Battery low on lock

It started playing up a few weeks ago, could no longer unlock, emergency unlock would not work. You could see it try but it got to the point where the cylinder would turn and gave up.

Reported 87% battery level

Did a recal, and it was fine for a couple of weeks.

Yesterday it decided to dive to 5% battery, from 84% last time I looked.

Put fresh Duracells in, still 5%.

Anyone seen this nonsense before?