During the past year, EMPF at ACI conducted a Thermal Battery Development project which was funded by the U.S. Navy ManTech. This Thermal Battery Development project is an example of the various partnerships the EMPF/ACI has with industry, academia and government laboratories. Specifically, this program showcases EMPF’s role in the identification and solution of manufacturing technology issues affecting batteries and power electronics systems used in DoD applications. Other power system projects ACI has conducted are the SDV battery, PRC-112 radio battery, MOFA reserve battery, and REPTILE power systems.

This U.S. Navy ManTech program was developed in response to military demands for munitions batteries with long shelf lives. The program goals were to demonstrate thermal battery manufacturability and to develop test data criteria for qualifying new thermal battery manufacturers. Specific objectives project were the identification of manufacturing processes that pose challenges to any new supplier of thermal batteries and the documentation of baseline test processes and test data to use as a reference for interested new market entrants.

Thermal batteries are single use batteries that are inert at room temperature. They are referred to as “Thermal Batteries” because their internal temperature must be raised above 400°C before they become electrochemically conductive (active). Another name for thermal batteries are “molten salt” batteries. These batteries belong to the “Primary Reserve” class of batteries. Primary batteries are discharged once and discarded afterward. A consumer market example of a primary battery is an alkaline flashlight battery. Secondary batteries, commonly called “rechargeable batteries,” can be reused after charging. Typical consumer market secondary batteries include lead acid batteries, NiCd batteries, NiMH batteries, and lithium ion batteries. Reserve batteries are batteries designed to be stored for years, even decades, without performance degradation. Reserve batteries are stored in an inert state and can be activated within a fraction of a second with no degradation of battery capacity or power. The demand for reserve batteries by the consumer market place is minimal.

Thermal battery cells typically have an alkali or alkaline earth metal anode, a salt electrolyte, and a metal salt cathode. A pyrotechnic heat source is inserted between the cells in the battery stack. At room temperature, the salt electrolyte is a solid. The battery is activated by a lighting an internal fuze that in turn lights the internal pyrotechnic materials. The pyrotechnic materials raise the internal battery temperature to between 400 – 700°C. Once the internal temperature is above the 400ºC, the electrolyte melts and the battery becomes active (electrochemically conductive). The time the cell is active and produces power is called the run time and it can be no longer than the time the electrolyte remains molten. Run times are usually under 10 minutes, but some designs allow for run times of up to several hours. The battery returns to an inert state as the battery cools below 400ºC and the electrolyte again becomes a solid.

Applications for thermal batteries
Thermal batteries are suitable for applications where the battery may be stored for years, or even decades, with no maintenance prior to use. They are good for applications where the battery must withstand extreme temperature storage conditions and high g forces. For example, this condition occurs when a munition is shot out of cannon. Yet despite this abuse, the battery must provide high rate power discharge within a fraction of a second after activation and have no degradation from the original performance design. Typical applications for thermal batteries are aircraft ejection seats, countermeasure devices, guided artillery, mines, torpedoes, guided bombs, and missiles.

Advantages and disadvantages of thermal batteries
The advantages of using thermal batteries are that they have long shelf life and do not need maintenance while in storage. Once they are hermetically sealed, they are electrochemically stable. Thermal batteries do not outgas or swell. They can be stored for decades. They can operate in a wide range of environmental temperatures (-65°C to +75°C). They can withstand high G forces and they deliver high power levels. After they are used and have cooled down, they are inert and can be safely stored prior to disposal.

The disadvantages of thermal batteries are that they are primary batteries and can only be used once. For critical applications, this raises a reliability issue because it is impossible to confirm the performance of each thermal battery prior to use. Samples from the manufacturing lot can be tested, but not the specific battery.

Another disadvantage is that when they are activated, the battery is very hot. Typical surface temperatures exceed 260°C and the internal temperature range is 400°C to 700°C. The munitions system must be designed to withstand the heat and yet not transfer heat away from the battery. Premature cooling of the battery will reduce run time and reduce the power available to the system. It is best to thermally isolate the electronics and the munition from the thermal battery. In addition, thermal batteries are susceptible to activation when shot or pierced by enemy fire. If enemy fire pierces a thermal battery, the pyrotechnic material may light and prematurely activate the battery which in term could activate the munition.

The final disadvantage is the cost and the limited supply of thermal batteries. There are limited commercial uses for thermal batteries and as a result there is a lack of commercial incentive to create large scale production lines with sophisticated automation technology. Thermal batteries are typically made on small production lines in small lots using a mixture of automated and hand assembly manufacturing techniques. For example, the electrode pellets are typically cold pressed. Cold pressing pellets limits how thin the electrodes can be made. Cold pressed thin pellets are not as robust as thick pellets and are susceptible to cracking and breaking. Thinner electrodes would improve discharge rate and voltage stability under discharge load. In addition, thinner electrodes would allow the smaller thermal batteries to be designed for miniature munitions electronics. There have been recent research and development advances to create thinner electrodes by utilizing hot pressing and plasma spraying methods. However, due to small sizes of orders for thermal batteries, thermal battery manufacturers are reluctant make the investment in manufacturing automation that would be needed to implement these advances, since there would be little or no return on this investment. Producers of raw thermal battery materials, such as the powders for the pellets, do not cater to thermal battery manufactures. Thus, the purity of materials can vary from batch to batch and the battery manufacturers must purify and lithiate the powders, which also adds to the cost.

The future
Recent advances in other battery technologies have made it possible to consider other battery chemistries for some thermal battery applications. For example, recent advances in lithium ion batteries have significantly raised the discharge rates that lithium batteries can be designed for. Along with advancements in reducing charge time, it is possible to consider using rechargeable lithium batteries for some critical applications where testing the battery prior to application is preferable and where the lack of battery maintenance is less critical.