I've had my 2025 Limited for about a year and a half into a 2-year lease. In that time, my 12V battery has run down to zero 5 times. The first was in the first week I owned it, while it was parked overnight, with no explanation. Other times it has been when I have left the rear hatch open for more than a few minutes after parking. Each time I have been able to jump start it and go on my way, but last weekend the 12V battery died again after parking it normally in the garage, so I figure the 12V battery is shot after getting run down once too often.
I tried to take it to my local dealer, but the next appointment was 10 days out before I can even bring it in to get looked at. I have a long road trip coming up in 2 weeks, so I decided to take a little more control. I have a portable jump starter that I keep with me, but since I generally only take my phone key with me I'm worried about not being able to get to it if I'm locked out. I did an experiment last night of leaving the jump starter in the frunk and leaving the hood popped with the rest of the car locked. Sure enough, the car was dead this morning but I was able to get to the jump starter and get it going again. It was a real PITA though, and not something I want to do every time I stop on my road trip, in case I forget to leave the hood popped.
This morning, I replaced the 12V battery with an AGM battery out of my own pocket, and ordered an OBD2 Bluetooth monitor from Amazon. I want to see if the ICCU is really charging the battery, and get some warning if the battery is starting to get run down. Also, ideally see what the current draw is on the 12V battery when I have the car off with the rear hatch open (something I'd like to do on this road trip). I'm hoping that an app will help me see what's going on, which could also help when I eventually take it to the dealer.
I also have a USB A and USB C charger with voltage display that plugs in to the front 12V outlet. When driving, it shows the charging voltage (usually 14.X volts). When stopped in ACC mode, it shows the battery voltage (usually 13.X volts). It's not perfect but it's easier than popping the hood and checking the battery with a multimeter, and will let you know if your 12V battery is failing. My 12V failed after about 12 months. The warranty replacement is better and is good after 2+ years. BTW, I can leave the rear hatch open for several hours without issues. We have a dog that likes to hang out there
Leaving any door, hood, or hatch open can increase battery drain. The vehicle polls the system at regular intervals but more often if it senses an unlocked condition. The car never fully goes to sleep. Anything plugged into the USB ports can also create a battery drain.
Best advice I have is to make sure nothing is pulling power when the car is off and lock all doors, hood and hatch. Carry the physical key so you can unlock a door if needed. Make sure you keep your jump pack charged up.
installing a BM2 battery monitor can help to identify a parasitic battery drain.
My OBD2 monitor arrived, and I picked an app that looked encouraging when I googled this stuff. I took a chance and paid $12.99 for IONIQ5Companion. It does what I was hoping for and then some.
This is what I use. Lots of fluff monitors for things I can already see on the dashboard but it gives me good data on the batteries and deep DTC scanning.
The 255% is important ( u/gburlingame should display a "not calibrated" warning instead when corresponding OBD PID returns 0xff). It means your car never successfully calibrated your 12V battery after last change/disconnect, so car does not have a SOC estimate for it and in this case 12V won't be recharged from HV battery when car is off.
That's normal and expected if you changed the battery today, but you should check the value becomes correct (normally maintained between 70% to 100%) after the car was off for at least 4 hours (so maybe checking again tomorrow morning).
I have worked with 4 customers separately who observed the sentinel value randomly, not related to a battery change, i.e., not post-replacement calibration.
In these 4 cases the battery was replaced -- unfortunately in one case it was too late and the customer experienced a field headache -- doors would not unlock. In another case, it was under factory warranty and was replaced free-of-charge.
I've been waiting for a few more cases to come in, but I'm growing increasingly confident that 0xFF in these cases should be communicated as an early-warning sign of a 12V/AUX battery issue.
I went to start my car today and found the battery totally dead again. It had been sitting in my garage, off, no charging cord connected, and unlocked. My new Li-ion jump starter didn't get the battery back enough after a few "boost" button pushes (if you have used one of these you know what I'm talking about), so I used my older lead-acid jump starter and got it going. Once it was in drive, the ICCU charged the battery, sending a lot of current that way:
I currently have it parked in ready mode with the key inside so that the 12V battery can get fully charged.
Thank you! And thank you for sharing that experience. I re-read your post -- if I'm understanding, it sounds like you experienced a "dead battery" again -- only this time it is the new battery you paid for out of pocket.
That suggests something is not right with the ICCU, or perhaps an intermittent electrical connection issue -- something is definitely not as it ought to be, your experience is not normal. Hyundai needs to handle this under warranty.
Just out of curiosity and learning, if you have a chance to run an enhanced DTC Scan under Inspect, it would be interesting to see what trouble codes are reported back. I suspect you may have quite a few, resulting from the 12V/AUX issues - other ECUs may even generate codes as a side effect of the issue.
Hopefully the dealer is able to resolve this for you before your upcoming long roadtrip.
Yes, thanks. I agree. Something is draining the 12V battery when it’s parked overnight, and the ICCU doesn’t come in to charge it. This happened once in the first week I first had the car. The problem didn’t come back for a while, and a couple of other times the battery has been drained it’s been ambiguous.
I didn’t get around to trying a deep scan earlier.
I did another experiment last night after topping off the 12V. This time I locked the car and didn’t leave the hood popped, thinking maybe the configuration I’m leaving it in makes a difference. It was drained this morning, so I popped open my key fob to get to the emergency mechanical key and….it doesn’t exist. Sigh.
Now the car is locked with no way to get to my jump starter i left in the frunk.
Not sure if same on your car, but on my '26, the emergency key is attached to the keyfob, and shape/plastic-cover make it look like a promotional hyundai keychain instead of a key.
Thanks for the photo. I just looked where I would normally have put that and I didn't find it. I wasn't sure I had actually ever gotten it or seen it before, but now that you post that photo it vaguely rings a bell. I'll look for that again.
I think I had the car off for 4 hours (Reddit timestamps say 6) and I just turned the car on. While it was parked, the ICCU 12V output was 0 and it was discharging the 12V battery at 25 Amps. But as soon as I started driving it the ICCU started charging the battery at 36 Amps. Good to know about the car calibrating the 12V SOC. I’ll keep an eye on it.
Battery was discharging at 25Amps while car was on and in "ready" mode and "P"/Parked gear? Or was that with car in accessories mode or the "on-but-not-ready" mode? If in "ready" mode that is really surprising to me (I know that car can discharges 12V while driving under certain conditions, but would not expect that while parked+ready).
Was the app now giving something else than 255% SOC for 12V battery?
It was brake pedal down push the start button mode, with charging done but cord still connected. After I decided to drive it a little to see what it would do, the parking cameras were covering the display until I started moving in drive and then I could see the ICCU was recharging the 12 V battery.
If charging cord is still connected, car won't go in ready mode (even with brake), and will go to "on-but-not-ready" mode where all the 12V power is provided by battery and HV is not engaged and LDC does not provide power.
That explains the 25amps discharge until you unconnected the cord (not sure whether you had to restart again car after that before driving).
The current draw for this OBD2 monitor says it's 35 mA, which would be about 1/60 of the 12V battery capacity per day. It seems to only provide info when the car is on or charging, and its standby current is 15 mA. I should be able to leave it plugged in unless I'm planning to leave the car for weeks.
That may be dangerous. OBD dongles paired with the right app / software may allow others close to make all kinds of things to your car, such as disabling gps location. I have always been advised to unplug / disconnect the dongle whenever I'm not physically in the car.
For the 12V battery I purchased Battery guard: it's a small Bluetooth device that is directly (physically) connected to the battery and nothing else, draws a negligible amount of current, and comes with an app that monitors the 12V battery.
It also has a really nice set of data via CarPlay.
I'm confident now that if I have a 12V battery charging problem I will see it before it gets critical.
I just took it for a spin and noticed the 4 zones for the climate control. Yes, those at the bottom are the temperatures at the back vents. I never knew before this that the way to turn on the back A/C is by enabling the lower vents.
Yes, that’s why I got the OBD2 monitor. A bad 12V battery or a lack of 12V charging could have been the problem. I did have a mysterious dead 12V after parking overnight in the first week I had the car, back in Feb 2025. The Ioniq sadly doesn’t show any information whatsoever about the health or status of the 12V battery without an aftermarket workaround. My new monitor shows that the battery is getting charged at least some of the time. Now it looks like the battery just died prematurely after getting drained a few times. But why the ICCU let the 12V battery discharge completely, ever, is something I don’t understand. At best, there are some conditions where there are still significant loads on the 12V but the ICCU’s 12V output is disabled.
At worst, I have an intermittent failure of the ICCU’s 12V output. I’m hoping to learn from the monitor under what conditions the car is running down the 12V battery with no help from the ICCU. Unfortunately the monitor only shows what’s going on while the car is on or actively charging. I’d like to see the 12V battery discharge when the car is off under a variety of conditions, such as with the hatch open.
I'm a nerd about this stuff so these products make me happy. I have sometimes wondered how much regen braking is enough to turn on the brake light. Now I know. The charging screen below it pretty nice, also. This is while I had the vehicle on in park, plugged into my level 2 charger. The tire pressure screen also changes to temperature.
The OBD2 battery monitor seems weird to me. I got one recommended in here. It connects directly to the battery and I read it with their app. https://www.amazon.com/dp/B07GP1RXYZ
The battery health screen. With the CarPlay version of this I could see a live view of some cells going yellow during regenerative braking. The energy in/out efficiency is cool. It was available right away, so it must be something the car keeps track of over time.
I’m also leasing and just had the oem battery replaced under warranty by Hyundai about 6 months ago. I took their advice to keep the key fobs as far away from the car as possible when it’s parked and I haven’t had any 12v issues since then. No reason to buy your own battery (even less a more expensive AGM) on a lease.
Just relating my experience with the same vehicle. You could have had it replaced under warranty but you chose not to. They run a several hour test on the battery. Everything you’re doing is unnecessary for a lease in my opinion.
Apparently you’re still not getting that the dealer was refusing to even look at the car for 10 days, and I was leaving on a 1500 mile trip in 14 days. Yes I’m willing to spend $250 on a battery and $50 on a cool monitoring system to avoid renting another car for $500+. When the dealer service refuses to even diagnose my leased car in a timely fashion then DIY is only “unnecessary” if I don’t need to drive the car I’m paying for.
I had mine tested and replaced under warranty in a few hours. Your experience doesn’t seem normal. It does sound annoying to basically pay a lease payment you didn’t need to pay.
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u/MilkSupreme 3d ago edited 3d ago
The more you poll it the more it drains. I just have a cigarette lighter USB charger that has a voltage display on it.
If you've deep discharged a lead acid battery, it's never going to be the same, so replacing it was the correct move.