Lights with a side switch consume a tiny amount of power, even if the light is off. With an old school clicky switch, the electric circuit is open when light is off, so drain is zero.
But this is not the case with lights like the TN4A. One problem is that the parasitic drain value of a light is rarely specified by a manufacturer. Another problem is that the parasitic drain value can vary significantly from sample to sample, even though the light output is identical.
So basically the parasitic drain is one big unknown when purchasing a light. And this is a bad combination with a light that you want to leave in the glove compartment and forget.
An easy remedy with a light like the TN4A is to slightly unscrew the battery tube, so you have an open circuit for zero drain. But you must keep this in mind.
