12V battery keeps going dead

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Apr 27, 2021
I have a 2018 Soul EV and it has a very intermittent problem that seems to be getting worse. Sometimes we will shut the car off to go into a store — maybe only 15 or 20 minutes — and when we come back, the 12V battery is dead. It has passed a load test with flying colors, and most of the time there is no problem. We have been very careful to be sure we are shutting down properly, not in accessory mode,

When it goes dead, I can put a voltmeter on it and I’ll get between 3 and 8 volts — clearly dead. I can easily get the car restarted with a small battery pack, and be on my merry way. Once started, it will recharge from the main battery, or I can recharge it by plugging in.

But the other day, it wouldn’t start after a brief trip to the grocery store. I jumped it, but when I got home (15 minute drive) the 12V was still dead — not even enough juice to open the charging door. When I put an amprobe around the battery cables before plugging in, it read 3A discharge rate.

Perhaps there is a relay somewhere that occasionally doesn’t make the connection between the batteries? When you are plugged in, does the 12V charge thru the main battery, or charge directly through the on board charger?

The 3A discharge rate would seem to indicate that something is still on, even with the 12V dead. I can’t hear anything running, and I thought everything except the propulsion motor ran on 12 volts. Can the DC-DC converter run in both directions, discharging the 12V into the main battery? Seems unlikely, but I can’t think where that juice is going.
The route for power into the 12V battery is through the dc-dc converter, which receives current from the traction battery when not recharging the car. When the car is recharging some of the current from the OBC will go to the dc-dc converter, and some to the traction battery, but at what stage this happens, and under what conditions is not clear.

The 3A you mentioned when the car was switched off and not recharging does not seem to me to have an innocent explanation, i.e. there is a fault. I suggest you go to the dealer for warranty attention.

EDIT: Just had a thought. It is quite normal for modern vehicles to register a sizable battery drae after switching off, fo a few minutes. You should close all doors, switch everything off (make sure preheating and charge programming is off), wait (say) 3 min) and then measure. Having said that, I still think 3A is excessive.
I think you have a bad 12V battery, and taking a 12V car battery down to 3V will kill it if it wasn't dead before. I'm surprised it's passing a load test, although I believe some modern testers don't actually put the battery under load and instead a doing some form of digital analysis.

IanL is right about the car taking a while to fully power down after being turned off, I'm sure I read in one of the manuals that it can be up to 48 hours. I think that's for it to get fully powered off, I think it has multiple stages of turning things off the longer it goes. It is noticeable that if just powered off for a short while then the infotainment comes straight back on as it has really just blanked the screen, but a longer while means that it has to actually has to boot up. Similarly I have the door mirrors that auto fold when locked and those will flap out as you walk to the car if it has only been a day or so, but if left for many days then they stay in until unlocked. 3A isn't much power (36W) if all the computers are still up and running and only have shut off the screens.

Definitely agree with leaving it longer with everything closed and locked while measuring the current drain, I would tend towards more an hour than 5 minutes, and see if that current draw drops.
Thank you gents — very helpful.

Had another event last night. The car was left on the charger (inadvertently to 100%, usually I stop it at 80 - 90%) until it shut itself off, but I left it plugged in. It was plugged in but not charging for about a day, and when I went out this morning it was dead. About 5 volts with about a 2A drain. I jumped it, let the 12V charge thru the main battery for a couple hours, then turned it off.

As you guys suggest, there was about a 2A draw immediately after turning off, but about an hour later it was down to about 0.2A. Voltage was at 13.2 after that hour, which seems pretty good. In fact, that voltage makes me wonder if the main battery is still connected in some fashion.

I don’t think the 12V battery is bad, because sometimes it will go for a week or two just fine. Other times it will drain quickly.

The only Kia dealer within battery distance can’t get me an appointment until late Feb. They only have one tech who has been to EV school. Called a Hyundai and Nissan dealer, but they won’t touch it. There is a local independent mechanic who seems interested in EVs, but not sure if I should try him.
CaptD said:
There is a local independent mechanic who seems interested in EVs, but not sure if I should try him.

Not sure that would help. He would need factory test data and the KIA dedicated test equipment (I think it is PC plus software, but afaik, the software is not in the wild).

As you guys suggest, there was about a 2A draw immediately after turning off, but about an hour later it was down to about 0.2A. Voltage was at 13.2 after that hour, which seems pretty good.

That sounds good to me, but check again a few times, to see if you can catch an intermittent problem.
Just wanted to get back and say that I think notfred was right — simply a failing 12V battery. The first time I had it load tested, it was still in the car and within a minute or so after turning it off. I suspect that it may have been still connected to the big battery in some fashion.

The second time I had it load tested (same Napa store) it was out of the car. It was holding voltage, but failed the load test. They sold me a plain vanilla sealed lead acid replacement, and everything has been fine since (about 2 months).
Good news. You'd think a battery which doesn't have to crank a cold engine would have a longer life, but I had to replace my late-2017 battery at less than 3 years. I fitted a Yuasa, which is what Toyota use, in the hope that it will do better than batteries KIA fit, which, incidentally, are more than twice the price.
Had the same thing happen with my 2016 after about four years of owning the car. Sorted it out pretty quickly and purchased a new battery that is still working well after 3+ years. I think we don't notice the 12 battery decline on an EV the same way we would in an ICE vehicle. In the ICE you get immediate feedback every time you start the vehicle with the large starter current and, with a failing battery, a slow cranking rate. Not so in an EV, the 12 volt battery simply dies and the car will not start. I don't think the 2016 has an idiot light to tell me the 12 volt battery is failing - simple measurement of voltage when starting the car would probably do it.
My wife drives a 2018. It was originally driven in March, 2018. So, assuming it is the original 12V and I will be away for a bit, I replaced it. You just don't get feedback when the 12V is about to go on an EV.

Better safe than sorry.