This is the Model 308 unit. Turn it over.
There are 3 screws that have to be loosened.
Remove the cover and set it off to the side.
The shiny round disc in the black holder is the battery.
Push the release tab (at the point of the screwdriver) all the way and the battery will pop up.
You have to push flat on the top edge of the tab.
Don't push down, push horizontally.
If you push too low the screwdriver will
prevent the battery from popping up.
You can use your finger nail if you do it right.
The battery's edge will jump up visibly when the catch is released.
Lift the battery out.
Next: Leave the unit unplugged with the battery out for at least 40 minutes!
This "re-boots" the computer chip and corrects a blank display or other problems.
Now carefully plug the unit back into the wall without the battery. It might be best to put
the back on loosely so the wires stay in place. The display should show all 8888s for a
second or two, then go to the default - 12:00:00 and start counting seconds.
Now just reverse your actions to put in the new battery.
Push down on the battery and it will snap into place.
The cover fits easily once all the wires and switches are where they belong.
Don't force things, light pressure is all that is needed when everything is lined up correctly.