The Battery Report
Device Sentinel reports two types of battery problems, the low battery (one below the threshold), and one that has held a steady charge its whole life, and is now falling toward zero.
The battery report is an HTML page in www/device_sentinel/, one file per day, named battery_report_YYYY-MM-DD.html. Fourteen are retained. Just as with the Daily Brief, battery_report.html, a copy of the newest report is generated so a dashboard Webpage card pointed at /local/device_sentinel/battery_report.html can display the current day's battery conditions and statistics.
One morning in my sensor fleet, a door sensor sat at 24 percent, and a soil sensor at 82. Both batteries were of similar age and more than a year old. By looking at the percentages, the soil sensor appeared to be in better shape. It was not. You see, the door sensor had 2 AAA alkaline batteries and the soil sensor a lithium coin cell, and the two do not decay equally. During its use, an alkaline battery decays at a uniform rate as it slowly empties, but a lithium battery will hold almost full charge for its entire life. Then, when the lithium battery is near its end of life, it decays rapidly from almost full to zero over a short time. A threshold is good for detecting when an alkaline battery needs replacement, but it will not give you enough warning for the lithium.
This is the first section of the report showing every readable cell in ten-point bands, as a bar chart. It is used to see, at a glance, the condition of the battery fleet.
This section of the battery chart shows the cells that are measurably losing charge, sorted by an estimate of the amount of time they have left. Each row in the chart shows the current battery level, its rate of decay, and an estimate of how much time it has till empty. Read it as a ranking, not as a date. What it reliably tells you is which cell to replace first.
The chart also shows you the cells at or below your low battery threshold, and the date each crossed it. The same devices your problem list and daily brief are already telling you about, gathered here so the page holds the whole picture.
This portion of the chart lists all cells that are above the threshold and holding steady. These are the batteries that currently need no attention.
Any device reporting an anomalous level, such as greater than 100%.
Some of your devices are mains powered and do not contain a battery. Other devices' battery entities are disabled and need to be turned on.