Help prove a point about car AC and Batteries

Your assumption (a) is correct - I don't know of any car made that doesn't run the A/C compressor off of a belt/pully system from the engine. If the engine isn't running the A/C compressor isn't doing anything. It isn't an electrical device.

Assumption (b) could be problematic. If you had the ignition switch on and had fans on full blast the battery COULD drain pretty quickly - especially if the car was just started and only run for a few short minutes before stopping again. It's possible that the battery was never fully recharged from starting the car and running the fans could drain it from there. (Car batteries are designed for short burts of lots of energy not long slow draining like boat or RV "deep cycle" batteries.).

That said it's not unusual for a car battery to be stressed by heat and die a premature death. I had to replace the batteries in both of the last 2 cars I bought new (both GM cars) when they were both less than 2 years old.

Fishin' has a good point in b). Did you have the key turned "on" or to the "accessory" position?

The latter will run limited items such as the radio, blower fan and cigar lighter. Some cars will vary on which things run on "accessory" position.

THe "on" position will turn on the dash lights, indicator guages, and all the above, thus draining the battery faster.

I am in disagreement that a good battery "could" have died if not having been run for long to allow recharging.

A good battery should have enough power to crank an engine for several minutes in case of a hard start.

It should run all the accessories for at least half an hour (that is, wipers, radio, lights, flashers, blower fan, etc) so it should not have killed your friend's good battery in ten minutes flat.

Your friend has either a weak battery (old or abused, or low on water), or an undersized battery (sometimes people will try to save money by putting the smallest battery that will do the job instead of buying a good battery with ample power).

Or, she just may have a charging system problem that's not recharging her battery fully--kind of keeping it on the edge of failure all the time.

Oh, fishin' is right--heat is bad for a battery, as is freezing (as in freezing solid), though heat is generally worse.

Ways to kill an automotive lead/acid battery:

1) let it drain to near zero
2) let it go dry
3) jump-start it
4) let it sit all season without periodic low-amp charging
5) turn the headlights on, then crank the engine (unnecessary amp-draw)
6) connect the jumper cables backward (this will also blow the alternator on the car being jumped)
7) overcharging (bad voltage regulator, or a manual charger left on too long)
8) refill the battery's cells with tapwater (use distilled water instead)
9) drop or jar the battery (this will loosen the plates, causing an internal short)
10) pry on the top-post terminals instead of using the proper tool to remove them.

Hope this helps.