The design of a battery involves several distinct
stages: (1) Requirements for the battery must be defined; (2) General battery
parameters, electrochemical system, active versus reserve, etc., must be
chosen; (3) Parameters such as cell size and circuit arrangement must be
established; (4) Finally, one must design the intercell connections and
battery packaging. Each step of this design process should reflect the
requirements of the system and of Navy policy for the safe use of lithium
batteries. Such a design process often involves consideration of many
different types of batteries for each configuration. The comments which follow
apply to primary batteries containing lithium anodes which operate at ambient
Choice of Electrochemical System and Battery
Technical Manual (TM) S9310-AQ-SAF-010 requires that
any lithium battery proposed for use in the Navy shall be supported by a data
package which demonstrates "the validity of the lithium battery selection." A
"valid" battery design will be appropriate in terms of electrochemical system
and cell design. The choice of possible electrochemical systems is often
governed by system requirements such as weight, volume, voltage, current, or
environmental parameters. Chemical systems which are capable of sudden violent
failure or venting of toxic gases should be avoided if they are not needed.
The cell/battery should be no larger than required. High-rate cell designs
should not be used in systems where low-rate designs will suffice. Cells which
have been designed for high rate applications may have limited active shelf
All cells should be designed with a method of venting
which will allow the safe release of internal pressure if the cells should
fail or be abused.
The relative merits of "active" versus "reserve"
batteries are system dependent. A reserve battery is one in which the
electrolyte is stored separately from the cell stack. When the battery is
needed, the user device initiates or "activates" the battery. In certain
circumstances, reserve batteries offer substantial benefits in shelf life,
rate capability, and safety. These benefits may be offset by penalties in
weight, volume, or cost. The use of a reserve battery is restricted because it
cannot provide any energy until it is activated. Reserve cells are designed in
many different configurations and sizes depending on the application and power
requirements. An example of a low capacity electrolyte-activated reserve cell
used for a fuze application is shown in Figure 9-10.
To meet voltage and current requirements, it is necessary to connect several cells into a battery. The question of whether several strings of relatively small cells in parallel are to be preferred to one string of larger cells is open to discussion. Multiple strings offer the reliability advantages associated with parallelism. However, they suffer from the increased risks associated with the possibility of a failure in a large collection of parts versus the increased hazards sometimes associated with the failure of large cells. The relative costs of this choice will be very system dependent.
Certain electrical design practices may be viewed as standard; many of these are required by Navy policy. Cells connected in series or parallel should all be of one chemistry, size, internal design, and manufacturer. Cells in any given battery should have similar age and storage history. Center-tapping a string for a voltage source is to be discouraged as a possible safety risk. Center-tapping as a source of current should not be allowed; it can be a serious safety hazard. Some designers argue that center-tapping is adequately safe if the cells involved are very small and battery housing is strong. Any string of cells which has a potential short circuit hazard should be fused in its ground leg with the smallest possible fuse consistent with electrical and environmental requirements. Separate fuses are required on each leg and tap. Practically, any string which will deliver more than a few hundred milliamperes or warms more than a few degrees when short circuited should be protected with a current-limiting device.
Any string of cells which has any potential for going into thermal runaway through electrical use or abuse should be protected with a thermal fuse or similar device. This will cause the circuit to "open" at elevated temperatures. Cells in parallel with other cells or an outside source of current should be protected from charging with a blocking diode. The installation of fuses on every cell in a string can offer additional safety protection, especially if short-circuiting a single cell might represent a significant safety hazard.
The value of using "shunt diodes" or current-limiting resistors on each cell in a battery must be decided on a system-by-system basis. These devices can reduce certain hazards, but their use can introduce possibilities for failure. Switches in battery circuits should be placed to minimize the danger of accidental activation. No switches should be installed in the ground leg of a circuit.
The mechanical design of any battery and its housing will be governed by environmental, safety and reliability requirements. TM S9310-AQ-SAF-010 requires that, for safety, multi-cell batteries "be constructed so that they are not interchangeable with commercial batteries used in consumer products such as flashlights or radios." All connections should be made using methods which have been approved by the manufacturers of the components being connected. For example, some cells and fuses are not designed for direct soldering unless special heat-sinks or tabs are used. Methods of mechanical connection should be appropriate for the stresses which they will receive. Because errors in assembling a battery could introduce safety risks, TM 9310-AQ-SAF-010 states, "in development programs, assembly of batteries by user personnel should be avoided."
In designing the physical shape of the battery and its container, one must consider problems of thermal management. As a battery is discharged, some energy is released as heat. When the rate of heat generation exceeds the rate at which heat may be dissipated to the surroundings, the temperature of the battery will begin to rise. If the battery becomes too hot, failure will occur. Heat generation can be minimized by careful selection of cell chemistry, design, and discharge rate. Heat dissipation can be enhanced through cell design and by including thermal pathways in the design of the battery and its housing. Mathematical models and the experience of the battery designer may be used to assess the possibility of thermal management problems for a given system. Actual experiments may be required to confirm the success of the design.
If possible, a separate compartment should be set aside for the battery. In all cases, care should be taken to ensure the battery is protected from sharp objects or rough edges. These hazards might pierce the cells or cut through insulation. The compartment should contain a pathway which will allow any gases venting from failed cells to escape before the pressure in the equipment exceeds 50% of its yield pressure. Depending upon the design of the equipment and its proposed use, cells may be allowed to vent to the surrounding atmosphere or the venting products may have to be safely contained within the equipment.
Battery size and shape shall be chosen to prevent
it from being interchanged with common cells or battery
Ample weight, and volume allowance and a
reasonable battery compartment configuration shall be designed into the
Thermal management of waste battery heat shall be
Electromagnetic Interference (EMI), Electrostatic
Discharge (ESD), and Hazards of Electromagnetic Radiation to Ordnance (HERO)
tests shall be conducted, as appropriate, with the battery installed in the
Venting paths in the end item shall be specified.
The end item shall contain a pressure relief vent to keep peak pressure
below 50% of the unit's yield point. If lithium battery powered equipment is
to be carried or used aboard submarines, all venting products must be
contained within the equipment.
If the battery is of the reserve type, careful
consideration and analysis shall be performed to determine the possibility
of inadvertent activation. Also, depending on the scenario of use, an
appropriate wet stand test shall be developed and performed on the
battery.The fusing and diode arrangements shown in Figure 9-11 and Figure