Davis Weather Station Battery Pack for Cambridge Bay
DavisWeatherStationBatteryPackforCambridgeBay
Under normal installation conditions the Davis weather station uses 3 alkaline C sized batteries for backup power. However in Cambridge Bay and other cold installation locations these batters cannot be used. Alkaline batteries will not operate below ‐18 degrees Celsius. They also perform poorly at temperatures approaching ‐18 degrees Celsius. This is shown in the extract from the Energizer Data sheet for their E93 alkaline battery. This document lists the batteries acceptable operating temperature to be between ‐18 and 55 degrees Celsius. Figure 0.1 Energiser E93 Datasheet Page 1 For colder applications lithium chemistries perform much better. This is shown in the following extract from the Energizer. This document lists the batteries acceptable operating temperature to be between ‐
40 and 60 degrees Celsius. Figure 0.2 Energizer L91 Datasheet Page 1 However, Lithium chemistries have different discharge profiles from alkaline battery chemistries. As such the full charge voltage and discharge voltage is different and must be monitored for state of charge taking the different voltage profiles into account. The performance specifications provided in the data sheets are slightly different between the two chemistries. However the datasheet for the lithium chemistry provides a comparison between the two. The following tables show that the open circuit (no load) voltage for the lithium cells is approximately 1.8 volts per cell for a very short period of time with low to no load. However, the voltage quickly drops to a nominal voltage of 1.5 to 1.7 volts at low (<10 mA) discharge rates. Figure 0.3 Discharge Profile for Lithium Chemistry The pulse response comparison shows that while the initial voltage of the lithium cells is approximately 1.8 volts the initial voltage of an alkaline cell is 1.6 volts. Figure 0.4 Pulse discharge Response Comparison between Lithium and Alkaline Chemistry of Similar Capacities. To summarise, in the case of the Davis weather station with 3 alkaline cells in series compared to 3 lithium cells in series the expected voltages will be as follows. This is highly dependent on ambient temperature. Where higher temperatures will result in higher cell voltages and lower temperatures will result in lower cell voltages. Once the cell reaches the Discharged cell voltage (while under no load) it must be replaced ASAP. Because if the Cells reach the lower limit of the Davis weather stations operating voltage the weather station may fail to operate from the batteries of a power outage occurs. Table 1 Lithium Chemistry Region of charge Initial discharge Nominal Capacity Region Discharged Cell Davis weather Station Lower Limit Max Min (Volts) (Volts) 1 Cell 1.8 1.7 1.7 1.5 1.5 1.4 1.1 1.0 Max Min (Volts) (Volts) 3 Cells in series 5.4 5.1 5.1 4.5 4.5 4.2 3.3 3.0 Max Min (Volts) (Volts) 1 Cell 1.7 1.5 1.5 1.2 1.2 1.1 1.1 1.0 Max Min (Volts) (Volts) 3 Cells in series 5.1 4.5 4.5 3.6 3.6 3.3 3.3 3.0 Table 2 Alkaline Chemistry Region of charge Initial discharge Nominal Capacity Region Discharged Cell Davis weather Station Lower Limit It is unknown what the maximum voltage reading that the Davis weather station may report under normal conditions however the Davis weather station is known to stop working correctly if the batter voltage drops below 3.1 volts. 
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