All of you guys are technically correct here so let's not get into a confusing tizzy about details...
The battery is used primarily to start the engine and also in most cases to provide field excitation for the alternator / (generator if it's older). Some voltage regs are that good (prestlite etc) that the output is good enough alone without the battery in circuit - provided the system will let you do it!!
The alternator in the a/c is really no different from that at Sizewell! It doesn't need a battery!!
