I did a quick search for "Car A/C horsepower", and Wikipedia suggests the typical energy consumption is ~4hp or 3kw. That seems a little high to me for a compact car, but let's roll with it.
3kw @ 12v is 250 amps.
A typical 90's and early 2000's Civic alternator is 75 amps, so you'd need around 4 alternators maxed out to run the A/C compressor and the car. Alternators are optimistically 50% efficient, so the load on the engine would be ~8hp, effectively doubling the fuel economy loss.
A group 51 battery has around 30 amp hours when new and fully charged. Running the A/C compressor would run the battery flat in 7 minutes, assuming zero losses. You likely wouldn't be able to start the car with it after a few as 4 minutes.
Also:
Quote:
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Lead-acid batteries can only be discharged up to 50% before irreversible damage occurs.
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So, let's make that 3.5 minutes and 2 minutes.
The last part is the maximum safe draw from the battery. I expect you'd need about 3 typical Civic car batteries to not damage it from the compressor current draw. The bright side? You'd get a few more minutes of run time.