Battery charging issue?

Hi, My mother in law has a Stannah 300 stairlift.
It stops every 6 inches or so when going up.
I have replaced Batteries which works for a while but then problem reoccurs.
Clearly it is not charging as it should.
Charger output appears to be 27v.
Does this model charge at top and bottom or continuously (as there are no charging points visible at top and bottom.)
Could it be a connection?
Any help welcome thanks.