Battery Pack Design
The purpose of a battery pack is to provide a convenient integrated power source for portable applications. The advantages of using custom designs are outlined in the section on Benefits of Custom Packs. The pack may fulfil several functions:-
- It enables higher voltage or higher capacity batteries to be built up from low voltage, low capacity cells.
- It houses a cell or a bank of cells together with the associated interconnections in a single convenient pack.
- It accommodates any necessary monitoring and electronic protection devices or circuits within the pack.
- It can accommodate additional circuitry such as indicator lights, heaters, cooling ducts and solar panels.
- It matches and meshes with the cavity in the product which the battery is intended to power providing both electrical and mechanical interfaces.
- It can provide unique electrical and mechanical interfaces to ensure compatibility both of the battery with the intended product and the charger with the battery.
Two examples of battery packs from Axeon Power are shown below.
The pack on the left is a 12 Volt 30 Ah Lithium Ion battery used for seismic instrumentation. It uses 32 pouch cells in a 4 series, 8 parallel configuration. The pack incorporates heaters which enable it to work down to -30°C and a solar panel which keeps the battery charged.
The pack on the right is a 3.6 Volt 800 mAh battery employing three Nickel Metal Hydride cells used in mobile phones. The gold plated connector is moulded into the plastic frame.
See also Cell Construction
Capacity and Voltage
With a simple series chain of cells, the battery capacity in AmpHours is the same as the capacity of the individual cells since the current flows equally through all the cells in the chain.
High battery voltages are achieved by adding more cells in a series chain. The voltage of the battery is the voltage of a single cell multiplied by the number of cells in the chain. This does not increase the AmpHour capacity of the battery , but it increases the WattHour capacity, or the total stored energy, in proportion to the number of cells in the chain.
Battery capacity can increased through adding more parallel cells. This increases the AmpHour capacity as well as the WattHour capacity without increasing the battery voltage. For batteries with parallel chains the capacity of the battery is the capacity of the individual chain multiplied by the number of parallel chains.
Whereas cell voltage is fixed by the cell chemistry, cell capacity depends on the surface area of the electrodes and the volume of the electrolyte, - that is, the physical size of the cell. If at all possible the number of cells in a pack should be minimised to simplify the design and to minimise potential reliability problems. Fewer cells require fewer support electronics. Thus parallel chains should be avoided by specifying the highest capacity cells available. Design issues for multi-cell batteries are considered further in the section on Cell Balancing.
NOTE Cells with different capacities or cell chemistries should not be mixed in a single battery pack.
Pack Design Options
The design of the outer package or housing of the battery depends to a great extent on the components it has to accommodate and the physical protection it has to provide for them. These components are not just the cells, but also protection devices, electronic circuits, interconnections and connectors which must all be specified before the final battery case can be designed. For high power, high energy batteries robust packaging is required for safety reasons.
The ultimate shape and dimensions of the battery pack are mostly governed by the cavity which is planned to house it within the intended application. This in turn dictates the possible cell sizes and layouts which can be used. Prismatic cells provide the best space utilisation, however cylindrical cells provide simpler cooling options for high power batteries. The use of pouch cells provides the product designer more freedom in specifying the shape of the battery cavity permitting very compact designs.
The orientation of the cells is designed to minimise the interconnections between t he cells.
Besides the cells many battery packs now incorporate associated electronic circuits. These may be protection devices and circuits, monitoring circuits, charge controllers, fuel gauges, and indicator lights. Electronics for high power multi-cell packs also include cell balancing and communications functions.
The packs may also be designed to deliver more than one voltage from the basic cell combination, although applications requiring multiple voltage sources are more likely to make provision for this within the application. See Multiple Voltages
In addition to the basic battery support electronics the battery pack may include other functions such as heaters to extend the lower working temperature or solar cells to keep the battery fully charged. These circuits in turn have their own control circuits.
Space, fixing points and methods and interconnections need to be allocated for all these electronic circuits.
Sofware is a major component of Lithium batteries, particularly for automotive applications. See the section on Battery Management Systems (BMS) . Control systems are required to keep the cells within their specified operating range and to protect them from abuse. Fuel gauging needs complex algorithms to estimate the state of charge (SOC). Communications with other vehicle systems are needed for monitoring the battery status and controlling energy flows.
Low power cells are usually connected together using nickel strips which are welded to the cell terminals or the case. Soldering is not recommended since the soldering process is apt to apply large, uncontrolled amounts of heat to the battery components which may damage the separators or the vents which are normally made of plastic. Modern computer controlled resistance welders allow much more precise control of the welding process, both limiting the amount of heat applied to the battery and localising the heat to a small desired area. Welding also provides a stronger, low resistance joint. The interconnecting strips often have complex shapes and profiles which may be stamped out of flat strip in a progressive die.
High power cells may use solid copper bus bars or braided straps.
The electronic components are usually mounted on a conventional printed circuit board (PCB).
Flexible PCBs may cost more than rigid PCBs but they can be used to reduce the overall product costs. Not only do they save weight and space but they also provide more packaging options and they simplify physical interconnections and assembly operations as well as eliminating the need for connectors. Connectors may in fact be specified to facilitate assembly and disassembly if the design requires that individual battery components need to be changed or serviced but there is usually a cost and reliability penalty associated with such designs.
The type of terminals or connections to the external circuits depend on, the current to be carried, the frequency with which the battery may be connected and disconnected and the design of the design of the circuit to which the battery will be connected.
For low power circuits, gold plated contacts are the terminals of choice for connectors which are subject to frequent insertions. Gold is hard wearing, it has low contact resistance and doesn't oxidise. Flying leads with spade terminals or snap on studs are also used for low power applications. Metal tabs are also used on pouch cells.
Terminals for high power applications are usually threaded metal studs to ensure a reliable connection. Safety requirements on high voltage batteries may also dictate shrouded terminals to prevent accidental exposure of the operator to dangerous voltages or of the battery to short circuits. Keyed or terminals or connections are also advisable to prevent connection to incorrect chargers or loads.
Thermal management is a major issue in high power designs, particularly for automotive applications. See details in the Thermal Management section. As part of the battery system, it may be necessary to provide air or water cooling ducts, pumps or fans and heat exchangers for high temperature working or heaters for operating in low temperature environments. The layout of the cells should be conducive to managing heat flows within the pack.
The battery casing has to provide the mechanical and electrical interfaces to the product it is designed to power as well as to contain all the components outlined above.
The simplest and least expensive packaging for small batteries is shrink wrap or vacuum formed plastic. These solutions are only possible if the battery is intended to be completely enclosed by the finished product.
Injection moulded plastics are used to provide more precision packs. For enclosed packs designs using a minimum of materials are based around which a plastic frame holds the components in place thus minimising the cost, the weight and the size of the pack. The overall product cost can be further reduced by using insert mouldings in which the interconnection strips and the terminals are moulded into the plastic parts to eliminate both materials and assembly costs. Overmoulding may also be used to encapsulate and protect small components or sub-assemblies.
Case for 3 AAA Cells
Case for a Single Prismatic Cell
Insert Mouldings Showing Cell Interconnecting Strips
In some designs the battery pack forms part of the outer case of the end product. The colours and textures of the plastic must match the plastics of the rest of the product even though they may come from a completely different supplier. These designs are usually required to incorporate a mechanical latch to hold the battery in place. Again this latch as well as the terminals must interface with plastic parts from a different supplier so high precision and tight tolerances are essential. ABS polymers are the materials typically used for this purpose.
Batteries for traction applications are usually very large and heavy and subject to large physical forces as well as vibrations so substantial fixings are required to hold the cells in place. This is particularly necessary for batteries made up from pouch cells which are vulnerable to physical damage. Automotive battery packs must also withstand abuse and possible accidental damage so metal casings will normally be specified. The metal pack casing also serves to confine any incendiary event resulting from the failure of a cell or cells within the battery and to provide a measure of protection for the user. At the same time the case must also protect the cells and the electronics from the harsh operating environments of temperature extremes, water ingress, humidity and vibration in which these batteries work.
Usually the complete pack is replaced when the battery has reached the end of its useful life. In certain circumstances however, for instance when the pack incorporates a lot of electronic circuits, it may be desirable to design the pack such that the cells within the pack can be replaced.
14.4V 12Ah Lithium battery pack in an off-the-shelf case
If the design requires provision for replacement of the cells the casing of the battery pack must be designed to clip or screw together. Normally the parts of the plastic housing will be ultrasonically welded together both for security and for low cost as well as to prevent unauthorised tampering with the cells and the electronics.
Thermal effects need to be taken into account and, tolerances must allow for potential swelling of the cells. Some Lithium pouch cells may swell as much as 10% or more over the lifetime of the cell. For this reason potting is not recommended. In low power designs groups of pouch cells may be shrink wrapped but for higher power applications plastic or metal frames may be used both to provide physical protection of the cells as well as to allow for swelling.
The battery pack should not normally be airtight or sealed since many batteries release hydrogen or oxygen during operation which could cause bursting of the pack or an explosion if the gases are allowed to accumulate. Lithium cells do not emit gases under normal circumstances, but in the case of failure and thermal breakdown, inflammable gases may be vented by the cells. Some form of ventilation or purging should be provided to avoid these problems.
Tolerances should also allow for the use of alternative cells from other manufacturers. While the cells may be "standard" sizes, there could still be differences between cells from different vendors.
High power batteries may need special ventilation or channels between the cells to permit forced air or liquid cooling.
The pack design must incorporate some form of identification to indicate the manufacturer's name, the cell chemistry, the voltage and the capacity as well as the country of manufacture. Most manufacturers will also include a date stamp and or serial number to assist traceability in case of problems. This information is usually provided on a label but it may also be printed directly on to the battery casing.
The costs involved in designing custom battery packs are often underestimated.
As an indication of the order of magnitude, some very rough cost estimates are given below. They assume that the manufacturer posesses all the necessary standard production resources and they include the pack maker's profit margin and warranty provision. Costs could be lower if the packs are designed and made in house, but then some investment in capital equipment may be required and possibly some recruitment and training costs.
- Design Engineering Costs
Low power batteries are usually designed for very high volume production and costs may be calculated to one thousandth of a cent. Most battery packs include some form of battery management electronics, even the smallest designs used in applications such as mobile phones incorporate integrated circuits mounted on a printed circuit board . The mechanical engineering effort however is the activity most often underestimated. It involves the design of precision thin section plastic parts and their associated complex moulding tools as well as metal stampings requiring precision stamping dies. Component sourcing as well as cell selection and qualification also add to the costs.
For low power packs these engineering costs could amount to $20,000 to $50,000.
High power batteries bring an additional set of challenges. Systems integration is much more complex due to the wider range of system functions and demands to be accommodated. For automotive applications the accuracy of the SOC estimation must be much higher and this may also require a major cell characterisation programme. The components are much larger and more expensive and the enormous energy content of the cells demands special safety considerations to prevent physical and electrical abuse and accidental damage. This requires robust steel frames and enclosures and fail safe electronics. Thermal management is also very important and designs may include both heating and forced cooling circuits. Expensive cable forms are needed to connect the cell voltage and temperature sensing signals to the BMS processing unit. All of these requirements add to the complexity, costs and timescales of the associated systems software.
Enginering costs for EV and HEV applications could be upwards of $200,000 and probably much more.
- Tooling Costs
High volume products may require multi-cavity moulding tools and progessive stamping dies. In addition automated transfer mechanisms and assembly jigs and fixtures will be required for the manufacturing operations.
All this could cost a minimum of $100,000. This assumes the manufacturing plant is already equipped with standard engineering, production and test facilities such as CAD and CAM, PCB assembly machines, conveyer belts, welders, presses, power supplies and electrical test equipment.
For manufacturing high power batteries, material handling and operator safety become major factors because of the heavy weight of the packs and the high voltages involved. Tooling costs may be double those needed for low power packs starting at $200,000 or more.
Prototypes could cost double the cost of volume production. Low volume purchases are more expensive and a considerable amount of manual work is involved. This is only significant for high power batteries.
- Production Costs
The manufacturing costs for low power batteries used in mobile phones could be as low as $2.50 whereas a high capacity EV battery could cost upwards of $10,000. In both cases the major cost is the cells. In small batteries this may be 80% to 85% of the total costs. Large batteries use more electronics and higher power components. They are also more labour intensive. For large batteries the cost of the cells could be between 60% and 80% of the total costs depending on the battery specification. Since most cells are sourced from Asia, shipping costs also contribute significantly to the costs.
Two conclusions can be made from this .
- Large production volumes are required to justify the development of custom battery packs.
- For safety reasons, batteries for electric vehicles involve very high unavoidable engineering development costs, even for a single vehicle.