Why does range drop when the battery weakens, and how does sleep mode extend battery life?
As the supply voltage falls, the output power and modulation quality of the power stage degrade; that is why the complaint the remote works but the range has halved is a typical symptom of a battery about to die. Sleep mode, on the other hand, drops the transmitter to a current in the microamp range as long as no button is pressed; since only a very small part of the total time is spent transmitting, it is this sleep current that really determines battery life.
The link between range and battery is direct. A 6 dB drop in output power halves the theoretical range exactly. A weak battery both lowers the output power and degrades the modulation, reducing the receiver's decoding success. That is why a weak battery usually shows itself first in range and only much later as complete failure to work.
Other important points:
- Increasing power increases range but also increases the current during transmission: a 6 dB increase means roughly four times the power, hence four times the transmit current.
- As long as no button is pressed the transmitter sleeps; a sleep current in the microamp range gives months of standby. For AXI's SAROZ remote control, sleep mode when not in use with a 23A 12V alkaline battery and months of operation in typical use are stated.
- On rechargeable Li-Po remote controls the internal resistance rises in the cold; the same battery does not give in winter the runtime it gives in summer. The ST100/ST200 remotes are Li-Po rechargeable, the SF100/SF200 remotes use 3 AAA batteries.
- The right behaviour is for the system to stop the user before range becomes uncertain. On the HCT402, when the battery level falls to a critical level, motion control is not permitted and the user is directed to charge with an LED and buzzer warning.
Full article: 433 MHz or 2.4 GHz?