This page describes tests of a 300 kHz Workhorse ADCP and of its alkaline and lithium battery packs. The principle result is that the lithium pack lasted about 3 times as long as the alkaline pack:

Lithium pack capacity: 5010 kJ/1390 Wh at 0° C
Alkaline pack capacity: 1620 kJ/450 Wh at 0° C

The packs were tested in a refrigerator using identical pulse loads and a small microprocessor that kept track of loads and voltages, which enabled the counting of joules afterwards. Figure 1 shows how the voltage changed with time for the two packs.

Figure 1. Voltage vs. time for the Workhorse lithium and alkaline battery packs.

The lithium pack’s voltage curve matches closely the curve from other packs using different construction, but the same cells. Figure 2 compares the Workhorse pack’s voltage curve with a voltage curve taken from an entirely different pack constructed using lithium c-cells. After the voltage and duration of the comparison pack were rescaled, the shapes of the two curves match each other closely. This is but one example of the consistency of this voltage profile.

Figure 2. Comparison of the workhorse lithium pack’s voltage profile with the profile obtained using a different pack. The comparison pack had a different mean voltage and total duration, but after scaling both, the curves line up closely.

The lithium pack’s internal resistance is far lower than the resistance of the alkaline pack. The resistances are shown in Figure 3. The lithium pack’s internal resistance is determined by the HLC’s resistance. The average lithium pack resistance is about 1.8 ohm, while the typical alkaline pack resistance was 14 ohms. With 9 cells, the average HLC resistance at 0° C is about 0.18 ohm.

Figure 3. Battery pack internal resistance during the test.

To understand the role of internal resistance, consider a battery pack supplying a 1 amp pulse. With 14 ohm internal resistance, the voltage drop across the alkaline pack is 14 volts, which reduces the voltage output from a 35 V pack to 21 VDC. When this happens, 40% of the energy supplied by the pack is dissipated in the pack’s internal resistance. In contrast, with 2 ohm internal resistance, the voltage drop across the lithium pack (2 V) loses less than 10% of its energy to internal dissipation.

How workhorses use power

A Workhorse in operation spends most of its time asleep, using negligible power. When it pings, it wakes up a fraction of a second early, then pings. It stays on long enough to receive and process the echo, then returns to sleep. Figure 4 shows the power surge associated with a 16 m transmit pulse on a 300 kHz Workhorse.

Figure 4. Power surge associated with a 16 m transmit pulse from a 300 kHz Workhorse. This pulse was measured with the ADCP powered by a 5 A power supply at about 35 VDC.

At 35 volts, the transmit pulse in Figure 4 dissipates about 6 joules of energy. A Workhorse Long Ranger using 32 m cells should dissipate 12-24 J in a transmit pulse. Both of these are tiny compared to the 30 kJ energy stored by the lithium pack’s 9 HLCs.

The lithium and alkaline packs respond differently to the transmit pulse. The Workhorse power supply stores much of the energy required for transmit in large capacitors. The capacitors recharge from the battery pack during and after the transmit pulse. Because the alkaline pack has higher resistance than the lithium pack, it takes longer to recharge the capacitor and it dissipates more energy through a voltage drop across its internal resistance. The Workhorse’s internal capacitor makes the ADCP relatively efficient when extracting power from batteries.

Ocean Batteries, 12344 Oak Knoll Road Suite E, Poway, CA, 92064
858-486-4077