This may happen if the width of text used for battery status information exceeds the area that macOS allots for information. As a rule, macOS will give traditional applications that use the menu bar (e.g. Finder, web browsers, graphics packages, etc) priority on what to display and will hide information used by menu bar apps like Upper Volta. You can avoid this by not choosing more than 3 or 4 battery indicators at the same time. In the future, I will look to warn users about having too many items selected if they are using a laptop on a lower resolution display mode.

About every 10 seconds except when your Mac is asleep.

Upper Volta reads values straight from your Mac's power controller (via macOS functions). Sometimes, as you switch contexts like going from video editing to sleeping and vice-versa, the power controller has to re-calibrate itself, which accounts for some of the strange time ranges reported. Fortunately, any strange values are usually resolved very quickly (within a minute).

At startup, Upper Volta will run a simple diagnostic by check your battery's health. If the response isn't "Good", then it will look at other signals such as your battery's age, temperature, charge cycles and others that suggest it may need to be replaced. There will be cases where you battery is functioning, but macOS views it as unreliable. In these cases, it pays to have it looked at by Apple.

NOTE: Sometimes due to workload and/or ambient heat, your Mac's battery may be placed under stress, which can sometimes cause Upper Volta to claim your battery has a problem when its actually functioning normally. Usually, rebooting your Mac will clear the issue. However, if it does't, you may want to look more deeply at your battery.