You posted under a photo that looks like the Utilitec 0144506
See image of timer and timer manual on following link.
http://waterheatertimer.org/Woods-timers-and-manuals.html#6-weekIf this is not your timer. add a comment
Batteries for most plug-in timers are internal rechargeable batteries.
They are not replaceable and cannot be accessed.
To charge the battery: plug timer into receptacle for an hour.
Afterward, unplug timer to set timer program
Next, plug lamp into timer, and then plug timer into wall.
Be sure that lamp is turned ON.
If battery does not charge, then display doesn't show. That means timer is busted.
About 6-8 of new timers are defective from the start.
If battery won't charge, then return timer for another one.
Most of these timers work for a season or two, and then you buy another.