1. Why is specifying the right battery pack so important for subsea instrumentation?
The majority of remotely deployed oceanographic devices depend entirely upon primary (non-rechargeable) lithium batteries for reliable performance.
These applications include GPS tracking, SARSAT/COSPAS distress beacons, animal monitoring, seismic devices, sonobuoys, transponders, and data loggers, to name a few When designing battery packs for larger instruments, you must thoroughly understand the unique power requirements of each application in order to maximize performance and cost-effectiveness. One prime example is a redesigned battery pack that reduced the initial cost and more than doubled the operating life of SOLO II autonomous profiling floats.
Utilized by Scripps Institution of Oceanology as part of the Argo program, SOLO II floats measure climate change by monitoring ocean temperature, salinity, and pressure. Considered the gold standard of the Argo program, SOLO II floats outperform nearly half a dozen similarly designed floats being deployed by an international consortium of nearly 30 nations that collaborate to create a global network of one profiling float for every three degrees of latitude and longitude.
Operating in 10-day cycles, SOLO II floats spend the first 9 days in a ‘standby’ state, drifting with ocean currents at a depth of 1,000 m. On the 10th day, high power is required to actuate a valve that lets in sea water to decrease buoyancy, initiating a controlled dive to 2,000 m depth and 200 atmospheres of pressure. Once at maximum depth, the battery pack powers a ballast pump that transfers oil from an internal reservoir into an external bladder, which increases buoyancy by expanding the float’s volume, beginning a controlled ascent back to sea level. The greatest amount of power is required at 2,000 m depth, with progressively less power required throughout the ascent. Upon returning to sea level, the collected data is telemetered via satellite before the next cycle begins.
2. What are the cost implications of choosing the optimal battery power solution?
Battery life dictates operational life, so the choice of power supply is essential to minimizing the cost of ownership.
For example, the previous generation of SOLO floats were powered by 4 DD-size spiral-wound LiSO2Cl2 battery packs that had an average lifespan of 5 years. As a result, 800 new floats had to be manufactured and launched each year as end-of-life replacements. At a cost of roughly $20,000 per float, this meant that $16 million was being spent each year simply to maintain the number of floats as a means of compensating for expected losses. Moreover, the true cost of replacing those 800 batteries was actually much higher when factoring in the added labor and logistical expenses required to deploy them at sea.
The battery pack was redesigned using 8 D-size PulsesPlus™ bobbin-type LiSO2Cl2 cells and 4 hybrid layer capacitors (HLCs) that store and generate high pulse energy. PulsesPlus bobbin-type LiSO2Cl2 battery packs more than doubled operating life due to their higher capacity and lower annual self-discharge rate.
3. How does passivation affect battery performance in marine environments?
While highly efficient, the upgraded ballast pump used in SOLO II drew more power than previous models, which presented problems for spiral-wound LiSO2Cl2 battery packs, which experienced excessive passivation after 9 days of inactivity, causing voltage drops and delayed response.
Passivation involves a layer of LiCl that forms on the surface of the lithium metal inside the battery when not in use to create a separation barrier that reduces chemical reactivity, thereby reducing battery self-discharge.
As the spiral-wound LiSO2Cl2 battery packs aged, the passivation effect became more pronounced as the separation layer grew progressively thicker, causing the voltage to drop below the minimum threshold required to power the ballast pump. As a workaround, engineers at Scripps had to perform multiple shorter dives to depassivate the spiral-wound cells prior to initiating a full dive cycle. This makeshift solution worked but was time consuming and personally taxing given that hundreds of floats were involved.
4. Was there a more effective solution to extend battery life in this case?
In search of a long-term solution, Scripps collaborated with Doppler Ltd. to test the use of PulsesPlus bobbin-type LiSO2Cl2 battery packs. Major advantages of bobbin-type over spiral-wound construction included higher capacity, a lower self-discharge rate, and lower initial cost. The major drawback of bobbin-type LiSO2Cl2 cells is their inability to deliver continuous high-rate current due to their low-rate design. This challenge was addressed by the addition of a Hybrid Layer Capacitor (HLC), which slowly draws and stores energy from the primary battery to generate high pulse energy on-demand. The HLC is also 90% energy efficient, further contributing to extended battery life.
5. How can you determine if your battery configuration is fully fit-for-purpose?
Scripps tested two variations of the PulsesPlus pack: the preferred version consisting of three packs, the alternate version consisting of two packs, with each pack containing a combination of 8 D-size cells and 4 HLCs. Both versions performed extremely well under a variety of operating conditions without experiencing any excessive passivation. The three-pack version was selected for its higher capacity, which significantly extended battery life.
The initial transition to PulsesPlus packs was to occur over several years. However, the redesigned pack performed so well that project manager Dean Roemmich decided to switch production entirely over to Pulses Plus batteries after one year. The benefits were demonstrable as the 8 D-size bobbin-wound packs were less expensive than the 4 DD-size spiral-wound packs they replaced. The redesigned pack was less expensive, solved the passivation problem, and delivering much higher capacity with a lower self-discharge rate to more than double the float’s operating life from 5 years to 12 years, resulting in a much higher return on investment.
Every application presents unique design challenges. Identifying the ideal power supply requires thorough due diligence as numerous variables must be considered. Consulting with an applications engineer who specializes in batteries is always highly recommended.
This feature appeared in ON&T Magazine’s 2025 June Edition, Deep-Sea Exploration, to read more access the magazine here.
