Fortunately you already have an AC generator built into your car, the alternator. All you need to do is turn it fast enough to generate electricity. Usually that's done by the car's engine, but you have to get the engine started first.
If you were REALLY smart you'd have brought a jump-starter with a built in battery and just jumped the car with that.
Put it right next to the Chilton's Guide, textbooks, spare gasoline and fluids, spark plugs, fuses, and all the other spare parts you also thought to bring with you before time traveling (or just have in your trunk because you're Ash Williams).
If you're smart, your time traveling car is a manual transmission. It's easier to repair and can be push started. A modest 5 to 10 mph is all that's required, rolling down a shallow hill or getting a few folks from the local inn to push. This provides enough mechanical energy to turn the alternator and generate electricity for the spark plugs to start the gasoline engine. Once the engine is turning, the alternator will charge the battery.
(If you're smart you'd also use a diesel set up to run bio-diesel. If you run out of gas you can use vegetable oil. It also needs no spark plugs, one less thing to go wrong.)
Push starting an automatic is much harder and might be possible around 30-40 mph, but I wouldn't risk irreparable damage to the transmission.
Instead, you'll have to turn the alternator manually. Most alternators need 500 to 1500 RPMs to charge and need less than 1 horse power. This can be done with leather belts (or adapt the ones already in the engine), simple gearing, and some way to get 1 horse power in medieval Europe... perhaps a draft animal, a wind mill, water wheel, some friendly peasants. I guess a horse will work, too.
Getting the RPMs just right isn't very important, any car alternator in the last 50 years will have a voltage regulator.
For payment there are any number of gadgets and tools in a car which would be worth a fortune in medieval Europe. A knife, lighter, calculator, clocks, flashlights, foam padding... even something as simple as a watertight plastic bag, or tiny springs and screws are amazing technology in medieval Europe.