Battery is good (fairly new, and tested yesterday) and alternator is good (tested yesterday), but after driving, battery is not fully charged. Fully charged should show at least 12.7 V, but I see typically 12.2 - 12.5 V right after driving. By the next morning, it drops to 12.0 V, and I recently had a case where the car would not start after a few days parked.

I verified that when first started, the battery is being charged. I observe about 14.5 V across the battery terminals after starting the car. However, it seems to stop charging prematurely, leaving the battery dangerously low after driving.

I checked current being drawn in the parked state, and I'm only seeing 60 mA or less (after a few minutes), so I don't think this explains the weak battery after being parked. It simply isn't being charged enough while driving.

Anyone know what the problem is, and how to fix it?

 

How about your secondary battery? And what is your main battery? (brand/date of manufacture/etc)

 

Main battery is Nationwide Superstart Premium Standard 91 T6 - 91PRM, purchased within the last year from O'Reilly Auto Parts (using their recommended size). I don't know the manufacturing date. Aux battery was replaced just two days ago with a new PowerSonic PS-AUX12 from Batteries Plus. I replaced the aux battery because the start/stop function also does not work for quite some time now (a year or more?). The old aux battery that was removed had a full charge (12.74 volts) when it came out, so I actually doubt that it was bad. However, being 7+ years old, I figured it was likely nearing end of life anyway and good to replace. Replacing the aux battery had no effect on the main battery charging situation -- the problem I have described here occurred the same before and after aux battery replacement. Start/stop still does not work (not as urgent an issue as the main battery charging problem).

Added 1/3/25: To try to understand a bit more what's going on, I disconnected the main battery and charged it up to 12.8 V (not necessarily full charge, but decent). Car sat ~30 min with no main battery, so fully powered down. Reconnected charged main battery, and tested car. For the first time in a long time, Start/Stop shows as "available" and works properly. After 30 min drive, battery is down to 12.6 V, so not yet clear if it is charging better or not. Will know by tomorrow after some more usage. The thought behind this test is that perhaps a battery needs to be fully charged when first installed, since system reboots and might measure battery voltage as a reference level relating to charging. We will see if installing with decent charge helps.

Added 1/4/25: Didn't solve the problem. This morning voltage was back down to 12.2 V. Car started OK, but start/stop no longer available (looks like it simply disables it when the main battery voltage is too low). Some additional data: When I start the car, battery voltage is 14.7 volts, which should be charging nicely. When I am done driving, if I check the battery voltage before I stop the motor, it also reads 14.7 V. This makes it appear that it has been charging the whole time while I was driving, but as soon as I shut off the motor, it falls to 12.6 V (not fully charged) and overnight it drops to 12.2 V. Does it charge only intermittently while driving? Is the car draining the battery at some point during the night when I'm not checking battery current? I may need to figure out how to use a data logger both the monitor voltage while driving (to see if it charges continuously as it should, at least until battery fully charged), and to log the current overnight to see if something happens that drains the battery overnight. Mysterious!

 

Follow up: Didn't get much help here on this forum, but kept working on things and eventually found that the battery was the problem. The replacement battery I had was installed in November 2023, and was only 4 months on the store shelf at that time (mfg date 7/23), so it should have been a good battery. However, I noticed that replacing the battery at that time didn't make the car fully happy -- the stop-start system didn't work. At that time I suspected I would also need to replace the aux battery to get that working, not yet suspecting that my new main battery was actually no good. Since the car worked OK except for the stop-start, I left it that way until this month. Replacing the aux battery did not fix the stop-start system, and the aux battery seemed to be fine when I replaced it (it was sitting at 12.8 V, which indicates a pretty good battery). I now know that when the stop-start system isn't working, the main battery may be the culprit, not the aux battery (although the aux battery may also need to be in good shape).

Since the main battery was showing a low voltage at rest, I suspected either the battery was bad or the car wasn't charging it properly. When I took the battery back to the store, they tested it and declared it good, so I then investigated the car's charging system. The store also tested the alternator and declared it good, so between those two tests, the testing at the store didn't make sense. I further checked the car's charging system by monitoring the main battery voltage while driving, and found it to be stable at 12.58 V for an entire 15 minute trip on the freeway. This should be charging the battery just fine, but the battery at rest showed low voltage. Based on this test result, I concluded that although the store's tester said the battery was fine, it most likely wasn't.

I purchased a new battery, this time a size 94 AGM battery with 850 CCA. After installing it and resetting the car's Battery Management System (BMS), everything is now fine. The stop-start function of the car has worked from the moment I installed the new battery, and after a day of driving (about 60 min total), the at-rest battery voltage has increased from 12.55 V to 12.8 V, so the battery is charging properly.

So what do we learn from this? Well, the internet "lore" is that you need a really big expensive battery for this car to work properly. Although that is what I now have installed, I doubt that has anything to do with it. Bottom line is that the new battery I bought in Nov 2023 actually was defective, with too low at-rest voltage and inability to hold a full charge. I doubt that the store really tested the battery properly (they tested it again when I returned it and still said it was good, although they were nice enough to give me my money back under warranty, so I didn't pay too much for the new upgraded battery). Although they used a tester capable of a cold-cranking amps (CCA) test, I noticed that the heating coils on the tester (used to simulate the starter load) never gave off any heat, so either they didn't do a proper CCA test, or the tester had alternative methods for testing CCA (such a pulsed internal battery resistance testing). In any case, the tester did not provide a useful result to identify the real problem.

For me, proving to myself that the car was charging properly (by monitoring battery voltage of 14.5+ V for the full duration of a trip) was the evidence to prove the battery was actually bad regardless of what the tester said.

And now the car is working happily.