Many users inquire as to why they see their battery level reported by sensors vary. A wandering battery level is not necessarily a sign of a battery issue. The manner by which recent Monnit sensors measure battery level can result in an inconsistent battery level reading.
How battery level is reported
Monnit sensors take a sample of the battery voltage to interpret battery level. The relation between the battery voltage and the percent reported in the iMonnit software can be reviewed in this article. The voltage of the battery in your sensor is sampled during the sensor Heartbeat while the sensor’s radio is active. When the sensor’s radio is active, the battery is under the heaviest load it will experience. As a result, the battery level can vary quite a bit while it is under load, and thus the battery level reported in the software will reflect this.
Why Monnit measures battery voltage while it is under load
The stable nominal voltage of a battery while it is not under load is generally much higher than the voltage while it is under load. Therefore, if you measure the battery while it is not under load, you may get the impression that the health of the batter is in better shape than it really is. This is why Monnit measures the battery voltage under load, as it gives a more accurate representation of the condition of the battery. While it does mean that the battery level reported on different Heartbeats may vary, the overall picture of the battery health is more accurate.
The larger picture
If you find that the battery level of your Alta sensor varies between Heartbeats, it is normal behavior. Rather than focusing on a single battery level report, looking at the battery level reported over a larger period of time will give you a more accurate picture of the battery performance.
Effect of radio environment, temperature, and usage
A noisy radio environment and the ambient temperature in which the battery is operating can affect the battery level, especially while it is under load. Therefore if your sensors are operating within a refrigerator or freezer, or if they are in a noisy radio environment (or have a low signal as a result of distance from the gateway), you might see this reflected in the battery level reported by the sensor. Also, the more frequently your sensor’s radio is active, the less time the battery has to recover from being under load. Therefore heavier usage (more frequent Heartbeats) can exponentially affect the battery level. The battery may be depleted more rapidly and the battery level reported may be more inconsistent and varied.
Differences between Gen1 and Alta
Generation 1 (Commercial) sensors did not measure batteries in this manner. This was introduced in our Alta line in order to provide a more accurate picture of the battery performance. This is why you may have Generation 1 sensors that appear to have more stable battery performance when compared to Alta. However, this is not a result of actual differences in battery performance, but rather the method (which is more accurate with the Alta sensors) by which the battery level is measured.
While the battery level variance is common, there are other issues that may indicate a problem with the battery or sensor. If you have questions about this, feel free to contact firstname.lastname@example.org.