Mini Cooper 07 Battery light

Hi there. My 07 mini cooper was at the garage 3 days ago
for a new o2 sensor. All went well. Today took it for a drive
and the battery light has come on. It then went off, and
come on again, then went off, and back on again. Repeated
cycle. It drives and sounds fine except for the constant
beep of it going off and back on again.  Not sure what
could cause it to come on and off again.  I haven't drove it
in 2 days and weather has been very cold. Not sure if this
could be the cause. Thanks.