If I use a 12v USB adapter, how long can I use it before the battery becomes discharged enough to not have enough power to start the car? How long can I run the radio (at a moderate volume) before I kill the battery? If I were to sleep in the car overnight and listen to the radio all night while I played with the phone for a couple of hours would the battery die?

 

Without knowing the wattage of the unit you are talking about, anything would be a guess. Best advice - don't do it. Leaving a cell phone or GPS powered up overnight will kill it.

If you listen to the radio for a few hours - yeah you probably would have problems. Listening to it overnight means you will be needing a jump or be ready to push-start it. The stock battery in the HHR is not exactly a huge battery - it does it's job and not that much more. Even working on the car with the doors open (dome lights on) for a couple of hours can kill it.

 

From the owners manual:
Accessory Power Outlet(s)
Accessory power outlets can be used to connect
electrical equipment, such as a cellular phone.
The accessory power outlets are located on the
instrument panel below the climate controls and at the
rear of the center console. There may be an outlet in
the rear cargo area on the passenger side.
To use the outlet, remove the cover. While not in use,
always cover the outlet with the protective cap.
Notice: Leaving electrical equipment plugged in for
an extended period of time while the vehicle is off
will drain the battery. Power is always supplied to
the outlets. Always unplug electrical equipment
when not in use and do not plug in equipment that
exceeds the maximum 20 ampere rating.

As far as leaving the interior lights on because of open doors/hatch. The HHR has a battery saver mode that will turn off the lights after 20 minutes of continuous door openage.

 

Also the radio will turn off in ten minutes. When I park In will sometimes listen to a extended story and in ten minutes it turns off. It doesn't matter if the key is in or out of the switch.

 

That is true Ron. The radio is hooked up to the cars RAP system.

Again, from the owners manual:
Retained Accessory Power (RAP)
These vehicle accessories may be used for up to
10 minutes after the engine is turned off.
. Audio System
. Power Windows, if equipped
. Sunroof, if equipped
The power windows and sunroof will continue to work
for up to 10 minutes or until any door is opened. The
radio will work when the key is in ON/RUN or ACC/
ACCESSORY. Once the key is turned from ON/RUN to
OFF/LOCK, the radio will continue to work for
10 minutes or until the driver's door is opened.

 

I would not push my luck, I've had one time the battery died, and it cost me a replacement battery.
So I whistle a happy tune...

 

"As far as leaving the interior lights on because of open doors/hatch. The HHR has a battery saver mode that will turn off the lights after 20 minutes of continuous door openage." That's what I thought - but that sure didn't work on my 2006 as I was swapping interior door handles to do a JB Weld repair on one of them. It started afterwards, but barely, with the dials and warning lights putting on quite a show. However, the battery was getting on in years, as wasn't exactly in top-notch shape no doubt. So, I've never quite trusted the Battery Saver function since then.

 

As said, the battery in the HHR does what its supposed to, but there is little to no reserve for anything additional. When I do anything to the HHR that keeps the door(s) or hatch open for any period of time, I hook a battery maintainer to the car, which is easy to do up front.

 

Would it hurt to leave the 12V USB adapter plugged in? What about leaving the adapter plugged in with a cable plugged in but not hooked up to a phone or anything?

 

The standby current on those USB adapters is usually pretty small (a few milliamps), I leave mine plugged in all the time, I suspect that the biggest drain is probably the LED status light on it.