2610 losing screen calibration, battery?

My Garmin 2610 is having difficulty with screen calibration. I have
trouble entering the letter A or the + when I try to zoom in. All
things on the left upper side (plus a few more).

If I perform the screen calibration, things then become OK. However,
soon I seem to lose the calibration. Is there an internal battery
that needs to be replaced? I have had this unit pretty much since
they came out.

B
.