The Plant-Pot Energy Budget
The Plant-Pot Energy Budget
Ada re-derives this chapter’s own numbers step by step, at full precision
ADA · CALCULATION AUDIT
The Plant-Pot Energy Budget
Motor duty cycle, sleep current, and rounded battery life, ~4 minutes
A Braitenberg controller can be simple, but the battery claim still needs physics bookkeeping: active motor current, daily duty cycle, and sleep current must all be in the same charge units before the lifetime estimate is trusted.
An IoT plant pot uses two LDRs and two DC motors to rotate toward a window, drawing about 280 mA while active for roughly 5 minutes a day and only 10 µA while asleep the rest of the time. Powered by 2500 mAh AA batteries, the chapter estimates a battery life near 107 days, crediting the motor’s duty cycle rather than the sleep current. This audit asks the question that estimate invites: once the charge ledger is carried through in full, does motor-on time really dominate the plant pot’s battery life?
Companion to the chapter Braitenberg Vehicles: Sensing to Action — every number here comes from that chapter.
See the relationship before changing it
The figure reads from left to right. The blue card is motor-active time. The middle card applies this page's rule. The green card is daily charge. Walk the arrows once: set the input, apply the rule, then read the result with its unit.
The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only motor-active time, so the numeric fixture does not switch without explanation.
Derive the baseline in four named moves
- 1
Name the input. The chapter baseline is 5 min/day.
- 2
Name the relationship. charge = 280 mA x active minutes / 60 + 0.239 mAh sleep
- 3
Substitute with units. 280 x 5 / 60 + 0.239 = 23.57 mAh/day
- 4
Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.
Predict, then change motor-active time
Try Predict the direction of charge = 280 mA x active minutes / 60 + 0.239 mAh sleep. Test another motor-active time, then compare daily charge.
Observe Motor time dominates the plant-pot budget while sleep adds a small fixed daily floor. Reset motor-active time to 5 and compare daily charge.
Explain Motor time dominates the plant-pot budget while sleep adds a small fixed daily floor.
Check yourself
What should you do before trusting a moved-control result?
What does this small model leave out?
Treat 5 min/day at 280 mA, 10 µA sleep current, and the 2500 mAh battery as fixed audit inputs; predict lifetime before following the ledger.
The ledger shows 23.333 mAh/day for motion but only 0.239 mAh/day asleep, producing 23.572 mAh/day and roughly 106.1 days.
Average consumption is current weighted by time; despite its short duty cycle, the motor's 280 mA draw dominates the microamp sleep state by almost two orders of daily charge.
The charge ledger
active charge = 280 mA x 0.083333 h = 23.333 mAh/day
sleep current = 10 uA = 0.010 mA
sleep charge = 0.010 mA x 23.916667 h = 0.239 mAh/day
2500 mAh / 23.572 mAh/day = 106.1 days, rounded at the end
The conclusion is unchanged from the quick estimate: motor-on time dominates. Reducing active minutes from 5 to 2 would save more energy than shaving a few microamps from sleep current.
Every number above is taken from the chapter’s own plant-pot energy example and re-derived step by step.
Technical boundaries: This two-state charge budget omits motor stall and start-up current, driver loss, battery voltage sag, temperature, self-discharge, usable-capacity limits, sensor sampling, and regulator quiescent current.