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After sitting stationary with just the radio on for 20 minutes today, I then attempted to start the the car.
An unfortunate error appeared with the reading , check brakes for errors.
Turned out the 12v battery was dead.
Fortunately I have a jump starter on board which got me out of trouble.
This has now happened 3 time with this battery during the last 3 months although not with this error.
Think I'll be buying a new 12v battery to keep in reserve.
 

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Interesting, So the car still has enough juice left to operate the radio and to give you the warning message, but not enough to turn the car on?

I'm wondering what is going on here, since the ionic does not use 12v battery to start the ICE, and to turn the car on, we just need enough power to start the computer. Clearly the computer is still running because it gave you the warning message, false error message, of cause. So maybe the voltage got low enough to cause the computer to operate weirdly, but not low enough for the system to become un-operational?
 

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Yes, the 12V Battery will go flat enough quite quickly with just the radio/aux on. Even leaving door open will do the same if you do it for long enough. When sitting in the car waitting for someone elase, I start the car and leave it in park, and turn off the lights if it is dark. There is almost no drain on the main battery when idle, and this keeps the 12 V battery charged.
I have had my 12 V batter replaced under warranty, the new one does seem to be a lot healthier then the original. It might have been a bad batch of batteries causing the numerous issues with 12 V batteries on this site, however I will wait to see how my new battery lasts.
The Brake failure and other odd errors occur when the 12 V battery is low. I suspect that different system stop working as the 12 V battery fall below a critical value.
Glad you carried the jump starter -- I have one too!
 

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It has been my experience that often new car batteries do not last more than two years. I found a cool device on Amazon-it plugs into the 12 volt outlet and has an led voltage read out and two USB charge ports. An Li-ion or Li-po might be a good replacement if you are going to keep the Ioniq for a while. It would also cut your battery weight by about 1/3-1/4, and you do not need one with high CCA because you do not use it to start the vehicle.
 

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because you do not use it to start the vehicle.
I know you know how this works, but a slight misstatement. The 12 volt system and battery indeed does "start" the car and runs all the control systems that makes the car work. But unlike ordinary cars, there is no 12 volt starter motor to turn the engine over. The 12 volt system (ECU) instructs the traction battery to start the engine via a high voltage motor (the HSG) that serves multiple duties, not limited to just tuning over the engine. It is this high voltage system that is in part responsible for the very smooth transitions turning the ICE on and off. In fact, many tradition ICE only cars are moving to a high voltage system (40 volts typically) to make on/off systems smoother, more reliable, and more efficient.
 

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You said exactly what I said. There is no 12v starter. The traction battery starts the ICE. It is just semantics to say the 12v battery initiates the starting process because everything in the vehicle is initiated by the 12v system.
 
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