I don't understand what the battery may have to do with it, but keys jamming
in the ignition lock is a known defect in early production Focus. It
happened to me. It requires a dealer or qualified locksmith to fix, since
the new lock must be set to the old key. Fortunately it's not too expensive
(maybe $200 U.S.).
The replacement locks seem to be better quality than the original ones, but
for safety, it's a good idea to squirt lock lubricant (not regular oil) into
the key slot once or twice a year.