Doorbell 2 disappointment

Bought a Doorbell 2 and official mains transformer and basically the battery was dead last week (down to 3%) HAD assumed it would keep it topped up. Recharged it last weekend and already down to 50% which is ridiculous, wish I had not bothered spending the £20 on the mains transformer as it is a waste of time if you need to take off the bell to plug it into an external charger !!!