[CruiseControl]

BM2 monitors should be attached to the + terminal of the battery, but a chassis grounding point instead of the - battery terminal. On the Ioniq 5, the system chassis grounding point is conveniently located on the firewall, above and behind the 12V battery.

If the BM2 you get is like mine...it's a stretch to reach the grounding post, with the provided hook-up wires. If you've got the tools and know-how, it may be worth your time to extend the ground wire of the BM2 by about 8", to make it easier to simply adhere it to a convenient flat spot atop the battery. Mine sits at a funny angle, hanging off the corner of the battery. (Adequate, but my Engineer's eye flinches every time I see it.)

My BM2 manual says to connect it to the battery’s poles. Your’s says something different? Later in the manal, there’s written instructions saying the same thing.

 

[HandyAndy]

The BM2 diagram doesn't take account of the battery monitoring gizmo on the earth-terminal of the battery. A Hyundai guru has stated that the BM2 should be connected between 12V + terminal and the car's bodywork, NOT direct to the 12V -ve terminal. reason is BM2 draws a current, in the order fo 10 mA, and if you connect it between 12V + & - posts then this gizmo won't see that extra bit of current.

Whether that tiny current is actually detectable I can't say, and maybe it's tiny compared to a battery's internal self-discharge, but to be most accurate it does no harm to connect to bodywork and NOT bypass that special gizmo.

 

[JerryP]

I was initially lazy and connected mine to just the terminals. I didn't have any problems, but it wasn't long before I decided to do it right, which is the way @HandyAndy described and for exactly that reason.

 

[CruiseControl]

The BM2 diagram doesn't take account of the battery monitoring gizmo on the earth-terminal of the battery. A Hyundai guru has stated that the BM2 should be connected between 12V + terminal and the car's bodywork, NOT direct to the 12V -ve terminal. reason is BM2 draws a current, in the order fo 10 mA, and if you connect it between 12V + & - posts then this gizmo won't see that extra bit of current.

Whether that tiny current is actually detectable I can't say, and maybe it's tiny compared to a battery's internal self-discharge, but to be most accurate it does no harm to connect to bodywork and NOT bypass that special gizmo.

Hmm, maybe I should move the negative connection then.

 

[tadda]

I have my negative lead from the BM200 connected directly to chassis ground such that it doesn't affect or interfere with the ICU 12v battery monitoring / sense circuity which is connected directly to the negative battery terminal.

 

[thomas1n]

Wow 🤩. Nice data, nice work there! Very cool. Will definitely have a nice AGM battery in our future.

 

[bkindel2]

Thanks everyone. New to EV and new Ioniq 5 SLE 2025. Been reading about the 12v potential issues. From my obd2 car scanner I’ve been seeing 11.7v on the 12v system with the car off. Will verify with multimeter later. Came home (in other vehicle) to Ioniq 5 in the garage with fan running on while parked and off. Have BM300 coming today to assess further.

Questions if anyone could help with would be much appreciated thanks so much:

1). Is there a way to see the 12 V system voltage via Hyundai blue link? If so, can you give me a hint how to get there?

2). If my 12 V battery is bad a month after buying the car, is it covered under warranty or should I just shell out the bucks for a better quality battery?

3). Any specific AGM 12v battery recommendations for the 2025 ioniq 5?

thank you!

First off, the 12v battery voltage isn't constant. At rest, a regular lead-acid battery should be around 12.6v. (AGM batteries run a bit higher.) The car's system will attempt to charge it (at 14v) when it gets much below that.

Of course, starting draws more current than one might assume (not like cranking and ICE, but still substantial). If the battery is already low enough, the car will refuse to start. (Been there, done that. If you don't already have a compact jump start battery in your frunk, get one before you need it.)

A reality of the conventional OEM batteries is that they really don't handle deep cycling very well. ICE alternators don't give them much of a chance after the initial crank to start the engine. My 2022 IONIQ 5 has received a number of firmware updates to improve the 12v charging logic, and they seem to have worked. My original battery still died after two years, and I replaced it with an AGM Die Hard, which has done quite well for 18 months thus far.

As for monitoring the battery while driving, I kill two birds with the same stone. I've been using a wireless phone charging mount on a stalk that plugs into the 12v "cigarette lighter" socket to hold the phone just below the dash board. It includes two USB-C ports, one of which goes to the charging pad, and the other is available for whatever. It also has a small display that cycles through the current battery voltage to the voltage and current draw of each USB port.

 

[79Bry]

Hmm, maybe I should move the negative connection then.

I've had my bm2 connected to the negative terminal for about a year. It's a much cleaner installation IMHO. I personally think it's fine. I've got 20+ years of experience as an EE. The bm2 draws about 1.5ma of current, this should be completely negligible for any IBS measurements (the factory installed battery sensor).

 

[thomas1n]

1). Is there a way to see the 12 V system voltage via Hyundai blue link? If so, can you give me a hint how to get there?

No

2). If my 12 V battery is bad a month after buying the car, is it covered under warranty or should I just shell out the bucks for a better quality battery?

3yr/36k However dealership may perform Hyundai test and it must fail for replacement. Our 22 with 18 month old batter had been dying. Wife jumped it to get it to the dealer and was dead when she shut it off. Dealer tested it for over a day and it "passed" Dealer told my wife she could take it...........however it would likely die again...she went ballistic but to no avail. I simplely dropped in an AGM

IMHO and that of others, just replace it with a better type battery

3). Any specific AGM 12v battery recommendations for the 2025 ioniq 5?

H5/Group 47. Walmart Everstart Platinum AGM 4yr nonprorated warranty 180.00 is the best deal

Thanks for the very helpful and specific answers

 

[Coastal]

Tonight our 2025 Ioniq 6 orange (12V charging) light was on when I went out to plug into to our Grizzle-E Duo to charge. I checked looking at the BM2 using my phone and it had just started to charge. I then plugged the car into the Grizzl-E and the orange light went out. Looking at the BM2 the car was still charging the 12V (the car was not yet charging HV battery as scheduled charging was set to start at midnight).

I hadn't noticed this behavior before, but it made me wonder. If the 12V needs charging and the car is plugged into an EVSE can it charge the 12V without waking the HV battery? I understood one possible reason that had been suggested for the orange light was to alert people people the HV was on with the car off?

Or maybe it is as simple as this useless light (which is gone on some newer models) is on when the 12v is being charged when an EVSE is not plugged in and is off when the 12V is being charged with an EVSE is plugged in?

I don't recall the detail on how the ICCU sources power to charge the 12V? Can it do it from a connected EVSE without waking the HV battery? If so a related question which we will likely never know the answer to is whether this is easier on the ICCU, or not?

 

[Mikael48]

I've had my bm2 connected to the negative terminal for about a year. It's a much cleaner installation IMHO. I personally think it's fine. I've got 20+ years of experience as an EE. The bm2 draws about 1.5ma of current, this should be completely negligible for any IBS measurements (the factory installed battery sensor).

The BM2 current might be negligible, but the car monitors several other parameters of the aux battery, gathering hundreds of data points, all stored for 30 days. A BM2 hooked directly to battery terminals might impact some of the measurements.

Hyundai GDS-M service diagnostic tool shows the 30-day history:

https://www.ioniqforum.com/threads/12v-aux-gds-m-battery-diagnosis-function.44153/?post_id=540031#post-540031

SST073 KDS Battery Diagnosis Feature

This bulletin outlines the KDS Battery Diagnosis function and diagnosis procedures. The Battery Diagnosis is an on-vehicle battery diagnosis system, utilizing the battery sensor data. The battery sensor which is located next to the battery negative (-) terminal, continuously stores, and saves...

www.kiaevforums.com

 

[Hitstirrer]

I've had my bm2 connected to the negative terminal for about a year. It's a much cleaner installation IMHO. I personally think it's fine. I've got 20+ years of experience as an EE. The bm2 draws about 1.5ma of current, this should be completely negligible for any IBS measurements (the factory installed battery sensor).

That's fine. But you have to be aware that a forum member who is a senior Hyundai technician disagrees with you. I know which advice I will be following.

 

[79Bry]

The BM2 current might be negligible, but the small battery sensor monitors several other parameters of the aux battery, gathering hundreds of data points, all stored for 30 days. A BM2 hooked directly to battery terminals might impact some of the measurements.

Hyundai GDS-M service diagnostic tool:

https://www.ioniqforum.com/threads/12v-aux-gds-m-battery-diagnosis-function.44153/?post_id=540031#post-540031

SST073 KDS Battery Diagnosis Feature

This bulletin outlines the KDS Battery Diagnosis function and diagnosis procedures. The Battery Diagnosis is an on-vehicle battery diagnosis system, utilizing the battery sensor data. The battery sensor which is located next to the battery negative (-) terminal, continuously stores, and saves...

www.kiaevforums.com

Most of what it reports is just some sort of calculation made based on the voltage and current it measures. The bm2 will not affect the voltage measured, and I already talked about the current. The only other measured value possibly affected would be the battery internal resistance, and yeah I'm not worried about something that's about equal to a 10k ohm resistor in parallel with the battery, it's going to be negligible as 12v lead acid batteries measure in the miliohm range.

 

[79Bry]

That's fine. But you have to be aware that a forum member who is a senior Hyundai technician disagrees with you. I know which advice I will be following.

Yeah you do you. I've read the "master" tech's posts before. Personally, I've had thread discussions with I believe the same tech on another N specific forum on this topic. To me he just seems to be regurgitating something he's read out of a Hyundai technical manual. I'm sure there are instances of some battery monitors causing issues, some different battery monitors can draw 10x or even 20x what the bm2 draws. I've looked at the technical data sheets for a few IBS's and made my decision based on that.

 

[Hitstirrer]

Yeah you do you. I've read the "master" tech's posts before. Personally, I've had thread discussions with I believe the same tech on another N specific forum on this topic. To me he just seems to be regurgitating something he's read out of a Hyundai technical manual. I'm sure there are instances of some battery monitors causing issues, some different battery monitors can draw 10x or even 20x what the bm2 draws. I've looked at the technical data sheets for a few IBS's and made my decision based on that.

As I said, that's fine. But this is not all about a tiny power draw at all. It's about potentially confusing a sensor without any good need. When the negative lead can be just as easily attached to a convenient chassis point, why would I deliberately ignore official advice, even if 'regurgitated' from an official manual, to possibly cause problems that are specifically warned about in the manual? And instead, listen to an EE with zero access to the huge amount of research done by the OEM and specifically mentioned in the manual?

Quite why you do that, and justify it on the grounds of a tiny consumption, when it's not all about that, beats me. And to attempt to persuade others, by claiming 30 years experience in totally different systems, to follow your lead and go against the official manual advice is puzzling. There is clearly zero benefit from ignoring the manual, and an OEM master tech. But there is a real and present potentially negative effect from following your lead. If there was no risk, as you state, then explain to me why the OEM had advised against doing what you say. Especially when it's an identical manual event to connect to the chassis rather than the battery terminal. Makes no sense to me.

But, as you say, you do you.

 

[Arob]

Or maybe it is as simple as this useless light (which is gone on some newer models) is on when the 12v is being charged when an EVSE is not plugged in and is off when the 12V is being charged with an EVSE is plugged in?

That seems to be the case. Odd distinction, but then the light isn't particularly useful anyway since opening the hood stops 12V charging.

 

[GregBrew]

I've had my bm2 connected to the negative terminal for about a year. It's a much cleaner installation IMHO. I personally think it's fine. I've got 20+ years of experience as an EE. The bm2 draws about 1.5ma of current, this should be completely negligible for any IBS measurements (the factory installed battery sensor).

IIRC, it was the Hyundai EV Master Technician that haunts this forum who pointed out the necessity to hook up the negative wire on the BM to a chassis ground point, rather than the battery negative terminal.

As a 46-year experienced EE, I deferred to the expert, and moved mine to the grounding point on the firewall.

 

[FrankB]

That seems to be the case. Odd distinction, but then the light isn't particularly useful anyway since opening the hood stops 12V charging.

I was under the impression that the purpose of the light was to warn that the main DC bus is hot and therefore touching the wrong thing could result in death. Perhaps even Hyundai lawyers feel that such a warning is a bit redundant when the car is being charged?

 

[Arob]

I was under the impression that the purpose of the light was to warn that the main DC bus is hot and therefore touching the wrong thing could result in death. Perhaps even Hyundai lawyers feel that such a warning is a bit redundant when the car is being charged?

What can you touch without opening the hood? If opening the hood stops DC charging of the 12V, kind of makes the warning unnecessary it would seem.

 

[EVmastertech]

Wanna know why you shouldn't mess with the negative terminal on the 12V battery? And also, why you need a fuse for that BM2 or other stuff you add that doesn't have one?

Most of what it reports is just some sort of calculation made based on the voltage and current it measures. The bm2 will not affect the voltage measured, and I already talked about the current. The only other measured value possibly affected would be the battery internal resistance, and yeah I'm not worried about something that's about equal to a 10k ohm resistor in parallel with the battery, it's going to be negligible as 12v lead acid batteries measure in the miliohm range.

The smart shunt sensor on E-GMP cars does more than just measure voltage and current, right?
I usually get grilled on this forum by some members, but if you wanna learn more, I'd be happy to explain how this sensor works.