The Collaborative International Dictionary of English v.0.48:

bats \bats\ (b[a^]ts), a.
   Crazy; insane; loony; demented; batty.
   [PJC]

WordNet (r) 3.0 (2006):

bats
    adj 1: informal or slang terms for mentally irregular; "it used
           to drive my husband balmy" [syn: balmy, barmy,
           bats, batty, bonkers, buggy, cracked,
           crackers, daft, dotty, fruity, haywire,
           kooky, kookie, loco, loony, loopy, nuts,
           nutty, round the bend, around the bend, wacky,
           whacky]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "bats":
   balmy, bananas, barmy, batty, beany, bonkers, buggy, bughouse,
   bugs, crackers, cuckoo, daffy, dippy, dotty, flaky, flipped,
   freaked-out, fruitcakey, fruity, gaga, goofy, haywire,
   just plain nuts, kooky, loony, loopy, nuts, nutty, off the hinges,
   off the track, off the wall, potty, round the bend, screwball,
   screwballs, screwy, slaphappy, wacky