I've just had this same issue where we came back to our car and it was "Dead". I found the information from SparkEVPilot (in quotes below) to be helpful in confirming that the big battery was indeed charging the 12v battery when we picked it up from the service dept. We had gotten the phone call that the car had been checked out and was ready, but I was concerned that since I had completely recharged the Bolt's 12v battery, everything would look hunky dory to the tech and he might miss any underlying problems. I checked the battery before starting and it was at 12.5v, then after starting the Bolt it went to 14.75 - so at least I know that there is some significant charging of the 12v battery occuring when the car is turned on.
I'm sure that the OP has already receive a lot of helpful information, but I wanted to offer my experience to let the next person researching this issue that they are not alone.
Some extra details on our experience -
1. Got dropped off after car pooling to dinner with some friends and the door button did not unlock the car, then the remotes got no response - like the battery was dead.
2. Could not get into the car. Called Onstar to see if they could open the car, but they couldn't.
3. Onstar asked if it was possible that we had both pressed our remotes at the same time and that could possibly wipe the remotes and prevent them from being recognised - we said maybe.
4. Went through a routine to try and gain entry to the vehicle, but the ONstar rep did not seem familiar with the process. Ended up figuring out how to get in by accident and then went through the "Reprogram the Key" routine - that may be necessary whenever the battery goes dead (according to ONstar rep). Nothing happened.
5. Then Onstar sent a message to Chevy Roadside to get us towed. Chevy Roadside was supposed to call us back but never did. Spent the time researching the idea of a dead battery in a Bolt online. Called Onstar again and this rep seemed familiar with the idea of a dead 12v battery and told us that if we could get Chevy Roadside or AAA to give us a jump, that the 12v battery would turn the car on and the fully charged big battery would allow us to drive.
6. We gave up on Chevy Roadside and called AAA. AAA had a similar ETA so we waited. Meanwhile, we notice a AAA flatbead truck sitting in the Target parking lot where we were stranded. Finally, the driver came out and agreed to try and help us. At first, he hooked up the cables and the alarm went off and we quickly did the routine to reprogram the key. The alarm stopped and the car was running, but when the driver disconnected his cables, the car died again after a couple of minutes.
7. They guy really wanted to help us and hooked his cables up again for about 15 minutes. That seemed to do the trick and after thanking and tipping the driver, I was able to drive the car home ( as short distance) but the max speed was 30 mph (so don't try to get on the freeway).
8. The warning light stayed on the whole time. When I got home, I hooked it up to a new cheap trickle charger overnight. In the morning, I started to hook the battery up to a heavier duty charger, but it was already fully charged. I started it and pulled it into the garage, but the warning light stayed on.
9. That evening, we took it for a test drive to see if it was still in the "Maximum Speed 30 MPH" mode, but it was not. My wife wanted to take it to work the next day. From driving it that evening, we received a text message that Chevy had run diagnostics on the vehicle and everything checked out. When the wife drove it to work, she reported that the check engine light had gone out. She still made an appt with service and everything seems to have checked out.
10. As I mentioned in the first paragraph, I checked out the 12v battery with a tester and it helped give us more confidence that the big battery was indeed charging the 12v battery once the car started. Problem mostly solved. We still don't know how the 12v battery got discharged. If something is out of whack electronically, it may be a while before we find our Bolt dead again as it probably takes much more time to discharge since the 12v battery does not have to turn a starter motor and only runs a bunch of gauges and led lights. Still, we are aware of the issue and hope to be better prepared next time.
Just having some idea that a typical AAA vehicle or even a good samaritan with jumper cables might be able to get us home was helpful. I recommend that you not follow our lead and end up in a worse situation or get in an accident. You should evaluate your own situation and act accordingly (and probably wait for a flatbed tow truck). Hope this helps someone.
"If the Bolt 12 volt battery charging function is anything like the Spark EV, then this is what I found that might help - I measured the 12 volt battery voltage on my 2014 Spark EV after the car had been powered off for a couple of hours. It read about 12.5 volts. Then I plugged the car into my L1 EVSE set at 8 amps and measured the voltage. It read about 13.3 volts. I removed the charging cord and, after a couple of minutes, remeasured the battery voltage. 12.5 volts. Then I started the car and remeasured the battery voltage. It measured 14.5 volts. I got similar results for my 2016 Spark EV using my L2 EVSE (240 VAC at 16 amps).
Numerous Spark EV electrical problems have been attributed to the 12 volt AGM battery and were corrected by replacing the battery. If the Bolt 12 volt battery charging function is like that in the Spark EV, ( same 12 volt AGM battery ) then the 12 volt battery only gets a small trickle charge when the car is plugged in. The main charging of the 12 volt battery occurs when the car is powered on and continues until the car is powered off.
I would suggest you get a good voltmeter and test the 12 volt battery for yourself; especially with the car powered off and then with the car powered on. If you do not see the voltage jump from 12.5 volts with the car powered off to about 14.5 volts with the car powered on, your battery is not getting charged. Check the 100A fuse at the side of the positive terminal or take the car back to Chevy, explain what you found and have them check the fuse and the charging electronics.
It really sounds like you have a blown fuse, a bad 12 volt AGM battery or a problem with the charging electronics. I would also suggest purchasing a BatteryMinder 2012-AGM battery charger and charging your 12 volt battery at least once per month for 10 hours or more - I charge mine overnight. BatteryMinder has an option that allows you to charge through the OBD2 port inside of the car and it works great. This is what I do for both of my Spark EVs."