Need Help Troubleshooting

Just a brief history.
In March of 2017, my gas cap broke off and got lost. It was 2 days before I got it replaced. It gave an error code, but went away.
Between April and June 2017 I spent a lot of 30 minute lunches in my car with it fully or partially (no engine) turned on. I say this because I now realize that may have depleted and ruined the accessory battery.

In the month of July, I went out to have lunch in my car and it wouldn't turn on in anyway. Several hours later I had someone bring a portable charger but it worked just fine.
As the temperature went up past 90 and 100 degrees all kinds of whacky things started happening.

-The ABS, VSC, Maint. Req., Check Engine lights, this light ((!)), the red triangle with an exclamation mark, a Problem code on the MFD accompanied by an red icon with the shape of a car and an exclamation mark on it.

-The brake system went out intermittently accompanied by a blaring ringing sound

-The AC system went out intermittently especially when the brake system went out.

-I filled the coolant and oil within the last month.

-I tried to make a short 20 min drive on highway and the engine just stopped. The electrical system seemed to work but it just stopped accelerating entirely. I pulled over on the should and it wouldn't let me put in in brake or turn the system off. It also burned through gas at an alarming rate. A wrecker came and charged the accessory battery for me (instead of charging a tow, nice guy) and I drove it to a mechanic.

-The mechanic tested it and found no error codes, but the accessory battery was completely drained. They charged it and everything worked fine. I drove it home though, and on the way all the lights came right back on. Since then the lights turn on and off intermittently, though the brake system hasn't gone out like before nor has it died on me or not turned on. The AC still quits sometimes (by quit I mean it isn't cooling things).

After a lot of research everything seems to point to the accessory battery EXCEPT that when the mechanic tested the battery it was seemingly still good. Does anyone have experiences with all these symptoms? Have you had these same problems caused by the accessory battery?

I'm also thinking the check engine light might be related to my replacement gas cap, but who knows?

 

Hi! Thank you for the reply

Gas cap is generic. Bought it at Autozone. Odometer reads: 170,700. The autoshop tried to read the codes but it gave him nothing. I've had them read at Autozone and it was a very non-specific "battery issue" message. I'd like to look into getting a voltometer and checking the hybrid cells in the near future.

 

good suggestion, thanks