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Can a bad auxiliary (12v) battery prevent EV system charging?

Posted: Tue Nov 23, 2021 11:34 am
by nickfank
I've got a 2016 with about 50K miles. It suddenly stopped charging. When I plug in the charger, the blue dashboard lights flash for a few minutes & then go off.

I had it towed to the dealer and their diagnosis is that the 12V battery needs replacing. That's reasonably inexpensive, and at 5 years, expected for a lead acid battery. But I'm skeptical... seems like if the 12V battery fault could cause a charging problem with the lithium batteries, I'd be able to find it previously discussed on this forum.

Has anyone else encountered this?

Re: Can a bad auxiliary (12v) battery prevent EV system charging?

Posted: Tue Nov 23, 2021 2:03 pm
by IanL
It's quite possible that the OBC requires 12V power to function. It seems likely that the propulsion battery is "off" when being charged, so it cannot power the dc-dc converter, in which case a dodgy 12V battery could be causing the issue. Most battery outlets will test a battery at no charge, if you want confirmation.

If you replace the battery and the problem persists, the dealer will be out of excuses, and will have to treat it as a warranty issue, I expect.

Re: Can a bad auxiliary (12v) battery prevent EV system charging?

Posted: Tue Nov 23, 2021 3:44 pm
by notfred
There was a firmware update a few years ago that changed the DC-DC converter behavior while charging.
For anything, the 12V needs to get the main contacts online to enable power from the main propulsion battery to drive the DC-DC converter. If the 12V is weak, you can get all kinds of weird behaviour, including brakes almost failing. It's always worth getting the 12V tested, and replaced if it is on the limit

Re: Can a bad auxiliary (12v) battery prevent EV system charging?

Posted: Wed Nov 24, 2021 6:33 am
by nickfank
Thanks for that additional info, especially the note about the firmware update! That may explain why people have not run into this frequently in the past.

I'll definitely report back on how this plays out so we have a searchable topic in the forum for those who run into this in the future.

When I took the car to the dealer, their first action was to apply 4 software updates, so it's very likely I didn't have the newer version that depends on the 12V battery being OK to start charging. However, now that I'm current on the software, it's certainly an issue, albeit one they may have inadvertently introduced.

It's painful to pay $170 to replace a $50 battery, but since the car is already at the dealership 25 miles from my home, I authorized the 12v battery replacement. Then they'll retest the charging problem. I think there's a good chance it'll still have the problem.

Re: Can a bad auxiliary (12v) battery prevent EV system charging?

Posted: Wed Nov 24, 2021 9:04 am
by nickfank
OK, just got an update from the dealership... Replacing the 12V battery didn't solve the problem.

Because I had the problem *before* the firmware was updated, I can't guarantee that this couldn't be a source of problems for anyone with current firmware. I've had the car since 2018, and didn't update firmware until now, so I know my version was at least three years old.

At this point, it seems like Kia is forcing the dealer to try everything before admitting that it's the OBC & hence a warranty issue. Since I know the 12V battery was original, I don't mind paying to replace it, since it was near the end of its expected life... But from this point onward, I'll be expecting Kia to do everything else under warranty.

Re: Can a bad auxiliary (12v) battery prevent EV system charging?

Posted: Wed Nov 24, 2021 11:49 am
by notfred
The 2016's and earlier are notorious for having bad OBC, mine blew earlier this year. I'm surprised that Kia isn't just straight replacing it.

I bought an extended warranty with the car, so I have to take mine in once a year to keep the warranty intact and it gets all the updates. There's been a few over the years, latest one was earlier this year on the FATC.