What the ....?

[21KonaER]

Hi all,
Came home from shopping yesterday and left the tailgate open for about half an hour - and the aux. battery discharged itself completely. The vehicle could not be started. I proved that my compact jump starter pack was able to allow the car to be started, but the aux. battery dropped out again shortly after. So, put my 20A charger on and let it fully recharge the aux. Please see June 17... pics.
Something similar happened last year when I left the tailgate ajar overnight. At that time I put it down to the 10W bulb draining the 12V battery and replaced it with a LED lamp (31mm festoon from memory). Thought this should sort the problem, even if the lamp stayed on for weeks.
But yesterday's event was bizarre. As if having the boot/trunk lamp triggers a high current discharge of the 12V battery. (Wish I knew what the Hyundai guys were on when the designed the aux. battery system - I'd order a kilo!)
Anyhow, the battery seemed to recharge OK and remain so overnight (charger disconnected at about 16:50). See June 17_18... pics. But notice, no aux. battery top-ups from the vehicle system overnight. Perhaps just above the threshold to trigger same?

Now to today, did a 90km round trip to 'the big smoke' (well, next big town...) with no problems. Notice that there were some vehicle top-ups while driving. On arriving home, left the tailgate open for just a few minutes while unloading and - there goes the aux battery charge again. Please see June 18...pics.

So, wise ones, what do we think the diagnosis is here? Is my 12V battery cactus? (The rate of discharge and 'supposedly' full recharge looks sus to me.) Do I have a problem with in-vehicle charging system? Does my LED lamp actually draw 30+ Amps???

BTW, my 5 year warranty is up in a month, so if my in-vehicle charger has problems, I'll have to flag it to the dealer very soon. Cheers.

 

[michael8554]

Put a current meter between the battery neg cable and the neg battery post.
Most are up to 10Amps, but only insert with the car completely OFF.
My Kona started and drove a couple of meters to the home EVSE.
But then failed to charge, 12V battery intermittantly down to 7V, loose or failing cell ?

 

[21KonaER]

Hi all,
I've finally found time to continue this story. Well I had to replace the 12V battery and all has been well since. But it's disappointing that the vehicle has used two batteries in 5 years, both needing to be replaced after 2.5 years. The vehicle was first registered in Jul 2021. The original battery expired in Dec 2023 and now the second one in Jun 2026. So I remain unconvinced that repeated (many times a day) charging to 14.70-14.75V is great for battery longevity.
Now another curious thing. When tested, the old battery didn't show as totally dead, after being recharged overnight and left to sit for a couple of hours.
A Kincrome 100A load tester gave the result of 'Weak" but not "Bad". And my digital battery analyser gave the results as shown in the attached images. Definitely not dead.
So, I'm still not understanding how leaving the tailgate open for just a matter of minutes triggered such repeated massive discharges. Any ideas?

 

[KiwiME]

...So I remain unconvinced that repeated (many times a day) charging to 14.70-14.75V is great for battery longevity.

Perhaps look up the absorption charging regime specified for the battery by the manufacturer? That didn't damage my OEM 'Rocket' which was still fine after 5 years. This info is from Century:

18 June
... left the tailgate open for about half an hour

17 Sept
So, I'm still not understanding how leaving the tailgate open for just a matter of minutes triggered such repeated massive discharges. Any ideas?

Historically it's been the most common way to kill the Kona EV's 12V battery.