How much driving to fully charge the 12V?

The 12V in our 2007 was replaced by a dealer a little less than 2 years ago. The car has not been driven much for the last year. I try to put at least 5 miles on it every other day. This may not be enough though. This morning my wife drove it to the post office about 1.5 miles away, parked it, and went to pick something up. When she returned there were all sorts of warning lights and it would not go into reverse. Eventually it did. The voltage of the 12V was measured in the fuse box under the hood maybe 15 minutes later and was 11.64V. Very, very low for no load. I then started the car (no warning lights, other than TPMS) drove the car about 7 miles and it was reading 12.29V at the same location. 150 minutes later it is down to 11.93V. None of the interior lights are in the "on" position. I think maybe we just didn't drive the car enough to keep the 12V reasonably charged. How much driving would that be?

I have a 12V battery charger and will pull the battery and fully charge it. (I don't trust this cheap made in China charger not to glitch and blow up something expensive if it was used with the battery in the car.) Hopefully it will still hold a charge, but maybe not if it has been south of 11.64V.

 

Typo, GSM for AGM?

The "Battery Tender Junior" we have will apparently only charge it up to 12.7V, but that will still be a lot better
than what it is at now. Started it a few minutes ago, hopefully this should be pretty quick.

Thanks.

 

Well something is way off, and it is probably that the battery has very little capacity.

Yesterday attached the "Battery Tender Junior" directly to the battery at 13:35 through the open hatch. Originally I was going to take the battery of the car, but then I got lazy and just left it in and hooked up. With the charger active (showing a solid red light for "charging") measured 11.65V. Lowered the hatch (struts are nearly shot) onto a block of wood and let it charge like that until 22:30. The light was still solid red but after unhooking the charger the battery read 12.65V. That is 9 hours of charging at a nominal 750mA. No way the charge level should have changed that much for so 6.75AH of charge.

A brief detour. How much current is drawn from the battery when the car is off but the hatch is open? I have a UT210 clamp on DC ammeter and this seemed like the right time to answer that question. So this morning I measured that, charger not attached, as 520mA. In another thread I had measured the current on a door sensor and it was over 300mA, so if the hatch is similar this is about right - 300mA for the hatch sensor and another 200 mA to drive whatever electronics wake up when a door is open. Then I folded myself into the hatch area, had my wife close the hatch, and kept measuring. Current dropped quickly to something in the 100 mA range (it was never steady) and then fell from there to 15mA where it stabilized. So the car does not have a large "off" draw, so long as all the doors and the hatch are closed. I also measured the DC current going into the battery when the charger was attached, which was about 800mA (but not quite stable). This tells us that yesterday's charge was even lower than expected, at a rate of only 300mA (800 in, 500 out), or
about 2.7AH.

At 9:10 this morning I measured the voltage again, it was 12.09V with the hatch open and the charger not attached. Hooked up the charger through a window (showed "red") and closed the hatch. So the car should only have been drawing 15mA from the battery instead of 520mA. Came back at 10:00 and the charger LED was green ("fully charged"), which should not be possible. Couldn't get back to it for a while and then made these measurements in quick succession:

10:40 (at the battery)
12.81V (hatch open, charger on)
12.68V (hatch open, no charger)
12.57V (hatch closed, no charger)
12.65V (hatch clased, no charger, 10s after the preceding)

10:48: (at the test point in the fuse box under the hood)
12:40V

In short, the 12V battery seems to have almost no capacity.

I noticed a small black tube which comes off the battery near the positive electrode and disappears down into the car. What is that?

 

I'm going to replace this battery next week. In the meantime, a few more measurements (all in the
fusebox under the hood)

3/6/2021
11:45 12.34V.
(Drove 11.4 miles in about 25 minutes)
12:22 (measurement odd, car made a whirring noise and the voltage dropped below 12V and then came back up,)
12:23 12.39V
17:00 12.27V

3/7/2021
10:45 12.14V
11:45 12.12V
(drove 10.3 miles in about 25 minutes, pop hood, lock car, wait 3 minutes)
12:15 12:54V
12:20 12:57V

So yesterday the battery didn't charge up a significant amount in the same amount of driving (distance and time) that today resulted in a reasonable amount of charging. Bizarre.

AGMs fail if they are overcharged. How does the Prius keep from doing that?

 

It shows that the battery does not charge consistently. In one case, not at all, in the other a shift of about 22% SOC, from roughly 62 to 82%. That last one is way out of spec too I think. For a 45 AH battery a change of 22% SOC would be 9.9 AH, and that was around .37 hour, which would need 26.8 A. No way the inverter was sending that much to the battery, so the actual capacity is much, much lower than 45 AH.