It is useful to estimate the remaining capacity of the Oyster Cellular batteries. The nature of the Lithium AA batteries (LiFeS2) recommend for the Oyster requires a capacity estimate to be made based on sampling the battery voltage after at least 3 hours at a low current. The Oyster attempts 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.

Firmware 1.8 and higher

Firmware 1.1 - 1.7

Notes

• This applies to Oyster Cellular devices only.
• These groups do not imply a linear relationship in each range. The voltages can only be used for rough classification into one of the given ranges.
• More accurate estimations are not possible without a fuel gauge (available in Oyster 2), because of the battery chemistry.
• 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 Capacity Testing

Predicting the battery life is quite difficult, due to the wide range of operating conditions.

The battery life is affected by:

• The type and quality of batteries used
• The type of modem (2G, 3G, LTE), and the average signal strength
• The average GPS signal strength available when installed
• The average temperature of operation
• The operating mode (periodic, GPS based tracking, accelerometer based tracking)
• The particular configuration of that mode
• The level of asset activity (trip quantity and length, loading / unloading jostling)

To help inform the battery life prediction and evaluation process, many discharge tests have been performed, and approximate battery cost constants extracted.