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Lithium Battery Primer
Deployment tips Design Performance Safety Shipping
Lithium Packs for Pulse Applications
Most oceanographic instruments are designed to minimize the energy they require, and to do this, they turn power on to their various subsystems only when the subsystem requires it. As a result, the dissipated power varies widely over time. Instruments typically spend most of the time in standby or in some relatively low power mode, punctuated with short pulses of high power. Such pulses, for example, power acoustic transmissions or pumps, which are critical subsystems of the instrument.
Traditional lithium battery packs use high-current cells to supply the power the instrument requires for these bursts. High-current cells are constructed in sheets that are wound in a spiral. These sheets increase the current the packs can supply, but overhead associated with the sheets reduces the cell's energy capacity. Low-current lithium cells are constructed more simply and store more total energy, but they often are unable to supply the pulses of energy required to operate the instrument.
Tadiran’s PulsesPlus battery packs take a new approach by combining low-current lithium cells and special rechargeable lithium cells (HLCs or Hybrid Layer Capacitors). The HLCs store sufficient energy to supply the high current pulses, and then the primary lithium cells slowly recharge the HLCs between the pulses. HLCs can supply the current because they have much lower internal resistance than the primary cells. By recharging the HLCs slowly, the primary cells lose less energy to internal resistive losses. This improves the pack's overall efficiency and allows it to produce more useful energy from the pack's primary cells. The combination produces effective and efficient power sources, but the system is more complicated than battery packs that use only primary lithium cells.
Figure 1. Schematic of a typical PulsesPlus battery pack. The pack is organized into ranks (one rank is surrounded by a red dashed line), each of which includes an HLC and several primary lithium cells. PTCs (Positive Temperature Coefficient) and diodes protect the pack.
Figure 1 is a schematic of the double-sized glider battery packs. The pack organizes its cells into ranks,. Each rank holds some primary cells, one or more HLCs and other components that protect the pack.
Figure 2. Battery pack equivalent circuit. In a typical pack, the capacity of the primary cells is of order hundreds or thousands of kJ, while the capacity of the HLC is only a few kJ. However, the internal resistance of the primary cells is 10’s of ohms, while the HLC resistance is typically a fraction of an ohm.
Figure 2 shows a circuit that is equivalent to Figure 1, which illustrates how the battery pack supplies pulses of high current. Because the HLCs have far lower internal resistance than the primary cells, the battery pack’s current under heavy loads comes primarily from the HLCs. Then the primary cells slowly recharge the HLCs between the high current pulses.
The key component in Tadiran's battery packs is the HLC, or Hybrid Layer Capacitor. While the primary cells store far more energy than the HLCs, the HLCs have plenty of capacity for relatively short pulses. For each battery pack application, the HLCs are designed to store enough energy to support the application’s pulse requirements, and its low internal resistance allows it to do so while sustaining the battery pack’s voltage. Cells with high internal resistance experience large sags in voltage as they supply pulses of high current. They store relatively little energy compared with the primary lithium cells (4 kJ for an HLC vs. 200+ kJ for a primary lithium D cell), but they have far lower internal impedance (0.1-0.2 Ω vs. 10-30 Ω for each cell).
HLCs exhibit capacitor-like storage over a relatively narrow voltage range (Figure 3), but cannot store energy below a threshold voltage. In their operational voltage range, they have equivalent capacitance on the order of 103 F (which would be exceptional were HLCs really capacitors).
Figure 3. Stored energy vs. voltage for an HLC1550A.
By releasing just 1200 joules of its stored energy, an HLC supplies a 1 amp current for more than 5 minutes.
Low current primary cells
The low current primary cells used in Tadiran PulsesPlus packs have significant advantages over standard high current cells including higher total capacity and safety. High current cells require large surface areas on the battery cathode and anode, and to do this, they are produced in sheets which are rolled up into the battery canister. This kind of construction leaves less room inside the canister for stored energy, compared with low current cells, which is why low current cells store more energy.
Why passivation is not a factor
When conventional high current lithium batteries passivate, their internal resistance rises. When a heavy load attempts to draw a pulse of high current, the output voltage sags by the voltage drop across the resistance. This voltage drop significantly limits the cell’s ability to supply the needed energy. Passivation is associated with an oxide layer that grows with time, and that dissipates with use. The effects of passivation can be minimized by using fresh cells or by drawing a large steady current from the battery just prior to use. Nevertheless, passivation can be a source of trouble and must often be carefully considered in design.
In contrast to conventional lithium packs, Tadiran PulsesPlus packs are unaffected by passivation. The internal resistance of the primary cells has almost no effect on the pack’s internal resistance because current pulses come from the HLCs, not the primary cells. Then, because the primary cells recharge the HLCs slowly and with low current, there is little voltage drop across the primary cell’s internal resistance, which means that little energy is wasted during recharge. If passivation increases the internal resistance of the primary cells, the increased resistance would have little effect. Furthermore, because the
Battery pack protection
PulsesPlus packs are protected against short circuit. If a pack is shorted, it shuts down. The pack is ready for service again as soon as the short is removed. Because high current lithium cells are capable of far higher discharge, they usually contain internal fuses. Once the fuse is blown, the pack is no longer of service.
The pack’s protection comes from its PTCs and diodes. A cold PTC has less than 0.1 ohm resistance. If an excessive current warms it up, its resistance increases, which in turn limits the current. Typical PTCs in these packs play little role for currents less than about 5 A, but they “trip” in a small fraction of a second under a short circuit. Diode prevent charging or reverse charging of the cells.
Hi voltage packs such as the Workhorse pack require extra protection circuit that shut the pack down when the voltage falls below a threshold. This circuit also provides additional protection against short circuits. You can short a battery pack without harming the circuit or the pack.
Ocean Batteries, 12344 Oak Knoll Road Suite E, Poway, CA, 92064
858-486-4077