Battery or structural component...or both?
TWI Bulletin, November - December 2009
There is an engineering opportunity in vehicle manufacture to develop an electrical power device which also contributes to the vehicle's structural performance. TWI has been working on one possible solution.
Paul Burling obtained a Higher National Certificate in Production Engineering with Merits and Advanced Mathematics from Cambridge College of Art and Technology. He has extensive experience in project management of large commercial projects world wide, which demand in depth knowledge of composite material and adhesives for military and commercial applications. His expertise covers most engineering technologies but he specialises in composite materials and adhesives.
There are many ways of storing electrical energy, but one of the most common and appropriate ways is to use an electromagnetic cell, a battery or capacitor comprising one or more cells. When using these devices it is important to understand how this electrical energy can be released, usually to perform work, whether it be mechanical, electrical or simply to generate heat. Managing this electrical energy can be difficult, especially if the system is to have high levels of efficiency. Paul Burling examines an alternative way of incorporating batteries, in a structure that will allow tensile, compressive, torsion, bending and shear loads to be transmitted throughout its fabric while still performing its electrical function. The obvious application is within the chassis of a car, but the principle could be applied to most air, land and sea vehicles.
This structural storage principle could also be applied to capacitors, as they can be similar in shape, but have a very different way of delivering electrical power. Such an approach not only allows mechanical loading, it will also allow cooling and high current dissipation. The Battrix concept, as it is known, was created by two engineers at TWI, Paul Burling and John Kell.
TWI has the technical expertise and knowledge to specify and join the selected materials for the structural electric cell casing. It also has the know-how and experience to join the end tabs for the anode and cathode, with minimal heat transfer, using either ultrasonic welding or laser welding. It is important to reduce the amount of heat input as this can reduce the efficiency of the battery cell and in some cases damage it beyond repair. TWI can also provide structural loading, testing and FEA on the Battrix system, especially when it is used with sandwich structures.
A battery comprises one or more galvanic cells that store chemical energy and make it available in electrical form. A battery consists of one or more voltaic cells. Each voltaic cell consists of two half cells connected in series by a conductive electrode. The electrodes are immersed in a solid or liquid electrolyte. Generally the materials are enclosed in a container and a separator, used to prevent the electrodes coming into contact with each other. The voltage developed across a cell's terminals depends on the chemicals used in it and their concentrations.
Alkaline and carbon-zinc cells both offer about 1.5 volts. Lithium cells can provide as much as three volts or more. They are light in weight, have lower per-use costs and have higher and more stable voltage profiles compared to their equivalent sized lead-acid cells. One of the most common forms of batteries is the lead-acid battery, delivering about 10,000W and 450-1100 amps. There are also 'dry cell' batteries available including nickel-cadmium (NiCd), lithium-ion (Li-Ion) and nickel metal hydride (NiMH).
Battery cells can be connected in parallel, or series (Fig.1) or indeed both.
Fig.1. Battery configurations - parallel and in series
A series combination has the same current rating as a single cell but its voltage is the sum of the voltages of all the cells. Most electrochemical batteries connect the battery cells in series. A parallel combination of cells can supply a higher current (the sum of the currents of the cells) but at the same voltage as a single cell. Typically metal tabs are welded to the anode and cathode of the cell so that the cells can be connected to each other.
The battery case includes a cavity for loading the cell and terminals for connection to the anode and cathode of the cell. A good conductive connection must be maintained between the end connectors and the terminals to ensure good battery performance. All batteries have a maximum current that they can produce. At higher current levels, batteries can produce excess heat which wastes some of their power.
An example of a 9V battery is shown in Figure 2. Beneath the packaging there are six small batteries producing 1.5 volts each in a series arrangement.
Fig.2. Typical 9V battery;
a) as packaged; and
b) with the packaging removed
The connection of these cells is often carried out by welding them together, an example of spot welds in a nickel cadmium battery is shown in Figure 3.
Fig.3. Spot welded terminals on a typical Nickel Cadmium battery
The connection is difficult to achieve without damaging the cells, so small spot welds are used during manufacture, typically using a laser. The performance of the battery largely depends on the quality of the connection. If a large current is drawn from this battery, the connection will get very hot and could fail.
Capacitors store electrical energy but are unlike batteries in that they do not produce new electrons. A capacitor comprises positive and negative terminals that connect to two metal plates, separated by a dielectric or non-conductive substance such as mica, ceramic, or cellulose. The dielectric dictates the kind and suitability of the capacitor for particular applications, for example for high voltage or high frequency uses.
When a capacitor is connected to a battery, the plate terminal that is connected to the negative terminal of the battery accepts electrons from the battery, and the plate terminal that attaches to the positive terminal of the battery loses electrons to the battery. When charged the capacitor has the same voltage as the battery. The energy stored by the capacitor is discharged in a matter of seconds.
Battrix interlocking connection
The matter of maintaining a high quality connection between the end terminal and other cells is vitally important. The connection's rôle is not just one of maintaining conductivity between the components, but of maximising the surface area of this connection, allowing good current flow. So there is a need to provide 'connectability', allowing the cells to work as a singular or collective power source. Ideally the cells so produced contribute structurally to the vehicle's chassis. So they play an external load-resisting role and as such, assist in the elimination of structural elements such as battery compartments and battery holders (see Fig.4).
Fig.4. Mechanically interlocked structure
The Battrix concept enables current to be discharged and recharged due to the large surface area of the terminals, assuming the correct chemistry is applied. The Battrix design permits cells or capacitors to be joined together. This reduces the weight of the device, and allows its volume to be tailored. The modular nature of the power unit gives the designer the ability to stack the batteries in parallel, in series, or both.
This is ideal for the transportation industry or any other sector that requires low weight, while maximising a given volume. Single or multiple devices can be added and run together assuming the combination does not exceed the current or voltage draw. The Battrix design enables the cells themselves to be cooled and warmed by air or liquid. If lithium polymer chemistry is used, the temperature can vary from -20 Celsius up to 90 Celsius on discharge.
The temperature at which it operates has a significant effect on the performance and life of the cell. In addition the Battrix structure is self supporting so traditional battery compartmentalisation can be eliminated in the design.
Further cells can be added in a modular way to increase either voltage or current.
Another advantage of such a construction is that redundant or faulty cells can easily be replaced when detected. This is vitally important. Many battery stacks are welded together using today's technology, and it is usual that one or more cells will vary in performance over a period of time. This can be attributed to a number of factors such as the cells not being balanced or subjected to overheating, especially in the middle of the stack.
Because the Battrix system is modular, it is possible to allow electrical devices to be connected to one or more cells so reducing the need for expensive wiring, or step down transformers. The geometric shape of individual cells can be designed to accommodate fitness for purpose criteria. The interlockability will enable large terminals (negative/positive, cathode/anode) to give good current dissipation.
The clearances between the male and female connectors or terminals are carefully controlled to achieve an intimate connection. The connections need to provide good conductivity to enable the current to flow from each node. Furthermore such connections should be of good tolerance so that there is relatively trouble-free assembly and disassembly of the cells.
The cell body or enclosure can be negatively or positively charged and this reduces the amount of wiring required. The nodes could either allow electrons to flow, or not, depending on the material used (as a conductor or as an insulator). This approach could provide discrete power to individual electrical devices as long as it does not exceed the battery's designed power output.
The cell's body structure may be manufactured by way of extrusion, co-extrusion, pultrusion, injection moulding, profile extrusion, deep drawing, direct metal deposition or any similar process. Considering basic alkaline, nickel-cadmium, nickel metal hydride and other similar batteries, steel or aluminium could be used to encapsulate the battery materials.
Processes suitable for forming the interlocking shape in the case vary from deep drawing and extrusion to general metal fabrication using welding and soldering. Indeed for many battery encasements, eg lithium-ion and lead acid, a polymer encasement may be more suitable than a metallic one, with copper, gold or other connectors used for the end terminals as required.
Such polymer materials include PTFE, HDPE, PE, and filled PVC. In addition encasement made from composite materials, such as polyester epoxy resins with metallic or non metallic reinforcing fibres, may also provide an ideal encasement material (See Fig.5).
Fig.5. Extrusion packed with chemical roll to produce one battery cell
Extrusion, pultrusion and injection moulding may be seen as one of the preferred methods of making the encasement. This continuous process would enable the manufacture of long lengths which can be sized to the individual encasement length, and post-worked as required. To enhance the profile extrusion, post work may be carried out, for example punching, drilling, or slotting.
The tolerance fit up of the interconnecting end terminals will usually be enhanced at this stage. An end cap can be welded or joined to the encasement and then the battery material inserted. Where required conduction connectors will be inserted into the end terminals and connected to the battery material. This may be performed before the end cap is first joined to the encasement.
Finally the remaining end cap can be joined to the encasement so securing the battery material inside the encasement. Once the single unit has been manufactured simply slot these together to achieve the required voltage and amperage for the intended application (See Fig.6).
Fig.6. Extrusions slotted together showing the anode connector and section of this as a structural unit
The materials that lend themselves to application of the Battrix concept are suitable for recycling and the power device structure itself can be designed for disassembly. There are well established methods for recycling batteries containing lead, nickel-cadmium, nickel hydride and mercury with those for the newer nickel-hydride and lithium systems being developed. Such methods include separation of the different materials prior to metallurgical processing, heat treatment of the device with the metals being recovered at the end of the process, vacuum thermal treatment and smelting. A range of materials used in the existing device can be reused as a secondary raw material.
Market applications
One of the fastest growing sectors that could use this technology is the automotive sector. The structure of vehicles will need significant strength with good rigidity with open surface geometries that are lightweight and modular with the ability to be upgraded and reformed (See Fig.7).
Fig.7. Typical automotive application showing a partly structural floor
This technology is particularly well suited to rechargeable batteries, especially those targeted at the electric vehicle market, such as lithium ion, nickel metal hydride, oxide and sodium-sulphur batteries. Batteries that have good charge and discharge capability, and are affordably priced, are seen as key in the development of electric car prototypes.
Energy saving in automotive systems will come from electrical power steering replacing hydraulic pumps, smarter electric fuel pumps, combined starter and alternators that will stop and start the engine automatically as appropriate. The introduction of hybrid powered vehicles (electric/petrol) will increase the demand for high power energy storage systems to cover the increase in electrical demands. The requirements of these storage systems necessitate high current transfer, long storage cycles, good thermal management, increased recyclability, reduced production cost, increased flexibility and functionality.
The technology offers the opportunity to switch cells on or off thus changing the voltage supply. This can be performed by conventional methods, for example by way of an electrical switch. This would enable the pulsing of electrical devices such as motors for short periods. Battrix offers an alternative approach to existing designs for structural units that can provide power. In summary the following make up the key benefits of the system.
Strengths
- Adaptable for use with the latest battery media
- Low weight
- High current transfer capabilities
- Exceptional thermal stability
- Environmentally friendly
- Increased recycle ability
- Increased flexibility and functionality
TWI is now heavily engaged with an automotive manufacturer in securing a supply chain for the manufacture of the Battrix cells structural sandwich floor, for a future electric vehicle.
