I Monitored My Ioniq 5's 12v Battery and Discovered What Really Happens When Your Car Sits Unplugged, The Data Surprised Me

Our 2022 Ioniq5 was fine until 1 month ago. We were due for our 45k service and had it booked (including iccu & vcu upgrades and an air cond compressor replacement) when it's 12v battery gave up during a clean in our driveway. No hint as to why. A zap from roadside assist got it charging again. 2 weeks of trepidation, we then had its service. Included a tick on 12v battery. 24 hours later, after its service, it failed to start, 12v battery dead, outside pizza hut. We had been trickle charging as our solar inverter had a fault. Another roadside assist has it alive again but post iccu upgrade we are cynical how long this will last. With 2 complete discharges of 12v battery we now have the ionic booked with Hyundai dealer to replace 12v battery hoping it will hold charge better...fingers crossed...

Meh, just use the car and stop worrying.
Leave mine for 2weeks at a time in airports, never had an issue.
If it ever failed, I’ll call the RAC to get it going.

I am leasing a 2024 Chevy Blazer EV and I had this happen just last month with only 5190 miles on the car. Went one morning to take to car wash and attempted to start when I stepped on the brake the car completely shut down.. I was unable to put the car into neutral for a tow to the dealership after 2 attempts by 2 different tow companies over 2 days, finally the after disconnecting the 12 volt battery and putting it on a charger we were able to put the vehicle into neutral for the tow.. The dealer had to replace both the 12 volt battery and the main battery as it had a dead cell.. It took 3 weeks to get the car back, Not a happy camper!!!!

A common issue at our shop. Not every one, but enough to notice a trend. Interestingly enough, every single Prius we've had in the shop has had the 12V die, and you need battery power to access it...

Knock on woodgrain I haven't had an issue with the 12v battery in my 2022 EQS. It was a used purchase so I'm not sure if it's the original battery or not. There is a 12v monitor in the app and I normally plug in every evening. I will monitor this a bit more but I've seen a Hyundai make it to a electrify America level 3 charger and his 12v battery left him stranded there.

This is absolutely true. I drive a hybrid, although it's not a full EV it still has the same problems as the EVS. If left sitting for 2 days or more 12-volt battery drain causes all kinds of havoc. The worst of which is when you get in the car to go drive the gasoline or ice motor won't even start to run . It will drive on the remaining Hybrid battery power, until depleted, then give a HYBRID SYSTEM FAILURE , PULL OVER NOW ! Warning, and there you sit !

Since you don't have a starter motor and you can't drive on the 12V battery why have it at all? It's more inefficient to convert the main battery voltage to 12V and charge that battery than to just have a single DC to DC converter from the main battery for all the 12V loads.

Your article is incorrect ("The 12-volt battery drain is quietly becoming the Achilles’ heel of modern EVs, and worse, not a single manufacturer seems to know how to fix it.") The MINI SE charges the 12 V battery from the traction battery - problem solved.

Your article is incorrect ("The 12-volt battery drain is quietly becoming the Achilles’ heel of modern EVs, and worse, not a single manufacturer seems to know how to fix it.") The MINI SE charges the 12 V battery from the traction battery. It also exhibits very little "phantom drain". The car can therefore sit for months and months without any issue whatsoever.

Just got my ioniq 6 back after two months in the dealers. ICCU and fuse replaced. Hyundai caught on the hop for this problem. They say there is a 1% failure rate but I think a lot more than this, judging by the amount of people I have across on the forums. My dealer told me a new revised edition ICCU went into my car and I was good to go.........we'll see!

2022 Ioniq 5 with 45,000 miles. I had left it unplugged several times for extended periods with no issues. But around January 2025 I signed up for a free service through my electricity provider that promised to give me useful feedback on my energy usage. A couple weeks later I was gone for a week with my car parked in remote long-term parking at O'Hare. I returned late at night to a dead car. Fortunately I had a jumper battery and was able to resucitate it. I cancelled the free energy usage monitoring service and haven't had a problem since.

Would it be wise for manufacturers add a secondary solar trickle charger to the 12v battery itself especially if it’s left out in the sun?

Yes I recently had to get a tow and fight to get a new battery on ‘23 EV6. I believe the problem (beyond hardware failure) is a mismatch between the battery spec and the ICCU charge controller logic. Frequent shallow charge seems shorten battery life. Also when the battery drops below 12v the car can go into a death spiral and become damaged.

I have a first generation 2023 Subaru Solterra. I left it unplugged for 3 months with the traction battery at 35% of full charge. At the end of the 3 months it started fine, without a 12 volt battery problem and minimal drain of the traction battery.

The reason is simple 12 volt lead acid batteries need to work hard they are designed that way if they are lightly used the plates get sulphated which eventually means they fail,you need an anti sulphation system to keep them healthy!

My 12V died at 1000 miles and was replaced. Died again at 3000 miles, and has spent last 3 weeks at the dealer, who can’t explain it. Hyundai recommended replacing the entire head unit (radio, telematics, etc) and part is on order. I’m not optimistic this will help. That’s why I carry a NOCO jump pack in the frunk (don’t keep it in the trunk, if 12V is dead you can’t open the trunk).

So just throw a small battery booster in the frunk and jump it when it misbehaves. That got me to the Hyundai dealer, (who replaced the 12-volt battery!) This was two weeks after the ICCU update, where they tried to reprogram it running on 12 volts and ran it down to 6.7! They recharged it overnight, but then replaced it two weeks later.

At about the 5 year point our Chevy Bolt failed...all of the 12 volt functions. I had parked in a large commercial lot and when I returned the car was dead

Had to call AAA to get a new 12-volt...waited over 3 hours in nowheres-ville

It's fine for the present...we'll see in the long run...the car is 7 years old and except for some of the early reworks (e.g. we had our entire primary power pack replaced to accommodate the fire hazard on the early Bolts...got 50 more miles per charge so there was a small silver lining

Since you explicitly invited people with 12V battery problems to comment, don't you think that will make it seem like everyone is experiencing 12V battery problems. Here's another data point. I lease a 23 Genesis GV70 EV, two years old. Driven only 15,000 miles in two years. All charging done via level 2 in my garage. The car often sits for a day or more unused and not plugged in. I usually leave the keys in the car in the garage and am constantly getting, "your car door is unlocked" messages which I ignore. I sometimes see an orange light blinking on the dash which indicate the 12V is charging from the main battery. I've had no 12V battery problems. I checked the Genesis GV70 and GV60 user forums and found 6 GV70 EV 12V battery problems since spring of 2023. However the GV60 EV forum had many 12V battery problem entries during that 2 year period. This 12V problem is an example of systems that get so complex that no one fully understands what is going on within a system. You think a simple 12V battery monitoring subsystem would at least pinpoint the problem but that would:
a) force manufacturers to admit there was a problem and
b) Add explicit extra cost to the car while ignoring the implicit costs of lost sales, more maintenance claims, recall campaigns, etc..

I recorded our IONIQ 5 12V battery voltage whilst we were on holiday for a month, last Christmas. The car was not attached to a charger. I have a graph. It shows that after 2 weeks the car pretty much went into deep sleep and the 12V was being topped up only once a week.

My 2007 Toyota Camry Hybrid, with 175K plus miles, still fires up every single time I press the start button. It has never failed to start in 18 1/2 years. Original Panasonic 12v battery in the trunk and original hybrid battery. Hybrid meter on dash shows battery to be in normal condition. Gas mileage is the same as when brand new.

We always replace our vehicle battery very 4-5 years after discovering that a weak 12v battery can shorten the life of a hybrid pack on the Prius’ we used to drive. However, the Hyundai EVs don’t even make it that long. We keep a little digital voltage gauge plugged in to monitor the 12v on our Ioniq 5 and observed issues at the 20 month mark. Had the 12v replaced under warranty. Never had an issue with our 2020 Bolt, though.

Find the antenna and disable it, then try your experiment again.

Remember when we found out that LG washer was sending out gigs of data?

My 2017 Bolt went eight years on the og battery. Then it randomly failed. Me dum dum. A new battery fixed it, though. We still love each other.

We own a 2017 Chevy Bolt, bought new by us. The propulsion pack was replaced by GM in early 2023 by recall. The last 2 years we have left the car, fully charged, but not plugged in, in an unheated garage - Seattle coastal type weather (20-40°f typical), for 3 months while we travelled. Zero problems at start up.
Your article surprised me. Did GM replace the lead acid startup battery as part of the recall? Otherwise, that's an 8 yo battery still working flawlessly.

Check out the 2018 Hyundai IONIQ, it had a section of the EV battery partitioned for 12V to solve the problem of having a 12V instrumentation battery. We had ours only a year with no problems at all... We upgraded to the 2019 plugin IONIQ and started having 12V problems within 9mos. I replaced the 12V battery for $500 and have been knocking on wood since.

This happened to me twice with my Gen 1 Ioniq when I forgot to switch it o

This happened twice with my Gen 1 Ioniq and both times it was because I lent it to Tesla drivers who forgot to switch it off. I have no idea why Hyundai puts a legacy on/off button in their EVs . It should be possible to inactivate with the key fob .

Don't have the car anymore but my 2019 e-golf's 12v died and I didn't lose everything. It still started and drove just fine but couldn't lock doors, roll down windows etc.

My wife had a 2018 BMW i3 with Rex. One of the most frustrating things was that there was no early warning to tell you that the 12 volt battery was going bad. All the error code that you could think of would just pop up on the display leaving you to wonder what could possibly be wrong. In our experience, the 12-volt battery lasted a little over 2 years. My wife's car left her stranded about a half hour away from our house and no tow truck company wanted to touch this EV. With all that being said, we are on our second EV, this time an Volkswagen id4. I have heard that this suffers some of the same 12 volt battery problems, however, the advantages of having a EV with a level 2 home charger are too hard to pass up.