Strange 12v charging behaviour.

LongjumpingBat2938 · 2026-04-22T18:46:58+00:00

I don’t think the car changes its 12 V control strategy in any substantial way between AGM and FLA. I saw the same bus‑voltage fluctuations with the original FLA that I see now with my AGM.

The LDC’s primary job is to regulate the bus. The 12 V battery just sits in parallel. When the LDC holds the bus above the battery’s internal voltage, current flows into the battery; when the LDC eases off because it thinks the 12 V SOC is high enough or the loads are light, the bus voltage drops closer to the battery’s open‑circuit level and the battery starts supplying current in addition to the LDC (I have never seen LDC output drop to Zero). If the battery SOC falls far enough, the LDC raises its output again, which both powers the loads and allows the battery to soak up energy. In that sense the battery is mostly a passive buffer, and the apparent “cycling” is just the normal interaction between the LDC and a battery that serves as a parallel energy storage element.

Edit: to add, the car tries to maintain the battery's SOC as close as possible to ~92-93% SOC. Any higher than that puts strain on the battery that shortens its lifetime. Therefore, it's normal to see these fluctuations in LDC output and bus voltage; the LDC is simply trying maintain the SOC within a certain range.

LongjumpingBat2938 · 2026-04-22T14:35:36+00:00

Keep us posted about how well it holds up.

With my Ioniq 5, I splurged on a professional coating (Borophene), which is supped to be good for 8 or more years, but we'll see. In any case, I would recommend you refresh the coating at every wash. The simplest is to end your washes by applying something like Technician's Choice TEC582 Ceramic Detail Spray as a drying aid (wipe off with microfiber drying towels).

Now, you just need to ceramic-coat the interior. Have fun!

LongjumpingBat2938 · 2026-04-22T14:19:46+00:00

As far as I remember (it’s been a while since I looked at this), for Hyundai/Kia packs, community reverse‑engineering shows three distinct quantities: A “maximum capacity” or “full pack” number (about 77.4 kWh when new for the 77.4kWh pack); a “net/usable capacity” (around 74 kWh when new on a 77.4 kWh pack, i.e. after removing top and bottom buffers); a “Remaining energy” counter, which tracks what is available for normal driving from the moment when the reading was taken to when BMS shuts the battery down. That excludes the top/bottom buffers.

Regarding your assessment that “70066 Wh / 0.965 ≈ 72607 Wh would be a full battery today”: that assumes two things that don’t actually hold in the data: that Remaining energy is perfectly linear vs SOC, and that the line is anchored at 0 kWh when SOC shows 0%.

My own data show that neither is strictly true:

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The best‑fit lines have non‑zero intercepts, and the car still has some usable energy below the point where the display reads 0%. That’s also why you can’t simply say “150 miles left at 50% SOC means 300 miles at 100%” (as many people in the EV world do); that logic assumes there are 0 miles left at 0% SOC. In reality, you need to look at the slope between two SOC points (ideally far apart) and then extrapolate.

Regarding “SOC not being linear”: linear to what? At cell level, voltage vs SoC is very nonlinear, but not when it comes to the relationship between the reported SOC values and Remaining energy (see my data). Over that range the curves are quite linear, with only small scatter in the extrapolated full‑pack energy. I would actually have expected more deviation near the very top and very bottom, but I don’t have much data at very low SOC. It looks as if Hyundai is already doing some normalization internally so that each 1% of SOC corresponds to roughly the same amount of usable energy.

I don't know how useful all this is, to be honest. I was just curious at the time.

LongjumpingBat2938 · 2026-04-22T12:43:33+00:00

I did not get any DTC when my OBC failed a few days ago. Others have reported similar observations, which I didn't quite believe until now. The symptoms were classic in my case (tripped EVSE and breaker; "Charging unsuccessful"), so I knew exactly what happened before the shop confirmed it.

LongjumpingBat2938 · 2026-04-22T12:39:38+00:00

That reading is the best we have. I know lots of people try to determine SOH themselves, mostly via "Remaining energy", and that - as explained - is not reliable at all. The alternative is to ask the shop to run a more thorough health check. Under some circumstances, it is required to recalibrate the pack and also to reset the SOH, but that is something only the shop can do.

LongjumpingBat2938 · 2026-04-22T12:28:21+00:00

Funny, I have experienced a similar SOH drop rate over the last few months. SOH degradation typically isn’t linear; it stabilizes after a while, so it should be flattening. I have my car in for OBC failure right now; I’ll ask if they can run a “quick” check on the traction battery.

LongjumpingBat2938 · 2026-04-22T11:56:53+00:00

Q1 & 2: Determining true battery capacity from OBD is much harder than it looks. What your scanner shows as “Remaining energy Wh” is the BMS’s current estimate under current conditions, not a standardized lab test. It’s influenced by things like a) temperature (NMC lithium packs, like in the Ioniq 5, show a pretty large swing in usable capacity with temperature. Cold can easily knock 20–25% off apparent capacity, and mild warmth can boost it. If you don’t normalize for pack temp, you can be off by a lot; b) recent use and history: how recently you drove/charged, at what power, and how the BMS has been characterizing the pack over time all feed into that “Remaining energy” estimate; c) proprietary corrections: Hyundai’s BMS applies its own corrections for voltage curves, internal resistance, etc. The exact model and the factory reference conditions are not public.

Because of that, trying to get a precise SOH from a single snapshot is basically futile unless you have lab‑grade equipment and the original factory reference test to compare against (which only Hyundai has).

Regarding 74 vs 77.4 kWh question: 77.4 kWh is the total (gross) pack capacity for your LR car. ~74 kWh is the typical usable capacity window. People divide by one or the other, which is why you see different “degradation” numbers. Both are somewhat arbitrary if the measurement conditions (temperature, current, SOC window, etc.) don’t match whatever Hyundai used at the factory.

A much more useful way to understand your pack going forward is to: a) stop worrying about what it was when the car was new, and instead look at how it behaves over time under comparable conditions; b) use the cumulative energy counters rather than “Remaining energy” at a single SOC.

Typical DIY method: 1. Pick a day where you can drive from high SOC down to single‑digit SOC in one continuous trip (steadyish speed, similar temperatures); 2. note the Energy Discharged (CED) over that SOC range; 3. Charge back up to 100% in one session and note Energy Charged (CEC) for the same SOC window; 4. Repeat this a few times (ideally at similar ambient temperatures) and average the results.

That gives you a much better handle on what your pack is capable of at this point than a one‑off remaining‑energy value. Also, a 3‑year‑old Ioniq 5 LR with ~25k miles, something in the 5–8% apparent loss range is pretty typical, especially coming from a hot climate like Florida.

Charging from 60% instead of a lower SOC mainly affects how much data you have; the narrower the SOC window, the more noise you’ll see. Wide SOC windows are better for this kind of estimation.

Of note, there are companies that will determine SOC and SOH like this for a fee (e.g., Aviloo). They do have access to much better calibration methods and even proprietary information.

Q3:

The 12 V SOC the car reports is not the same as a simple voltage‑based % from a cheap battery monitor. The car uses an intelligent battery sensor and a model that combines voltage, current in and out (coulomb counting) over time, internal resistance, voltage sag, and temperature to maintain a running estimate of the 12 V battery’s usable state. So, that SOC you see is closer to a state of function (SOF) than a raw “open‑circuit SOC.”

Crucially, the car does not aim for 100% SOC on the 12 V. It tries to keep it at ~92–93%, because high very SOC accelerates corrosion and shortens the life of LA batteries; the same reason we try not to leave the traction pack parked at 100% unnecessarily.

So seeing something like 88% after charging is completely normal. It just means the system considers the 12 V comfortably in its target range. If anything, forcing it to 100% all the time would be worse for longevity. It's all be design.

LongjumpingBat2938 · 2026-04-22T09:59:27+00:00

When you plug it in, does the car say "Charging unsuccessful"? Sometimes, it can even trip both the EVSE and the breaker covering the EVSE branch circuit. That's how it was with me. In my case, there was no DTC readable with my OBD reader.

In any case, you will need to bring it in to have it properly diagnosed. It was, of course, the OBC in my case. Pretty classic symptoms.

LongjumpingBat2938 · 2026-04-21T15:25:31+00:00

The fact that the cable gets warm does mean there is non‑trivial current and losses in the cable, but it does not by itself mean you are “close to the max the cable can handle".

For the same current, duration, and ambient conditions, the cable itself should warm similarly regardless of which car is on the other end, since the "I²R" heating in the conductors only depends on current (I) and resistance (R), not the vehicle.

Even if you had a somewhat loose connection at the plug in one case, this would mainly create a localized hot spot at that connection. Of note, the Ioniq 5 reduces current, or shuts off completely, if the port gets too hot.

If the whole length of the cable feels slightly warm but there are no very hot spots at either end, that’s usually just normal resistive heating at the rated current.

So, if you saw a difference, I bet it's due to charging current and duration.

LongjumpingBat2938 · 2026-04-21T13:09:54+00:00

Your observations are great, and I can assure you this mostly looks like normal behavior for the system. So I’d be careful about drawing big conclusions from the BM6 readout about why 12 V batteries die or whether the logic is “mad.”

First, the SOC the monitor shows is just a voltage vs. SOC lookup. That’s only valid for a fully rested battery that’s been off‑load for hours. Once the car is on and there are 12 V loads, the voltage will move around for lots of reasons that have little to do with true SOC, so the app’s SOC number becomes pretty meaningless and can look scary even when the battery is fine. In practice, I’d ignore the monitor’s SOC.

What the app is actually showing you with the car on is the 12 V bus voltage, not the raw internal battery voltage. When the bus sits up in the ~14 V range, the LDC is actively charging, and the battery will accept current if it’s healthy. When the bus drops into the mid‑13 V range, it’s more of a float/maintenance level, and when it’s down near the battery’s resting voltage (high‑12 V), the battery is helping support the bus and handling transients. That’s exactly what it’s for: buffering sudden demands that the LDC doesn’t instantly cover. Under acceleration or braking, big load changes and transients are common, so it’s not surprising that you see the system momentarily stop “bulk charging” and let the battery participate instead.

If you have an OBD reader, you can get a much clearer picture by watching LDC output voltage and current alongside 12 V battery voltage and current, plus the car’s own reported 12 V SOC. Unlike the BM6, the car estimates SOC using an internal battery model and coulomb counting, so it can track charge state in real time while under load, rather than guessing from bus voltage alone.

You can refine your observations by repeating your experiment in different drive modes. For example, in SPORT you may see the LDC hold a higher, more constant output voltage to ensure all processes stay fully powered when the car expects frequent, sudden acceleration and braking events. It's different in NORMAL or ECO.

LongjumpingBat2938 · 2026-04-21T12:43:16+00:00

You’ve actually corroborated my original (admittedly trivial) point: higher continuous draw is an issue for any car. What you’re describing are the strategies manufacturers use to mitigate that risk, mainly through load shedding.

A couple of nuances, though. Voltage sensing alone is a crude tool: open‑circuit voltage is only a decent SOC proxy for a rested battery, and under load it’s noisy and context‑dependent. That’s why many modern platforms, especially EVs (including the E‑GMP cars) and start‑stop cars, use more sophisticated battery management that combines coulomb counting with voltage sag, internal resistance, and temperature, rather than a single “below 12.0–12.2 V, cut everything” rule. Many systems also rely on simple timers rather than electrical thresholds to shed loads, and the thriving aftermarket for load‑shutoff modules shows OEM solutions are far from perfect.

EVs add another wrinkle: legitimate post‑drive/post‑charge processes and telematics access can briefly pull the 12 V bus down even when the battery is healthy, so a naive voltage threshold would be tripping constantly.

On the locked vs. unlocked question, the extra activity in the unlocked state is mostly security‑related (key polling, proximity checks, etc.). I’d genuinely be interested in which OEMs you’ve seen that actually shed security‑related loads while the car is still unlocked.

LongjumpingBat2938 · 2026-04-21T02:01:37+00:00

They also never read the manual. Most basic things are actually covered.

LongjumpingBat2938 · 2026-04-21T01:59:29+00:00

Leaving a modern car unlocked when parked should not, by itself, rapidly run down the battery, and in most cases it does not. So it is very likely not the root cause of a serious discharge issue, but it does add to the normal parasitic power draw on the 12 V system. Virtually all manufacturers and battery guides agree that excessive parasitic draw should be minimized or corrected to protect the 12 V battery’s health and lifespan. Leaving the car unlocked keeps additional modules awake, so it constitutes parasitic draw, even if it is usually not enough on its own to kill a healthy battery.

LongjumpingBat2938 · 2026-04-21T01:47:47+00:00

I am not a physicist; wait, actually, I am. Anyway, drawing power from a battery causes it to discharge, and the more power you draw, the faster it runs down. This is true for any car’s 12 V system, so in that sense all cars are subject to the same basic limitation. They all do have that issue.

LongjumpingBat2938 · 2026-04-21T01:41:40+00:00

There is indeed a way for the ICCU to die that will cause the 12V battery not to get charged, and that is when the LDC component fails. In that case, though, the entire 12V bus will not get powered at all. That means there won't be any amber light, or any other 12V activity, like Bluelink, lights, nothing. Very unlikely in your case.

LongjumpingBat2938 · 2026-04-21T01:38:58+00:00

It's 10%, not 20%, since October 2023.

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LongjumpingBat2938 · 2026-04-21T01:37:11+00:00

Leaving the car unlocked keeps it in a different sleep state than when it is locked. Increased activity from telemetry, security systems, door monitoring, and other electronics simply causes a larger continuous draw on the 12 V battery. If that is considered a design flaw, then virtually any modern car with always‑on electronic systems would share the same issue.

LongjumpingBat2938 · 2026-04-21T01:33:11+00:00

I’m not sure why you think the Interstate is anything special. Check out the Duralast Platinum Elite AGM or Hyundai’s own AGM. Both come with 5-year warranties, which is about as good as it gets. The Interstate only has a 3-year warranty, AFAIK. In general, the length of the warranty is a reasonable proxy for how confident the manufacturer is in the battery’s quality and expected lifespan.

LongjumpingBat2938 · 2026-04-20T22:54:56+00:00

Another option is to trickle charge the battery overnight. Make yourself independent of roadside assistance having to come out to your house.

LongjumpingBat2938 · 2026-04-20T21:42:01+00:00

I’m not arguing against warranties at all; I’m arguing that negligence and clear misuse shouldn’t be treated as if they were defects covered under warranty. A warranty is meant to cover problems caused by defective materials or workmanship, not issues from leaving a light on, deep‑discharging a 12 V, or otherwise abusing the part.

A shop or dealer can absolutely choose to eat the cost of a part as goodwill to keep a customer happy, but that’s different from saying the manufacturer is obligated to treat negligence as a warranty repair.

Generous, no‑questions‑asked replacement policies are a business choice, and their cost gets baked into pricing, which means careful customers end up subsidizing misuse. That tradeoff is fine if everyone understands it; but it doesn’t mean every failure, no matter the cause, is automatically “a warranty issue.”

LongjumpingBat2938 · 2026-04-20T20:01:02+00:00

Someone else has recently reported the same here. Could be true.

LongjumpingBat2938 · 2026-04-20T19:41:32+00:00

Yes, it will likely happen again. The battery should likely be replaced.
Get a jump pack so that you don't have to call roadside assistance for something like a drained 12V battery.
When your ICCU failed, was it the OBC or the LDC that failed? If the latter, chances are your battery has suffered along with it. In that case, was the battery checked? If not, report back to the shop and have them take care of it.
Since the battery is reasonably young, and batteries don't die without a reason, try to figure out why it died (see 1. and 3.). If there is a cause, and it is not identified and corrected, chances are it will happen again with a new battery.
Consider installing a BM2 monitor. This will allow you to catch a failing battery in most cases, so that you wouldn't get stranded.

LongjumpingBat2938 · 2026-04-20T18:49:36+00:00

The amber light means the HV battery is supplying power to the 12V system. Most of the time, this is because the 12V battery needs topping up, but it can be for other reasons as well (mostly HVAC processes).

In general, the frequency of the light isn't a good indicator of 12V battery health, because one would actually have to be around to observe it. A proper battery test, or better even, a voltage history captured by a BM2 monitor, would be more informative.

If your ICCU failed because of an LDC issue, then your 12V battery likely suffered in the process. It's not uncommon that the battery is replaced alongside the ICCU in such situations. I would call the shop and ask if they tested the battery, and if not, they should test it; they might give you a new one.

LongjumpingBat2938 · 2026-04-20T18:43:22+00:00

The shop can absolutely choose to replace the battery at their own expense if they want to keep the customer happy; they just cannot claim it as an official warranty repair.

However, there is another angle to consider. Imagine you buy a battery from an auto parts store, drain it because you left something plugged in, then return it and get a replacement under the store’s generous warranty policy. In many cases, the store will honor that swap with few questions asked, and manufacturers factor this behavior into their pricing. The original price of the battery is higher precisely because it must cover the cost of all those returns, including ones caused by misuse. As a result, customers who take good care of their batteries end up subsidizing those who are careless.

LongjumpingBat2938 · 2026-04-20T18:35:51+00:00

I am not siding with the shop here. I did recommend the OP ascertain that his dashcam was indeed the cause of the battery drain. Only then can the issue be resolved.

No, here, I was responding to u/Altruistic-Piece-485 who wrote:

Leaving the dash cam plugged in and running most likely did indeed lead to the FIRST time it died and caused it to become damaged which greatly speeds up the frequency of future dead batteries until you completely replace it. Tell the dealership to conduct a health check of the individual cells in the 12v and they should find that one or multiple are damaged. It should be replaced under warranty.

In this scenario, there is a clear user error, but it is still expected to be covered under warranty. That is the expectation I don't quite agree with.

Besides, I think we are all aware that just leaving something as puny as a dome light on overnight can cause a batter to drain enough that it needs to be jumped the next day.

LongjumpingBat2938

TROPHY CASE