horstmann economy7 clock loses time

Hi everybody.

I'm facing this issue with my horstmann economy7 device used to automatically turn on the heating pump for water heating. The device I have is not a digital one, but has a rotatory disk which keeps track of time.

The device is set up correctly, in the sense that the time window in which it is turned on is aligned with the night/day intervals of my electricity provider.

However, what I'm experiencing is that - even after setting the clock to the proper time - it loses time and as a consequence it starts and stops the heating of water in wrong periods.

I think it might be something related to the internal battery that may need to be replaced. Di you have any experience like this? If yes how did you solved it?

Thanks