Where the Meter’s Decade of Charge Actually Goes
Where the Meter’s Decade of Charge Actually Goes
Ada re-derives this chapter’s own numbers step by step, at full precision
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.
See the relationship before changing it
The figure reads from left to right. The blue card is energy per report. The middle card applies the page rule. The green card is affordable reports. Walk the arrows once: set the input, apply the rule, then read the result with its unit.
Derive the baseline in four named moves
- 1
Name the input. The chapter baseline is 0.3 mAh/report.
- 2
Name the relationship. reports = 1.24986 mAh/day / energy per report
- 3
Substitute with units. 1.24986 / 0.3 = 4.17 reports/day
- 4
Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.
Predict, then change energy per report
Try Predict the direction of reports = 1.24986 mAh/day / energy per report. Test another energy per report, then compare affordable reports.
Observe Poor coverage raises radio cost and cuts the affordable report rate by the same factor. Reset energy per report to 0.3 and compare affordable reports.
Explain Poor coverage raises radio cost and cuts the affordable report rate by the same factor.
Check yourself
What should you do before trusting a moved-control result?
What does this small model leave out?
Ready: use the stated baseline inputs, then compare each displayed result.
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 = 4.166reports/day, so “about 4” - Deep-coverage pit, 1.5 mAh/report:
1.24986 / 1.5 = 0.833reports/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.