When the pen is dead or low, there must be an explicit signal, not a silent stop

What it is

When the pen battery is empty, or low enough that firmware kills the radio, the user-side view is “I stroked and no ink”. If neither host nor pen says it is power, people treat it as a broken pen, a drop, or a frozen app. The signal must appear before or as response stops, and it must name power, not a generic “unavailable”.

Why it happens

Active pens power a resonator, a force sensor, or Bluetooth from a cell. Below a firmware threshold they first drop capability (pressure off, lower report rate) then power down. If the drop is undeclared, the pressure curve looks broken. After power-down the digitizer may still see a passive capacitive contact (the nib as a finger), so position remains without pressure or buttons—“it draws, but oddly”. If the host polls charge, the UI can show a percent or “please charge”; many pens report charge only when docked on a magnet, so the host is blind while writing. An LED on the pen sits outside writing fixation and is often missed. Silence is worst in an exam, a signature, a meeting note—no second pen, no time to debug in settings. The signal should also distinguish charging (already on the magnet) from still-in-use (minutes left), or people unplug the pen “to see what is wrong” and interrupt the charge.

Studying it

Put the pen in low, critical, and off bands. While writing, log whether ink stops, whether copy appears, how people attribute it, and whether capacitive fallback remains.

Independent variables: charge band, percent reported, LED vs on-screen hint, capacitive points after power-off. Dependent measures: correct attribution, time wasted before charging, unsaved content.

Where it stops holding

A batteryless EMR passive pen does not have this path. A replaceable AAAA cell may have no percent protocol, only a coarse voltage. Notification fatigue (low every day) makes people kill alerts; the threshold should be close to actually dying. When the host itself is dead, the pen’s hint is invisible—that is the other device.

Applying it

• Show charge, or “low, about n minutes of writing”, on the writing surface; at power-off keep one explicit cause.
• When capability drops (pressure off), change the UI; do not keep drawing fake pressure.
• On magnetic charge, switch to “charging” so people do not unplug to debug.
• How to check: from normal use to power-off, the screen should show low then stop, and attribution should be able to pick “power” not “broken”. If a capacitive trail remains after power-off, say “no pressure, please charge” rather than pretend all is well.

Related

• Same group: C5.17.1 Pairing can drop mid-session · C5.17.3 Magnets and silos reduce loss but do not replace find-my · C5.17.4 After a pairing drop, writing should degrade to finger touch
• Adjacent: C5.07 Division of Labor Between Pen and Finger · C5.01 Pressure Sensitivity
• Search: stylus battery · low power warning · silent failure