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.
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.