Balance Batteries is a new company which is, like the name says, in the business of producing batteries (writes Wayne Ward). It seems simple enough, but many of the companies we hear about are cell producers and leave the integration and design and manufacture associated with it to the end-user.
Company founder Doug Cross has a motorsport background, and spent time working for Toyota on Formula One engine design during the V10 era, followed by a spell at Renault covering its championship-winning years and the transition from V10s to V8s. After that, he co-founded Flybrid Systems, the company which developed flywheel energy storage for motorsport and other applications.
Balance Batteries offers several standard specifications of batteries with different discharge characteristics and energy densities, and with a neat integrated cooling system (Images courtesy of Balance Batteries)
Balance Batteries has developed a range of products to suit all kinds of battery EV (BEV) and fuel cell EV powertrains. Cross was keen to show that these are real, physical products and not simply a CAD assembly. They all share a common architecture.
The first point of note is that Balance Batteries is not a consultancy or in the business of repeating a design process at the customer’s expense for every new application. It aims to help the customer decide which of its standard products best suit the intended application and then sell them that product for them to install.
Although there is a lot of fine detail to be addressed in a successful cell integration, the key points to address are how to connect to the cells, how to connect the cells to each other, how to remove heat from the cells and how to incorporate the battery modules into a product (a race vehicle of some kind in our case). Cross explained that Balance has funded the development itself and tests all of these elements; a customer simply has to use the mounting points and fluid connections provided.
This table shows the different characteristics obtained by connecting cells in different configurations. This is for 52 Ah cells
Balance Batteries has also developed a modular concept using commercially available cells, in which the company packages the batteries using a novel cell interconnection method and a neat parallel-flow cooling solution.
Its products aimed at BEVs are the most relevant to motorsport owing to their combination of power and energy densities, although Cross explained that Balance Batteries is involved in the Viritech Apricale project, a hydrogen fuel cell hypercar that has different power and energy requirements from a BEV
The development process has been very detailed in order to optimise the heat transfer from the cells, which are of the flat pouch type. Cross explained that while cylindrical cells suit high-volume applications very well, a lot of the processes involved in integrating them into a battery are expensive to implement in terms of capital expenditure for equipment.
The pouch cells are supplied without connection tabs. Balance Batteries has designed its own simple tab geometry, and these are welded to the battery terminals to produce a cell ready to incorporate into a battery module. The method of creating an electrical connection between cells is particularly neat and purely mechanical, and would allow dead cells to be changed quickly and easily compared with a welded module, in which cell replacement is difficult.
There are 12 cells in each of the Balance modules, and between each cell there is a cooling ‘fin’, a thin assembly comprising two outer plates and a central water jacket, produced in aluminium. Manufacturing development here has focused on the reliable joining of the three components, quickly progressing from bonding, through various iterations to an undisclosed joining process that has proven to be reliable. The fin assembly is only 2 mm thick, and the very thin components make a lot of the obvious candidate joining methods difficult to implement reliably.
Between each cooling fin and cell is a layer of electrically insulating but thermally conductive foam. This is necessary to provide the full-face contact required and, given the foam’s compliance, there is an equalisation of contact pressure, and thus cooling potential, across the whole face of the battery.
Cross said it took a lot of development to select the correct type and thickness of foam to provide the required cooling and good, even contact across the whole face of the cell. It is a mechanical and practical development process; it would not be possible in FEA to arrive at the method of getting the even pressure distribution across the face of the cell that Cross was able to demonstrate with photos of pressure-sensitive film – the indispensable ‘Fuji paper’ that many of us have used over the years when developing cylinder head joints.
Even though the cells are mass-produced, there is still a degree of difference in them owing to the production method which can, without a suitable degree of compliance in the system, lead to high contact pressures in slightly unpredictable areas of the cell face. The compliant thermal foam is the key to compensating for these differences.
The method of providing the load to this sandwich has also gone through various iterations. The image of the module above left/right shows an iteration that was featured in our sister magazine E-mobility Engineering; it uses wide elastomer elements to provide the clamp load to the sandwich structure.
Cross explained though that these large elastic bands can be prone to creep, with consequent loss of load. A new, creep-free clamping method is therefore being developed. A specific range of contact pressure of 10-14 psi (69-97 kPa) has to be maintained across the face of the cell, and that applies throughout the cell’s life. The cells can experience a lateral expansion of between 5 and 20% over their life (depending on the make-up of the cell) and the clamping method needs to accommodate this expansion while maintaining the required pressure.
Each cooling fin is connected in parallel to a modular manifold at each end of the battery and, as far as the customer is concerned, the integration into the vehicle requires only a simply fluid entry and exit connection to allow water-glycol to flow through the cooling fins. The connections show Cross’ motorsport background: the simple and compact push-in fittings are widely used in top-level motorsport as a lightweight alternative to the usual method of using a threaded fluid connector.
A range of configurations within the standard module package are available to the customer. First, there are cells with different energy storage capacities and discharge rates. As is usual with cells, those with the highest discharge rate have the lowest storage capacity for their volume, so there is always a trade-off between power density and storage density. The cell geometry is an industry-standard package, which will allow Balance Batteries to take advantage of the latest developments in cell technology without having to redesign its modules.
The second major level of configurability is the cell connections. The cells can be connected in various ways to produce a range of output voltages and maximum currents. The 12 cells can easily be connected with one parallel path (12S1P) or two parallel paths containing 6 cells in series (6S2P). Balance also supplies modules connected in 4S3P and 3S4P connection configurations. The table above left/right shows data for the various connections in modules containing the 49 Ah cells that are aimed at motorsport.
The structurally efficient packaging gives good gravimetric performance. Based on Balance Batteries’ 49 Ah cell modules, the fully assembled 12-cell module, ready to fit, weighs 10 kg. The energy density for the complete battery module is 215 Wh/kg, which is better than many unpackaged cells.