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.
Below is the discharge profile of Energizer Ultimate Lithium L92 (AA) batteries.
Due to this flat profile, it is hard to say that a certain voltage means a certain percentage of battery remaining. Each percentage range is fairly broad. However we have developed a variety of methods to aid in achieving the best possible battery capacity estimates. Device firmwares handle most of this automatically in the background.
Battery Meter - Coulomb Counter
The Remora2, Falcon, Eagle and Oyster2 are fitted with a battery meter (coulomb counter) that tracks energy (mAh) used by the battery.
A coulomb counter is a device which measures the total amount of charge flowing out of the battery. This can be used to estimate the battery percentage, and remaining battery life.
This is superior to voltage alone. Considering the discharge profile graph above, for the first 600mAh per cell of capacity used, the voltage changes minimally. So we can't make accurate capacity estimates when we only have voltage to go off. However the battery meter would be able to detect this and therefore tell what percentage has been utilised - i.e. 600mAh/1200mAh = 50%.
Despite the excellent battery capacity estimates the battery meter provides, these estimates still 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. So the estimates aren't infallible.
How the battery meter generates the capacity estimates is detailed below.
Oyster2 and Falcon
The device 'guesses' as to whether the installed batteries are Lithium Iron Disulphide (LiFeS2), or Lithium Thionyl Chloride (LTC) based upon the battery voltage reading. LiFeS2 are 1.5V nominal per cell compared to LTC 3.7V
i.e. 3 x 1.5 LiFeS2 batteries will result in around 5.2V total supply voltage when fresh. The LTCs will read around 12V.
Once this is determined the following assumptions are made:
- LTC - capacity of 1650mAh
- LiFES2 - capacity of 3100mAh (Energizer Ultimate Lithiums are assumed)
So how do we know the percentage?
If the coulomb counter measures 1650mAh has been used, and the battery voltage is low enough to suggest LiFeS2 batteries, then we can say there is 1650mAh remaining, i.e. - (3100-1650)/3100 * 100% = 50%
14Ah capacity of the 2 x D Cell LTC batteries is assumed.
Eagle 4G 4x C Cells default mAh are:
- Alkaline Cells - 7000mAh
- LTC Cells - 6000mAh
Other battery types and adjusting the capacity assumptions
The assumptions for battery mAh and capacity can be changed under this parameter tab in System Parameters if required.
In general, these should remain unchanged (do not touch these parameters unless you know what you are doing). However if you are finding you are regularly getting greater/less capacity from your batteries, or you are using a different type of batteries with a different total capacity - they can be adjusted.
Since other battery powered devices in our range do not have a battery meter, the battery estimate must be made on voltage alone. We recommend Energizer Ultimate Lithium for the following devices:
- Yabby GPS + Yabby Wi-Fi
- Oyster v1
- Oyster Sigfox
- Oyster LoRaWAN
- Yabby LoRaWAN
- SensorNode BLE
- SensorNode LoRaWAN
The above all use 3 x AA 1.5V Lithium Batteries. The Oyster Sigfox can however be used with Alkaline batteries if needed - percentage -> voltage ranges for these batteries can be found here: Oyster Sigfox Battery Curves.
The nature of the Lithium AAA batteries (LiFeS2) recommended requires a capacity estimate to be made based on sampling the battery voltage after at least 3 hours at a low current. The devices attempt to sample the battery voltage at a low current, and the highest recent sample is stored in Analogue 1. This can be used to make some rough estimations of the remaining capacity. Given all devices make use of 3 x 1.5V Lithium cells - and this is the voltage that is being measured, the ranges below is the same across all devices which use 3 x cells (AA or AAA is the same).
The below voltage ranges therefore apply to all the above devices.
'Battery Good' Status Flag
|4.85 – 5.30 V||50% + remaining||Set (1)|
|4.20 – 4.85 V||0 – 50% remaining||Clear (0)|
The Guppy BLE and Guppy LoRaWAN only use 2 x 1.5V Lithium Cells, so the total voltage is always 2/3 that of the other devices. So the capacity for these devices becomes:
|3.25 - 3.5V||50% + remaining|
|2.8 - 3.25V||0 - 50% remaining|
- This will be less accurate if the device does not regularly sleep for 3 hours. For example, hourly heartbeats may not allow the battery voltage to recover completely, leading to a lower estimate.
- Further to the previous point, the voltage will not recover completely if the device is usually colder than 5 degrees C. The low temperature stops the battery voltage from recovering completely, providing a lower capacity estimate.
- Analogue 5 (Loaded Voltage) is an instantaneous sample of the battery voltage, and varies considerably with device activity. This is not useful for battery capacity estimates.
Battery Good Flag and Alerts for Low Battery
The 'Battery Good' Flag is bit 1 of the digital status flags (Field ID 2) - or Digital input 25 in Telematics Guru.
The Remaining Battery Percentage is reported in Analogue 6 in the device data and in Telematics Guru.
Cellular devices devices use a 'Battery Good' Flag. It is set '1' when the battery is 'good', and cleared '0' when the battery is not good (see table above) This applies to all cellular devices - both battery powered devices, and the internal back up battery of powered devices. The device firmware selects an appropriate voltage level to 'unset' the Battery Good Flag. This means integrators can simply set up a low battery alert to fire when this flag is unset - and it will operate consistently across all DM devices.
This is how the Telematics Guru 'Low Battery' alert works.
For the devices with the battery meter (Oyster2, Remora2, Falcon), the low battery flag is unset when the remaining battery percentage drops below 20%. For the Eagle this occurs at 15%.
How do I monitor and alert on battery level in Telematics Guru and 3rd Party Platforms?
3rd Party Platforms
For devices with a battery meter - the remaining battery % is in Analogue 6. A low battery alert can be set at any level, or use the low battery flag - which fires at 20% remaining.
For other devices, use the 'battery good' flag from the status flags in Field 2. This flag uses the voltage ranges in the table above. Alert when this bit is '0'.
Predicting Battery Life
Devices with the Battery Meter provide an excellent way to estimate how long batteries will last in a given application.
We can simply run the device in the intended application, with the desired settings, and take note of the remaining battery percentage over time.
So for example, if we run an Oyster2 in the intended application for 4 weeks, and 4% of the battery is used in this time, therefore we know that the usage rate is 1% per week, and we will get 100 weeks of life.
Telematics Guru's Battery Management page handles this all for us, and displays the estimated run time for all devices in a grid. Battery life is impacted by a wide variety of factors and the actual usage as measured by the Battery Meter is the true real-world usage which has all these factors accounted for.
To get a rough estimate of battery life when assessing the suitability of a device for an application, the following battery estimate calculators can be used: