Sony HVL-MT24AM Operating Instructions Manual page 12

Checking Batteries
The lamp on the control panel comes on or blinks when the batteries are low.
 If nothing comes on when the flash ON/OFF button of the macro flash
controller is pressed, check the orientation of the batteries.
1
lamp on
Changing the batteries is
recommended. The flash can still be
used in this state if (Flash-ready
lamp) on the rear of the unit is lit.
Only lamp blinking
Flash cannot be used.
Insert new batteries.