How do you charge a battery on a 1993 Audi A3?

According to the owners manual the battery is under the back seat, and we can't seem to get to it.How do you charge a battery on a 1993 Audi A3?the same in my dads manual, he owns a 56plate a6 avant. but our battery is easily accessible as it is in the spare wheel compartment. have a look. or Audi have built it in so that only a dealership can change it!!