Canon AE-1 - Manual (Page 29)

How to Check the Battery
The charge level of the battery is checked
by pressing the battery check button on the
top of the camera Carefully watch the meter
needle in the viewfinder.
Press the battery check button and if the
meter needle in the viewfinder rests below the
index, power level is sufficient. If the meter
needle rests above the index, the power level
is insufficient. If this is the case, replace the
battery with a new one of the prescribed type.
When a new battery with full voltage is used,
the meter needle in the viewfinder indicates
close to the 2.8 f/stop.
When the battery power wanes, it takes more
time for the meter neelde to stop fluttering.
Because of the special circuit, the meter
needle's swing depends on the state of the
battery. The meter needle will rise as the
battery power wanes until it reaches the f/5.6
position where there is no longer any more
power. Press the battery check button until
the meter needle rests still.
When the battery is just about to fail, the
meter needle in the viewfinder rests close to
the index.