“How do you make a battery last ten years?” Bella the Battery asked. “The secret is PSM – Power Saving Mode. When Sammy finishes sending his data, I tell the radio to completely shut off. My power drops from 15 milliamps all the way down to 5 microamps – that is three thousand times less power! I barely use any energy while sleeping.”
“Two timers control the whole thing,” Max the Microcontroller explained. “T3412 is the big timer – it tells me how long I can sleep before I need to check in with the network. I usually set it to 24 hours for a water meter. T3324 is the small timer – it tells me how long to stay awake after sending data, usually just 30 seconds to wait for a reply.”
Sammy the Sensor jumped in. “eDRX is for when I need to be reachable sometimes. Instead of sleeping completely like PSM, I set an alarm to wake up every few minutes and check if the server has a message for me. It uses more power than PSM, but at least the server can send me a firmware update or a new configuration command.”
“Here is a real example,” Lila the LED said. “A smart water meter with a 5 amp-hour battery, sending one 100-byte reading per day with PSM enabled: sleep current is 5 microamps for 23 hours and 59 minutes, active current is 200 milliamps for about 5 seconds. That math works out to over ten years of battery life. That is the power of proper power optimization!”
