The Remora reports 3 different battery level statistics:
1. The battery voltage (sampled at very light load)
2. A battery good digital input, which triggers (on to off) at about 7% remaining life
3. A battery capacity percentage, which only works for Alkaline batteries
Alkaline batteries drop in voltage as they discharge, allowing the Remora to calculate a battery percentage using the lowest observed battery voltage. The voltage follows an S-shaped curved during the discharge, starting at about 6.3 V, and ending somewhere between 4.0 and 4.4 V.
Lithium batteries on the other hand will maintain a high battery voltage throughout their life, right up until they are depleted. For Lithium Thionyl Chloride batteries, which the Remora detects by measuring the high voltage, no battery capacity can be calculated, and the battery voltage is useless for gauging the state of charge. However, the battery low indicator will still trigger near the 7% mark, as the Remora detects an increase in the battery's internal resistance. This is discussed further at the bottom of this article.
Lithium Iron Disulfide batteries (typical non-rechargeable AA lithium batteries) cannot be used in a regular Remora as they don't come in C-cell format. However, were you to use them in a customized Remora, they would be detected as regular Alkaline batteries, and would show 100% battery capacity and no battery low indication. There would be no way to gauge the battery life other than estimating based on the trip count, upload count, and GPS statistics.
How long the Remora will still be able to operate after the battery low indicator is set depends on usage, but is usually a couple of weeks. If you are using Alkaline batteries, you can set an alert on the battery percentage analog input, at whatever threshold you like. It is not recommended to set an alert based on the battery voltage, as it can fluctuate a bit with temperature and duty cycle.
Creating a battery low alert in Telematics Guru:
The recommended way to alert for low battery level is to use the "battery good digital input". This will work on both alkaline and LTC batteries. It will fire at 7%.
This is the input to use (highlighted in yellow):
And you would use that input in an alert like this:
How is the Internal Battery Good flag set for LTC's?
This explanation is specific to LTC batteries.
The Internal Battery Good flag is set based on the following explanation. Remember, if set or 1, the battery is fine. If clear or 0, the battery is low.
The flag is set/cleared based on this.
- First, the device needs to decide if it has LTC batteries. It looks at the open circuit voltage (analogue 1). If the voltage is sampled below 8V, it thinks they are alkaline. If above 8V, they are LTC.
- If they are determined to be LTC’s, it then looks at the loaded voltage (analogue 5), which is the lowest voltage seen when the modem is switched on (in rush current).
- In the first 500 uploads, it clears the low battery flag if the loaded voltage is below 9V - this is because the batteries have quite a high internal resistance out of the box, they loosen up.
- After the first 500 uploads, it clears the flag if the loaded voltage is below 11V. The LTC’s loosen up (resistance goes down) after some time, until they near flat, when it goes up a lot.
The key point is that the open circuit voltage recovers quite successfully on LTC’s, so open circuit voltage isn’t that useful. The loaded voltage is more useful, because that indicates the internal resistance has gone up, which happens when they are nearly flat.