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I ran across some more tech information on 12 volt battery systems in electrified cars. While the Kia PHEV does seem to have intermittent charging while on, some cars do appear to have constant DC/DC 12 volt (nominal - usually more like 14v) voltage while on (watched a realtime Prius Prime diagnostic) to power car systems and charge the 12 volt battery.

Another interesting tidbit explaining the relative life of ICE lead acid batteries versus EVs is that they are used to start the motor on ICE only cars. This high current condition helps to break up deposits that have formed on the battery plates. This helps explain why using a smart trickle charger for regular maintenance on EVs performing a similar break up of deposits can lead to avoiding early failure of 12 volt batteries.

While it is certainly well known that low voltage conditions on lead acid batteries shorten their life (and this does appear to happen on a number of EVs including the Kia PHEV), it appears a major factor in the reduced life of EV 12 volt lead acid batteries versus ICE cars is simply not using the batteries in a similar way. Thus using a smart trickle charger will help extend life, as many users report in EV forums.
 

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In my previous post (#38), I pointed out that I had put my PHEV 12V battery on a smart trickle charger, and two days later when I first took the car out for a spin, I saw the BS+ alert. I think there's something else going on here.

At the same time, I think yticolev's advice to occasionally put the 12V battery on a smart charger is probably a good idea.

Today I took my PHEV in to the dealer to get this checked out. The good news is that they didn't charge me anything for checking out my car. The bad news is that they concluded the car was normal (even though I told them that BS+ alerts come on every time I take the car out now, and that didn't happen when the car was new and the owner's manual says that they should not come on frequently). The technician's "solution" was to turn off the alert, but I told the Service Advisor I'd rather keep it enabled, so they turned it back on. I'm not sure if they had disabled BS+ (which anyone can do) or if they had used some sort of programming to disable just the alert. When I drove out of the dealership, the BS+ alert came on (but I sort of expected that, based on what they told me on the phone). I also noticed new behavior on my navigation system, which leads me to wonder if they installed a software update (no mention of that in the paperwork though).

According to the paperwork that was handed to me, the State of Health of the 12V battery was 100%, and the State of Charge was 87%. I'm kind of thinking that 87% SOC on the 12V isn't necessarily normal, but it might be.

On a related note, I have a suspicion that my fuel pump might be running for several minutes while the car is parked, for no obvious reason. See What's that humming sound? for more on that possibility.
 
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