HP97 The journey begins

[mfleming]

Modern batteries don't really fit the old voltage decline under load as cell discharges model. A voltage trip point would be pretty fiddly, and it might occur without informing the person if you're drawing current when off to maintain state.

Probably too late in the design for this suggestion but you could calculate the amount of energy consumed versus battery capacity, then use appropriate setpoints for warning vs. charge now annunciation. Normally you'd measure voltage drop across a low series resistor, but a simple formula could be a static discharge constant plus sum of lit segments. Not particularly accurate without a bit of tuning and tweaking, but good enough as a "horseshoes and hand grenades" level approximation. And it would be repeatable, inspiring confidence.

Perhaps use a low priority heartbeat interrupt handler to calculate and accumulate, reset when battery under charge?