How to stops flickering in high current draw Circuit
Instead of 







simulate this circuit &ndash; Schematic created using CircuitLab

try







simulate this circuit

where the zener voltage is about 1/2 the normal battery voltage.

This is about as simple as it gets. If you don't want the waste of battery power in the zener, you'll need do something more sophisticated, such as a voltage regulator or constant current drive for the LEDs.