Predicting the battery life of Lithium batteries is quite difficult due to their chemistry. The batteries will output a constant voltage until nearly the end of their life, and suddenly this drops off. Meaning early warning, and accurate gauging of battery life is quite difficult based on voltage alone.
The Oyster2, Falcon and Remora2 are fitted with a coulomb counter, which tracks energy (in mAh), used by the battery. This can be used to estimate the battery percentage, and remaining battery life. However these estimates rely on an assumption of the total capacity of the batteries. This can vary per battery type, and even on a single type quite significantly due to temperature variations.
The Oyster2 'guesses' as to whether the installed batteries are Lithium Iron Disulphide (LiFeSO2), or Lithium Thionyl Chloride (LTC) based upon the battery voltage reading. LiFeSO2 are 1.5V nominal per cell compared to LTC 3.7V
Once this is determined the following assumptions are made:
LTC - capacity of 1650mAh
LiFESO2 - capacity of 3100mAh (Energizer Ultimate Lithiums are assumed)
GIven this knowledge, the remaining battery % can be calculated.
The mAh usage is counted from start up, so while the coulomb counter is far better at measuring the battery level than voltage alone, it is not infallible.
If batteries are swapped, or the device is reset the reported percentage will be incorrect.
The assumptions can be changed under this parameter tab in System Parameters if required.
Low Battery Flag and Alerts for Low Battery
Battery powered devices use a 'Battery Good' Flag. It is set '1' when the battery is 'good', and cleared '0' when the battery is not good.
Check the DM Data Fields Document section on the Digital Input field to see where it is displayed in the status flags if you are using your own platform.
For the Oyster2 it is set when the battery percentage is above 15%, and cleared when below this. Note that firmware newer than 2/3.4 will start to use 25% to be more conservative.
So this is when the standard "Low Battery Alert" in Telematics Guru will fire.
The Battery Percentage is also reported in Analogue Input 6, and is very useful for setting alerts as they can be set at any level and not just the pre-defined 'battery good flag' level.
This is where the percentage should be read from in a 3rd party platform.