Measuring battery voltage is more complicated than it sounds. Exactly when and how a battery is measured will influence the results. The results will be different if you measure the voltage loaded or with no load. This is why if testing batteries we recommend using a battery tester that applies a load before taking a measurement instead of simply a multimeter.
As Lithium batteries age, their internal resistance increases. So when we draw a fixed current from them, the voltage under load reduces over time. Additionally, if the device sleeps for some time (i.e. longer than 6hrs) - the battery voltage can 'recover' somewhat.
The Oyster3 reports 2 voltages:
- Analogue 1: Battery Voltage
- Analogue 5: Loaded Voltage
- A1: Battery Voltage
- This is sample when there is little load on the batteries, and the reported voltage is the highest voltage seen in the last 3 days. This provides some smoothing, as battery voltage can get a bit 'noisy'
- A2: Loaded Voltage
- This is sampled at a period of high load (during an upload when the modem is on) - and the lowest value is reported.
Below is a discharge curve of LTC batteries in an Oyster3.
- It can be seen that at the end of the battery's life, the loaded voltage dips sharply (as the internal resistance has risen sharply)
- The drop off does not occur for some time, which is why it is difficult to accurately predict remaining battery capacity based on voltage alone, for the first 80-90% of the battery's life, it is constant so we can't make a judgement
- The Oyster3's firmware has in-built battery level monitoring functionality - and will unset the 'battery good' flag at these thresholds for each battery chemistry
- LTC - 7V
- LiFeS2 - 4.85V
- Alkaline - 3.6V
- This graph underlines why when the batteries go flat, the voltage in OEM/TG will still read around 10V for LTCs, and around 4.6V for LiFes2 batteries. Even though these voltages are above the 3.8V minimum required to power the device, as soon as the battery is loaded this voltage drops below this and can't power the device long enough to get an upload through.