Duodometer

The iPhone Duo runs on two battery cells — one in each half of the phone. iOS shows you a single percentage and not much else. Duodometer shows you the rest.
Built specifically for the Duo's dual-battery system, Duodometer gives you:
• LIVE COMBINED CHARGE — the level and charging state iOS reports for both cells together, in a big, clear gauge
• CHARGE BALANCE BETWEEN THE CELLS — see the estimated charge in the 2,962 mAh main cell (inner half) and the 1,921 mAh cover cell (outer half), and how the Duo's stored energy splits between them: about 61% main, 39% cover of the 4,883 mAh total
• DRAIN & CHARGE RATE — how fast you're using or gaining charge, in percent per hour, from readings the app records while it runs
• LEVEL HISTORY — a chart of your battery level over time, sampled on-device
• THE DETAILS — estimated mAh and Wh remaining, Low Power Mode status, and the phone's thermal state

A NOTE ON PER-CELL NUMBERS
iOS manages the Duo's two cells as a single battery and gives apps one combined percentage — no app can read the individual cells directly. Duodometer estimates each cell's charge by applying that combined level to each cell's rated capacity (from Apple's EU product data sheet), which matches how Apple's energy rebalancing keeps the cells working as one. Every per-cell figure in the app is labeled as an estimate. For measured per-cell health, iOS keeps that in Settings → Battery Health.

PRIVATE BY DESIGN
Duodometer collects nothing. No accounts, no analytics, no ads, no tracking, no network connections. Your battery history is stored only on your phone and is deleted with the app.