Where to post this? Not sure which forum fits, hopefully a mod will fix it.

I bought a little automotive digital clock with illuminated display (red segments). It has + and - power leads, a little setting button on separate leads, and a compartment for a 3-volt button battery, but no installation instructions at all. I am fairly handy with this sort of thing, but not sure how to proceed in this case and the clock was sold (Ebay) with no specs of any kind. 

I figured there would be some kind of dual power connection as with radios to keep the clock chip going when the car is not running with a circuit through the ignition switch to light up the display. This does not seem to be the case. Although the clock is advertised as being suitable for cars and motorcycles, there is only the one power connection with the unexplained battery compartment. 

If I hook the power through the switch, the clock loses its setting when the switch is off (unless the button battery has something to do with this). The only other option is to connect it directly to the car battery, meaning the display is always on. It is LED but will this not drain the battery over time? The display is pretty bright and there are several segments.

I am not sure how to proceed. Does anyone know the correct way to do this?