drawing a little more power from the battery), and that read 12.5. So, maybe that's a "full battery" reading for this gizmo. Time will tell with repeated tests like this one. Then I went back and tested with the multimeter again at the harness. This time it showed around 12.8, which I'm guessing makes sense, given everything I just did.

It's hard to know how long this supposedly new battery sat on the shelf before Toyota gave it to my mechanic to install. I also can't easily find the manufacture date on the battery, given how hard it is to maneuver in its installed location. But, I'm hoping that these readings indicate that the battery is healthy. Out of curiosity, I'm going to take a reading at the harness every day for the next few days while the car is sitting, to see how much the battery loses each day. Unless I'm mistaken, I'm guessing it's normal for that to happen (since it's drawing a tiny amount of power from the car all the time). I mean, one should not expect a battery to remain at full charge voltage as days tick by, right?

 

So, here's a family straightforward question: After fully charging a Prius AGM battery with a battery charger, what should a multimeter show after sitting for a couple hours? How about overnight? And then after sitting for 2 days? After 3 days? After 4 days? What should these voltage readings be for a newer, healthy battery (keeping in mind that one might be remotely unlocking and locking the doors before taking measurements each time, which will draw a bit from the battery as well)?

 

But I've been reading that that will actually be a residual "surface charge" from the charger, and that if one leaves it for a few hours, or overnight, the reading should be a bit less.

Anyway, I think you're right that just checking voltage can't necessarily tell you the health of a battery. I'm getting an oil change soon, and I'll ask them to test it then, since I don't have the equipment.