What Encryption Costs the Battery
What Encryption Costs the Battery
Ada re-derives this chapter’s own numbers step by step, at full precision
ADA · CALCULATION AUDIT
What Encryption Costs the Battery
A medical wearable team weighs heavyweight AES-256 encryption against the lighter ChaCha20, which the chapter says adds about half the battery drain. On the same 180 mAh cell rated for a 7-day life, AES-256 leaves about 85% of usable capacity and ChaCha20 about 92.5%, stretching runtime from 5.95 to 6.48 days. This audit re-derives those days from the daily draw to ask what encryption really costs the battery — and how much runtime the lighter cipher hands back.
Companion to the chapter How Different Roles Define IoT — every number here comes from that chapter.
See the relationship before changing it
The figure reads from left to right. The blue card is usable battery capacity. The middle card applies this page's rule. The green card is runtime. 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 usable battery capacity, so the numeric fixture does not switch without explanation.
Derive the baseline in four named moves
- 1
Name the input. The chapter baseline is 92.5 %.
- 2
Name the relationship. runtime = 180 mAh x usable fraction / (180 mAh / 7 days)
- 3
Substitute with units. 0.07 x 92.5 = 6.48 days
- 4
Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.
Predict, then change usable battery capacity
Try Predict the direction of runtime = 180 mAh x usable fraction / (180 mAh / 7 days). Test another usable battery capacity, then compare runtime.
Observe A cipher that leaves more usable capacity extends runtime against the same daily load. Reset usable battery capacity to 92.5 and compare runtime.
Explain A cipher that leaves more usable capacity extends runtime against the same daily load.
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: This chapter says swapping AES-256 for the lighter ChaCha20 stretches a medical wearable from 5.95 to 6.48 days on the same 180 mAh cell. The claim treats each cipher as a fixed slice of usable capacity, so let me re-run it from the daily draw.
A 7-day nominal life on 180 mAh implies a daily budget of 180 / 7 = 25.714 mAh per day. Now apply each cipher’s effective-capacity fraction:
- AES-256 at 85%:
180 x 0.85 = 153.0 mAh, giving153.0 / 25.714 = 5.95 days - ChaCha20 at 92.5%:
180 x 0.925 = 166.5 mAh, giving166.5 / 25.714 = 6.475 days, which rounds to 6.48 days - Runtime gained:
6.475 - 5.95 = 0.525 days, roughly half a day, or0.525 / 5.95 = 0.088, about 9% more uptime.
That half-day is the whole security-versus-power tradeoff made concrete — the lighter cipher does not change the physics of the cell, it simply hands about 9% of the runtime back, which is the number a wearable team must weigh against the security margin it gives up.
Every number above is taken from the chapter’s own material and re-derived step by step.