If you have the ORIGINAL battery - I would not THINK about it, I'd replace it before the sun sets tonight!

In today's cars (not just the XF) a bad battery can do WEIRD stuff that can send you off on a wild and expensive goose chase! My Dodge Durango did just that a few years back!

Drawing that many amps off a storage battery IS a problem - modern storage batteries are designed to deliver HUNDREDS of amps for a few seconds to start the engine - they do NOT like to provide 20 amps for two hours. It's all in the chemistry of the cells. If you ran a "deep cycle" marine battery designed for trolling motors, leaving the lights on would be fine. So yes, if it's an old battery, you may have brought it's eventual failure closer.

In any event, I use a "Sure Start" battery in my XF that I got from O'Reilly's. Under $100 - I plan on replacing it every three years to avoid expensive trouble. (A battery every three years is cheaper than a tow and a battery every, say, five years!)