For oceanographic deployments, there are two important safety considerations. The top priority, of course, is people safety, but the deployment is important too. Because the current is limited, these lithium packs are among the safest batteries you can use, and because they are hermetically sealed, they are far less likely to leak than alkaline packs.

Limited current makes safe battery packs

Conventional battery packs used in Doppler profilers must supply high current to provide the power required for acoustic transmissions. A battery pack that supplies high current can rapidly dissipate its stored energy which in turn generates a lot of heat. The result can be fire, eruption or even explosion.

Our battery packs store the small amounts of energy required for acoustic transmissions in special rechargable lithium cells. While these cells store only a tiny fraction of the energy stored in the primary cells, they can supply short pulses of relatively high current. The primary lithium cells slowly recharge these cells, which they can do with relatively low current. The low current lithium cells used in our battery packs cannot dissipate their energy quickly. This means that they are far less likely to start a fire, erupt or explode.

Lithium cell construction

Our low-current lithium cells use a bobbin-type construction. Lithium foil is pressed against the inside wall of a metal can. The cathode is placed against the lithium with a thin separator layer between the two. The cathode consists of a porous Teflon-bonded carbon powder, filled with a non-aqueous electrolyte.
 

Figure 1. Cutaway drawing of a lithium cell. The battery’s metal case is outlined in blue. The glass-to-metal seal at the top completes the hermetic seal .

Compared with typical aqueous electrolytes, the non-aqueous electrolyte has a relatively high impedance. The high impedance electrolyte, the large separation between cathode and current collector and the small surface area of the anode all combine to limit the maximum current the cell can produce.

High current lithium cells use a different construction in which the anode and cathode are constructed in sheets which are spirally-wound around a central core. This process increases surface area and decreases the distance over which current must pass through the electrolyte, both of which combine to reduce the impedance and thus allow more current to flow. This design has consequences. The first is that, because the layers are thin, there are opportunities for local shorts (hot spots). In contrast, the large separation in our cells between the lithium anode and the current collector makes an internal short extremely difficult to create. A second consequence of the spiral-wound design is that more of the battery volume is taken up by the separator layer, which reduces the energy storage of the cell. A third consequence is that the rate of self discharge is higher.

Leakage

Battery leakage is a concern with alkaline battery packs, especially for long deployments. There are a number of reasons why our lithium packs are less likely to leak than alkaline packs.

The first reason is the lithium cell’s hermetic¹ seal. Figure 2 is a cutaway drawing of an alkaline cell which uses a seal that consists of a vented plastic grommet. An alkaline battery requires a vent because there are circumstances in which it must release gas byproduct. Our lithium cells can be hermetically sealed because gases that are produced inside the cell are quickly dissolved in the electrolyte, and because the speed of the chemical reaction is limited by the inability of the cell to produce high current.
 

Figure 2. Cutaway drawing of an alkaline cell. The plastic grommet and vent at the bottom to not make a hermetic seal.

¹ Hermetic: “Completely sealed, especially against the escape or entry of air.” (back)