Where the Meter’s Decade of Charge Actually Goes

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

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math-foundations
calculation-audit
emerging-paradigms
Ada ADA · CALCULATION AUDIT

Where the Meter's Decade of Charge Actually Goes

A utility fits 200,000 residential water meters with battery-only NB-IoT modules, each buried in a pit or basement and expected to run 10–15 years with no maintenance visit. The chapter turns that decade into a daily allowance: a 5000 mAh cell over ten years is about 1.37 mAh/day, of which standby draws only ~0.12 mAh and the rest funds reports costing 0.3 mAh in good coverage but 1.5 mAh in a deep pit. This audit re-runs that budget to find where the meter’s decade of charge actually goes.

Companion to the chapter M2M Case Studies — every number here comes from that chapter.

Ada: The battery table above turns a ten-year promise into a daily allowance. I want to check every step, then find where the charge really goes.

Start with the allowance. A 5000 mAh cell over ten years:

  • 5000 / (10 x 365) = 5000 / 3650 = 1.36986... mAh/day — the chapter’s ~1.37

Now subtract standby. At ~5 microamps drawn all day:

  • 5 uA x 24 h = 120 uAh = 0.12 mAh/day
  • Left for reporting: 1.36986 - 0.12 = 1.24986 mAh/day, about 1.25

Spend that reporting budget at each coverage level:

  • Good coverage, 0.3 mAh/report: 1.24986 / 0.3 = 4.166 reports/day, so “about 4”
  • Deep-coverage pit, 1.5 mAh/report: 1.24986 / 1.5 = 0.833 reports/day, so under one

Here is the part worth pausing on. The per-report cost rises 1.5 / 0.3 = 5x from good to deep coverage, and the affordable report rate falls by exactly the same factor: 4.166 / 0.833 = 5. Coverage, not the schedule, sets the ceiling. And standby is almost a rounding error: it consumes 0.12 / 1.36986 = 8.8% of the daily charge, leaving roughly 91% for the radio.

The audit confirms the chapter’s headline and sharpens it: about nine-tenths of the meter’s decade is spent transmitting, and a deep pit multiplies that dominant cost fivefold — so the lever that decides whether the SLA is met is the measured coverage of each pit, not the sleep current everyone tends to optimise first.

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