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

Once upon a time, a dead 12-volt battery was the calling card of forgotten ‘87 Cutlasses in apartment complex parking lots and third-hand Civics rotting behind barns. But in the electrified utopia promised to us by the likes of Hyundai, Rivian, and General Motors, it turns out that same little lead-acid gremlin is still haunting the garage like it’s 1982. This time, it’s just wired into a $50,000 car. 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.

Roadside Rescues For EVs

EVs are fast, efficient, and smoother than a politician’s apology. But buried in their digital veins is a dirty little secret… most of them still rely on a humble 12-volt auxiliary battery to keep systems like locks, lights, and telematics alive.

When it goes dead, you don’t just lose headlights, you lose everything. And unlike an internal combustion car, you can’t just pop the clutch or boost it with your buddy’s Tacoma. You’re toast until roadside rescue shows up with jumper cables and pity. For reasons unknown, the 12V battery drain issue is something no manufacturer can solve, including Hyundai, GM, and Rivian.

What The Hyundai Ioniq 5 Offers

• The 2025 Hyundai Ioniq 5 offers a range of powertrains, including a high-performance Ioniq 5 N variant with 650 horsepower, capable of accelerating from 0 to 62 mph in just 3.4 seconds. ​
• Equipped with an 800V charging system, the Ioniq 5 can charge from 10% to 80% in approximately 18 minutes using a 350kW charger. The Long Range RWD model offers an EPA-estimated range of over 310 miles. ​
• The Ioniq 5 features a spacious interior with a flat floor, allowing for flexible seating arrangements. It includes advanced safety features like a 360-degree Surround View Monitor and has achieved a 5-star Euro NCAP safety rating.

This is not a hypothetical nuisance. It’s a widespread, borderline epidemic that manufacturers have yet to publicly own up to in any meaningful way. Online forums like r/Ioniq5 and r/Rivian are littered with tales of EVs going flat overnight, parked innocently in garages, only to become $50,000 paperweights.

That’s where a methodical, data-hungry Redditor named u/PrivatePilot9 enters the chat. Frustrated by the guesswork and dealer shrugs, he decided to monitor his 2022 Ioniq 5’s 12V battery over the course of a week, post-ICCU firmware update, and what he found was both enlightening and baffling.

“I am an active r/homeassistant user, and one of the data points that the amazing Bluelink integration provides is the 12V battery status. So, I started logging it, updated about every 20–60 minutes. It's also important to keep in mind that some of this data disproves (or at least partially discounts) the fear that many have that polling the car via Bluelink too often is automatically going to flatten the 12v…”

Bluelink, it turns out, isn’t the villain. Even under constant monitoring, 2 to 3 data pulls per hour, the battery held fairly steady… as long as the car was plugged in. 

An Eyebrow-Raising Moment

Once the Ioniq was left unplugged, things got unpredictable. The voltage began to fall, not drastically, but enough to raise eyebrows. And while the traction battery eventually kicked in to resuscitate the 12V system, it did so with all the urgency of a housecat deciding whether to acknowledge your existence. 

“It then went up 2% overnight (despite not being plugged in)… so it’s apparent that the car decided to maintain the 12V from the traction battery at that point,” 

Not exactly confidence-inspiring.

There’s something profoundly absurd about a state-of-the-art electric crossover playing voltage roulette with its brainstem. According to u/PrivatePilot9, 

“So, despite being polled extremely frequently via Bluelink, the battery never goes below 85% at which point the ICCU seems to happily just maintain it there with the traction battery…” 

That’s good news. But it also underscores how tenuous this balancing act really is. A missed software cue or logic delay, and suddenly you’re staring at a blank dashboard, wondering how you’re going to pick up the kids from school.

The Most Troubling Part Of It All 

Perhaps most troubling is that this problem is not tied to old or abused batteries. In fact, it's quite the opposite. 

“The 12V AGM battery in my 2011 Volt lasted 12 years and was still serviceable… I’m not sure why the Ioniq 5 is so hard on batteries in comparison.” 

That’s the mystery. Despite less physical load and more intelligent energy systems, EVs seem to chew through 12V units faster than an ‘05 Taurus. Is it the ICCU logic? Poor charge cycling design?

Inadequate prioritization of standby loads? Nobody, not even the automakers, seems to know for sure.

What Causes The Battery Drain?

• Devices like dash cams, GPS trackers, or OBD-II dongles can continue to draw power from the 12V battery even when the vehicle is turned off. Over time, this continuous drain can deplete the battery, especially if the vehicle isn't driven regularly. ​
• Some EVs rely on a DC-DC converter to maintain the 12V battery's charge using the main traction battery. If this system fails, due to software glitches or hardware issues like a faulty Integrated Charging Control Unit, the 12V battery may not receive adequate charging, leading to depletion. ​
• Leaving an EV parked and unplugged for extended periods can result in the 12V battery discharging. Even when not in use, certain systems may continue to draw power, and without regular charging, the battery can become depleted.

And this isn’t just a Hyundai problem. Across brands, from Rivian to GM to Ford, owners are encountering eerily similar symptoms. As one Ioniq 5 owner noted, their 12V battery completely failed at just two years of age, a lifespan so short it wouldn’t even raise a child to toddlerhood. The issue is so prevalent and so perplexing that forum posts have become de facto technical manuals. 

“It’s very unusual in any EV application for the 12V battery to not last many, many years. Clearly, there is (or hopefully, was) something very wrong with how the ICCU was behaving pre-update…”

The real kicker? These aren't exotic, temperamental machines. These are mass-market daily drivers. Hyundai Ioniq 5s, Ford F-150 Lightnings, Chevy Bolts. We’re not talking about the Pagani Huayra. These are practical, theoretically reliable appliances meant to usher in the next era of mobility, and they’re being tripped up by a component designed before Elvis got drafted. 

Maybe in the not-so-distant future, we’ll all have a laugh about it, how the first generation of EVs could slingshot you into the future, but might leave you stranded in your driveway on a rainy Tuesday because the car forgot to feed its 12V battery overnight, it makes you wonder if other solutions are out there. 

Ever been left stranded because your EV’s 12V battery just randomly gave out?

Did it make you miss an important appointment, or maybe even a date?

Drop your story in the comments below. We want to hear what happened.

Image Sources: Hyundai Newsroom 

Noah Washington is an automotive journalist based in Atlanta, Georgia. He enjoys covering the latest news in the automotive industry and conducting reviews on the latest cars. He has been in the automotive industry since 15 years old and has been featured in prominent automotive news sites. You can reach him on X and LinkedIn for tips and to follow his automotive coverage.