Cordless telephones use rechargeable battery packs as power sources. These telephones draw significantly less power in standby mode than during calls. Nonetheless, all their functions gradually discharge their batteries, from displaying time and date information on an LCD screen and retaining phone numbers in a dialling directory, to signalling incoming calls and backlighting the display and button controls. To avoid running out of battery power during or while waiting for a call, users need to know when and whether the telephone needs recharging.
Auditory Low-Battery Signals
Low cordless-telephone battery power causes several problems before reaching the point of failure. Call quality and volume may drop, especially if the user ranges far away from the base station. Most cordless phones send a distinctive beep or chirp through the handset at 15- or 30-second intervals to alert the user that battery strength has dropped below the level of functionality. This chirp is different from tones used by telephone companies to signal special call features. Once the low-charge chirp begins sounding, the battery may be able to continue supporting a call in progress for several minutes before running out, depending on the distance between user and base station and interference from other objects or devices. The audio may drop out for the length of each chirp on the other end of the conversation.
On-Screen Display Signals
The LCD display screens on most current cordless telephones include a variety of information, such as date and time readouts, context-specific function labels, menu items, dialling progress and whether the phone's ringer has been switched off. Most phones also include some form of a battery-strength indicator, usually a set of bars or bands that fade out one by one as battery power drops. Cordless-phone displays also provide a visual confirmation of the low-battery chirp signal, displaying the words "low battery" or flashing an icon designating that status on the screen. These visual indicators are subsidiary to the chirp signals because they aren't visible unless the phone is away from the user's ear.
Placing a cordless telephone in its charging base or cradle allows its battery pack to regain a viable charge over a period of hours. The same indicator that displays battery strength while the phone is out of the cradle also shows charging progress. Some cordless phones include a lighted indicator on the charging stand to signal charge-cycle completion.