Deployment Duration.

Figure 1 compares the duration of a Workhorse Lithium pack with a standard alkaline pack. The exact ratio of lithium to alkaline durations depends on the duration of the deployment and the water temperature, but the lithium packs should enable deployment durations of 3-4 times those supported by an alkaline pack. Alternatively, you can collect 3-4 times as much data with the same deployment duration.

Figure 1. Voltage vs. time for two Workhorse ADCPs, each set up the same but one with a standard alkaline pack and the other with a lithium pack.

Voltage and profiling range.

The voltage of a lithium pack is more constant than an Alkaline pack. The lithium pack’s voltage is generally lower than the alkaline pack, but still stays well above the Workhorse’s 20 VDC minimum voltage. The difference in voltage has no effect on the operation of the instrument because the internal power supply uses a switching regulator, and it takes energy from the battery efficiently over a wide range in voltage. However, it does play a small role in the profiling range. This is because the power amplifiers that drive the acoustic transmission use power at the battery voltage.

Figure 2 compares the voltage of a Workhorse Lithium pack with a Workhorse alkaline pack over a complete deployment. The scale on the right of Figure 2 shows how the battery voltage relates to the transmit power. Relative to the typical Workhorse Lithium pack’s voltage, a fresh alkaline pack’s voltage produces a 3 dB increase in transmit power, about a factor of two. The relationship between transmit power and range is highly variable, but a 3 dB increase in power corresponds to an increase in range of order 10%. However, the voltage of a fresh alkaline pack falls quickly, and the typical profiling range with an alkaline pack will be at best a few percent greater. This is such a small difference in range that it would be hard to detect. An advantage of the lithium pack is that you can expect the range to remain relatively constant during the deployment.

Self-discharge.

Batteries lose capacity just sitting on the shelf. An alkaline pack can lose more than 5% of its capacity in its first year, and more than 20% in 5 years. Over 5 years, our lithium packs will lose less than 5% of their capacity. This difference is particularly important for multiyear deployments.

Passivation

Passivation is a process in lithium cells in which a layer forms on the anode, and which impedes the flow of current. As the layer thickens over time, large currents cause a substantial drop in the output voltage. Because the cells we use supply only low current, passivation is not a performance issue.