Tap-Mounted LCD Support

My Cleansui On Tap Water Purifier- CSP601E-  has an LCD screen but it does not light up when I use it. Why not?

The CSP601E On Tap Water Purifier has a battery-operated LCD screen which provides a numeric countdown of cartridge life, flow rate monitor and battery life indicator. It will only light up when the product is used in filtration mode. This is to conserve energy and provide a longer battery life.  The low-battery indicator will only light up when the battery needs to be replaced.