So yesterday was a real bummer. I walked to AutoZone to get one of those portable batteries, and then hoofed it to my car, and when I pressed the battery test button standalone, it said, "FL" (which means, "Full", not "Fail", I looked it up), but when I connected it to my car, it said, "LO". After doing a little bit of research, I think a confluence of events caused my battery to discharge and freeze (it said it only takes a 8/10 volts down to be considered "discharged", and therefore would freeze at water temperature rather than -74 or whatever it is really), so I don't think a jump is going to help, I think what I need to do is warm up the battery, which is impossible where my car is, and I'm not carrying the battery home. I also think that the portable battery got cold between where I bought it and the car, cause I was just carrying it, not trying to keep it warm, not realizing that that was a thing. (It said in the manual not to use it if it's frozen, but I don't know how to tell if it's frozen.)
So...is it true that turning on the headlights for a couple minutes will put some current through the electrical system and warm it up? I was thinking of doing that, and then of course turning it off, and then trying to jump start my car with the portable battery (which I will try to keep under my coat or on a bus). Or should I just give up and have it towed to a shop? 'Cause it's not getting above freezing any time soon.
Also, how long would I need to drive to charge up the battery, assuming it's charging? Is 20-30 minutes enough?