Aux. battery charge voltage

[21KonaER]

Hi all.
Read a thread here a month or so back about 12V battery discharge problems, utility mode etc. It mentioned the aux. battery being charged to 14.72V, which seemed high to me. So I bought & fitted a BM6 monitor. And sure enough, it shows my battery regularly being charged to 14.75V.
Yesterday was my first visit to a dealer for a basic service and I asked if this charging voltage could be lowered using their scan tool etc. The answer: "No, it can't be adjusted." Is this true? Or do they perhaps not know how?
My understanding of lead-acid batteries is that normal 'dumb' alternators typically charge to 14.3/14.4V (as do chargers 'bulk' charge rate, then 'float' at 13.7/13.8V). Anything over 14.5V is an 'equalise' charge, to be done only infrequently.
The dealer also recommended the 12V battery be replaced, although it is dated 4-12-23 (d/m/y format here), so it's only 18 months old. This only strengthens my opinion that it's being very overcharged, frequently.
All opinions welcome.

 

[KiwiME]

... which seemed high to me.

Well, consider that you have a car designed by professional engineers and maintained by the dealer's trained technicians. You're asking them to change an important design parameter because you think it's too high. Did you present them with the relevant specifications from the battery manufacturer or other evidence? That's what anyone at Hyundai would have to do get a such a change order signed off.

I don't think charge voltage has been adjustable since the days of the Morris Minor but if you install the BM6 on a modern ICE car you'll see much the same behaviour as the Kona EV. Bursts of 14.65 ('bulk' charging) when the car is coasting or braking and 13.1-13.8 otherwise. Sealed calcium maintenance-free batteries ('CMF') use a higher charge voltage than the old lead-acid with caps where you top-up with distilled water.

A good start to determining what's required is the battery manufacturer's website. Next you can check with Century-Yuasa which does have charging guidelines.

My original Rocket battery made in 2018 was still working fine by late 2023 when I replaced it out of boredom with a Century AGM 'stop-start'. I keep the Rocket charged up in case I need it.

If the dealer is suggesting renewing the battery it's only fair that they tell you why so you can make an informed decision. Your BM6 graph looks reasonably normal but it's easier to discern the details if only a 24-hour period is shown.

 

[hieronymous]

Your Kona EV 12v maintenance charge regime is normal for this model. Unless you have experienced low voltage issues, your OEM starter battery does not need replacing. The pic below is from my March '19 Kona EV, with the original Rocket 12v still fault-free into its 7th year..

 

[electriceddy]

If the dealer is suggesting renewing the battery it's only fair that they tell you why so you can make an informed decision

Exactly

 

[electriceddy]

sure enough, it shows my battery regularly being charged to 14.75V.

The only change I notice between my previous models (2019 and 2021) and this new one (2025) is the actual charge cycle is less timid in its shut down procedure.
Before, the regular cycle would shut off once the drivers door was unlocked. On this 25 MY, the cycle continues (orange lite stays on) even when I am detected by the door handle- outside door handle lite comes on before motion detection unlocking, even after unlocking and entering car the orange lite stays on.
I was brave today and vacuumed the front area of the cabin without turning on the car or putting it into utility mode, which I usually do to avoid excess 12V battery drainage because the doors are open. The orange dash lite stayed on during the entire procedure.
I have several times additionally entered the car when the recharging cycle is active - only to have the cycle stop once the start button is depressed (ignition on).
I have not noticed any major differences in the charging frequency or voltage, the usual - every ~ 4 hours first 24 hours, every 12H after that for first 24H of being off for ~ 25 minutes @ 14.73V.
Not withstanding that last statement, looking at history: yesterday I do see it came on from 10:07 to 10:51 (44 minutes) without driving the car for that 24H period. Also April 16th duration 48 minutes and April 15th duration 64 minutes. So maybe the recharge cycle is a bit more aggressive when the car is off for more than 24 hours. ie longer charge duration but less frequent. I will keep an eye on those days-few and far between as I drive my car (or power it on) almost daily.
Using BM2.
Curious as to your installing the BM6 monitor as opposed to the BM2? does it have more features? I do notice the 3 day menu on your attachment, mine just has 1,7 or 15 days display also I don't think my BM2 has temperature - at least I can't find it.

 

[JoeS]

12v battery temperature I would consider of minimal concern in our electric cars, compared to the heat generated inside an ICE vehicle engine compartment. OTOH, this might be 'interesting' in your Canadian winters.

For myself, I go to the trouble of simply keeping the 12v on a smart float charger on any of my EVs which are not used daily (using a fused Anderson Powerpole connector pigtail). Doesn't seem to matter to my Kona 12v BMS, which mindlessly wakes up and does its normal charging routine anyway (I really do believe it does not actively monitor the 12v). With a 13.7v power supply attached as a 'float charger', the Tesla MS85 goes fast asleep and the main battery loses zero SoC for weeks on end (while its 12v vampire load continuously draws less than an amp nowadays); otherwise, it would wake up a few times a day to recharge that AGM 12v battery. The i-MiEV does not monitor its 12v battery but has negligible draw (~12ma) as long as I remember to remove the bluetooth OBDII, but I keep a float charger on any of my four i-MiEVs that aren't actively used. One of the i-MiEV daily drivers has a NOCO LiFePO4 12v 8Ah (in lieu of the 33Ah FLA) battery that's still going strong after 3-1/2 years of daily use, with the caveat that the car is garaged and our temperatures rarely drop below 0°C in recent years AND the measured upper voltage limit of the i-MiEV OBC is 14.4vdc. The Kona's measured 14.78v max peak charging voltage disqualifies using LiFePO4 for our application unless the 12v battery itself has a very smart overvoltage limiting BMS inside.