High capacity oceanographic lithium battery pack

Doppler Ltd. oceanographic battery packs use Tadiran's TL6930[1] low current lithium thionyl chloride primary cells combined with small rechargeable cells called Hybrid Layer Capacitors (HLC).

Low current cells use bobbin electrodes,[3] a simpler design consisting of an inner anode, an outer layer of metallic lithium, and electrolyte between the two.

Most oceanographic instruments use little power most of the time, and they require high currents only for short durations.

Examples are instrument systems that telemeter data through Iridium, and acoustic modems (e.g. from Benthos[4] or Link-Quest[5]).

HLCs are actually rechargeable lithium cells that behave like huge capacitors over a narrow voltage range (Fig.

Combining the primary lithium cells with HLCs enables the pack to deliver more of its stored energy to the instrument system.

The pack includes diodes in each branch to minimize the possibility of reverse charging the cells.

PTCs, or positive temperature coefficient thermistors act as resettable fuses.

The warmth increases their resistance which produces even more heat, and the PTC quickly "trips" and shuts off most of the current.

Any attempt to charge a lithium cell is potentially dangerous, and turning the pack off at about 75-80% of its starting voltage prevents this from happening.

These battery packs are commonly used in RD Instruments[8] Workhorse Sentinel[9] and Long Ranger[10] and in Nortek's[11] AWAC.

In some circumstances, a battery that is stored at a cold temperature could produce a test result that indicates it has been used, even though it holds its full original capacity.

The safety circuit determines that a pack has been used when its voltage falls below a threshold, or if it detects a minimum number of pulses.

The safety circuit causes the pack to become "depleted" when it falls below its end-of-life voltage and turns off.

Fig. 1. Left. Lithium cell voltage profile, compared with an alkaline cell. Both profiles assume depletion using the same constant power. Fig. 2. Right. Comparison of stored charge vs. voltage for HLCs and large capacitors. Over narrow voltage ranges, the HLC's stored charge increases as voltage increases, with slopes that look like 500 F -1400 F capacitors.
Fig. 3. Schematic of a typical Pulses Plus battery pack. The components are: 1) Primary lithium cell, 2) HLC, 3,4) Schottky diode, 5,6) PTC, 7) Safety circuit.
Fig. 4. Safety circuit behind a plastic window on a lithium battery pack.