Hi Andrze,
I too think day only is the proper thing to do, but the manufacture had something in mind when they made the day/night unit. I thought they were all day only and was surprised to see they even had a battery. BTW, when the battery goes dead, it becomes a direct short to the solar cells and the thing won't run day or night, but, if you simply remove the dead battery, it works again in the day only mode.
Cheers,
JJ