The Sleep Floor Sets the Battery Life

Ada re-derives this chapter’s own numbers step by step, at full precision

ADA · CALCULATION AUDIT

The Sleep Floor Sets the Battery Life

A battery-powered ESP32 environmental-monitor node sends a reading every 300 s, drawing 160 mA during the 0.2 s WiFi transmit but only 0.8 mA asleep the rest of the time. On its 3000 mAh pack that averages 0.92 mA and lasts about 136 days, clearing the 7-day target by 19x. This audit rebuilds the average current term by term and asks whether the headline 160 mA transmit peak or the 0.8 mA sleep floor really sets that battery life.

Companion to the chapter Capstone Projects — every number here comes from that chapter.

Ada: The panel above headlines a 160 mA transmit current, but I want to check which term actually decides the 136-day battery life. I will rebuild the average current from the chapter’s own duty cycle: the radio transmits 0.2 s out of every 300 s.

• Active duty cycle: 0.2 / 300 = 0.00066667 (about 0.067% of the time)
• Transmit contribution: 160 mA x 0.00066667 = 0.10666667 mA
• Sleep contribution: 0.8 mA x (1 - 0.00066667) = 0.8 x 0.99933333 = 0.79946667 mA
• Sensor contribution: 20 mA x 0.00066667 = 0.01333333 mA
• Average current: 0.10666667 + 0.79946667 + 0.01333333 = 0.91946667 mA, which rounds to 0.92 mA

Feeding that into the 3000 mAh pack: 3000 / 0.92 = 3260.9 hours, and 3260.9 / 24 = 135.9, so about 136 days, clearing the 7-day target by 136 / 7 = 19x.

Now the part worth noticing: the sleep term is 0.79946667 / 0.91946667 = 86.9% of the average, while the headline 160 mA transmit peak is only 0.10666667 / 0.91946667 = 11.6%. Because the radio is awake just 0.067% of the time, battery life is governed almost entirely by the 0.8 mA sleep floor, not the transmit peak, so on this device shaving the deep-sleep current buys far more runtime than a faster or lower-power transmitter ever could.

Every number above is taken from the chapter’s own material and re-derived step by step.