If its intermittently 'faulty' it could be the battery holder losing contact with the circuit board as a result of duff soldering!!
Mine is/was intermittent - took it to my mate who is a locksmith/cobbler - he cracked it open and all sorts of bits of foam fell out. Then we discovered the hopeless soldering job holding the battery holder on by only one of its three contacts.
A liberally folded piece of card, carefully placed and secured now holds it in place until I get around to soldering it properly.
If I remember - battery is a CR2032 - cheap as chips - pay no more than about a quid or so for a decent brand.