12V Battery Drains If Car Left Plugged In After Charge Complete

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New member
Oct 24, 2019
I bought a low mileage (9600 miles) used 2017 Kia Soul EV about a month ago. I enjoy the car a lot, but I have noticed that if I plug it in and leave it plugged in for 6 or so hours after it has fully charged the 12V battery drains and I can't start the car without first charging the 12V battery.

This didn't happen for the first couple of weeks I had the car as I drove it enough that the standard 120V OEM charger would barely have time to get it fully recharged by the time I used it again the next day. But I bought a level 2 charger that charges 3 times faster and now the charge is done in just a couple hours and the car sits plugged in over night, but already fully charged. I realize I can schedule the charging using the UVO function, but I haven't gotten around to that yet. I thought it was a problem with the L2 charger so I returned it and bought another brand/model, it does the same thing. I have even had this issue with the OEM charger if I drive very lightly one day and plug it in over night, the charge doesn't take long and then by morning the 12V is dead.

After the first time this happened the OEM 12V battery refused to recharge and my charger showed a fault. I am just outside of the 2 year warranty on the OEM 12V battery so I bought a brand new battery, but the issue still remains.

I have searched the forum but the only close matches I see are referencing the french forums and don't seem to have a solution to the problem.
I would have said bad 12V battery at first, but now that you have replaced it, it means that you have something draining the 12V battery.

I think you need to start looking for where the power is going, pulling fuses and checking current flowing.
Thanks for the suggestion, I will try that this weekend and report back.

Last night I unplugged the L2 charger before I went to bed, the car was at 95% charge and I measured the 12V battery voltage after I unhooked the charger it read 13.25V. I checked first thing in the morning after leaving it unplugged all night and the 12V battery was still reading about the same voltage, so no drain if the charging cable is unhooked.
The expert on problems with 12V battery was the French user SoulEV2016.
He posted numerous comments on this forum including.
SoulEV2016 said:
...Be careful to understand that if the car is 100% charged and the charge cable connected, it's the AUX battery that provides all electricity of the car ...
From 12V Battery Details?

Hence it seems that there is a large phantom drain on the 12V during the night if your cable is left connected.
Perhaps one of the lights is permanently on.

There are even more detailed analyses on the French forum by him including - Le Kia Soul EV ne démarre plus ? la batterie 12V est cuite

Sadly he hasn't posted here in a long time.
I have experienced this as well. From what I am thinking, is that when plugged in to a charger. The 12v system is activated and stays activated to run the high voltage charging system. While it is charging the high voltage battery, the DC to DC converter maintains the 12v battery. But when the main battery is fully charged. The DC to DC converter also shuts off. But the 12v system remains active, draining the system instead of shutting down to low power mode.

To me, this is a major design flaw. If the system is plugged in via a level 1 or level 2 charger, It should maintain the DC-DC converter via a voltage monitor to turn it back on if the 12v drops below a certain level or provide a small trickle charger to maintain the 12v system.