Auxiliary battery showing 12.0V, replace?

A couple hours isn’t enough. If you don’t have access to a charger, you’ll have to leave it in ‘ready’ for about 15 hours.

it should be fine, but if that doesn’t get it to 12.4 or better, replace it.
If it does, then get a free load test at auto zone. they might even charge it for free, idk

 

A couple of hours might not be enough for the onboard system to fully charge a dead battery.
Use a real battery charger or a tender with a capacity of 2 amps or more.
Leave it connected for 12 hours minimum.

You can not trust the onboard display to give you an accurate voltage reading.
An external voltmeter is needed for that.

 

Standard load tests are not worth much for these batteries. Those pull out a certain current (based on the CCA rating) for a short time and look for a voltage dip which is too extreme. That makes sense when a battery must turn a starter, but these don't. And yes, if an AGM fails that it really is bad and must be replaced. These batteries go bad by losing capacity, not because they cannot put out enough current for a starter. When new they are 46 Ah. The battery in my car now has only around 12.5 Ah left. This based on an AGM SOC table and measured voltage before and after a 15 minute load at 5A. Also it charges from very low to full much too quickly. Yet this battery passed an Autozone battery test, and it will pass a HF Viking battery tester, this one:

12V Digital Battery and System Tester

The issue with having such low capacity is that it is even more sensitive to an error (leaving the lights on), to short trips, and to just sitting unused.

 

The onboard system is not great because it is only available when a great deal of current is coming out of the battery, which depresses the voltage. It isn't the resting, open circuit, voltage, which is what you generally want. READY mode is even worse, since all that shows is the inverter output voltage.

Forum consensus is that the best way to measure the auxiliary battery's voltage is to leave the hood unlatched and to measure the voltage the next day at the jump point (and any convenient chassis bolt.) It will have sat long enough for the surface charge to have bled off, and the car will not have recently been activated (by unlocking it), which turns on various systems for various times, producing an unknown load on the battery. If the car uses SKS leave the fob far away to avoid a similar unknown/unknowable load.

If you must know the voltage immediately after driving the car, I suggest the following. Pop hood. Roll down the driver's window. Turn off car, close all doors, and lock the car. All car lights should be off. Wait at least 5 minutes. 10 is probably better. Reach through the window and turn on the headlights for 30 seconds (remove surface charge), then turn them off again. Don't open the door, or worse, unlock the door! Wait a couple of more minutes (the voltage on the battery will be rising from the headlight load for a while.) Measure voltage at the jump point. If the voltage is still rising you didn't wait long enough. If the voltage goes higher than 12.8V then the headlights were not on long enough to remove all surface charge.

 

Whew...here iis what I would do...

Remove battery from car. Take ot to Auto Zone -- or reasonable other store.

Have them test/charge/evaluate battery for free.

Depends on on results, but...

Sorry for the typos. My compromised hand is...
Kris

 

My sense : regardless of the battery’s duties, the load test of CCA is a good indicator of the battery’s health and reliability.

 

THIS is why you can not depend on a single quick test, no matter what tester is involved.
The test load needs to be applied several times with a few seconds in between to see if the subsequent tests show a significant drop in the reading.
But almost nobody does that.
So many attempts at testing are just a waste of time.

 

Most "battery experts" don't really know how to test a battery.
And some stores will automatically fail EVERYTHING.