Designing the Optimal Battery System More Than Price
In every large energy storage application it is the battery that most determines a project’s success. However, as the battery is one of the most expensive components its selection is commonly based on initial price or the lack of knowledge about another technology’s capabilities. While an upfront inexpensive battery makes the selling process easier the reputation of a company is based on educating customers as to their products benefits and delivering products that meet expectations. With so many battery options available (chemistry and suppliers) it is important for a company to become educated as to the various solutions and their characteristics. Drilling down to determine what each product’s true value is the best way to build a strong customer reputation and grow business.
As with any new product it is important to start with a plan. A detailed specification listing all of the battery requirements goes a long way in determining which chemistry, and ultimately solution, is best for an application. In addition, this will direct the BMS and enclosure designs. In general the more information about the application the better. Even subtle details can make a large impact in the design. Here is the minimum amount of information necessary to start a design though commonly these lead to further questions and discussions.
- Battery voltage (minimum and maximum)
- Discharge rates (typical and maximum)
- Runtime between charging
- Opportunity charging available
- Maximum charging rate
- Total energy required per day
- Number of days per year of operation
- Ambient temperature range
- Physical size and weight constraints
- Application requirements (agency testing, shock & vibration, electrical connections, etc.)
With the battery requirements fully understood a design needs to be put together for each viable cell option. These battery designs should optimize each cell’s characteristics in accordance with the main requirements of the project. This is typically where companies make the mistake of eliminating chemistries or manufacturers before their true benefit is understood. As there are many chemistries and manufacturers they all have something to offer whether it’s price, safety, quality, availability, energy density, power density, fast rechargeability, high cycle life or thermal performance.
To fully determine which solution is optimal for an application all of these variables need to be included in the design so comparisons are made based on optimized designs and not just on cost per watt hour. For example, if an application requires a lot of charge and discharge cycles an inexpensive lead battery will end up costing more than a Titanate battery over time as the Titanate has a lower cost per cycle. While putting these comparisons together takes time and effort it insures an optimal solution is made based on quality, performance and price. An optimal solution will insure a strong customer reputation while eliminating the potential for competition to offer a better solution.
Understanding the system requirements and which cell is being used allows for a BMS to be properly designed and programmed. While the cells are the heart of the system the BMS is the brain. A good BMS accounts for making sure the cells are properly protected, accurate and critical information about the battery is available, cells are being exercised to maximize their life, and the battery system works flawlessly with the host system. While these details need to be defined as much as possible in the battery specification it often requires refinement during the development of the systems (battery and host) to finalize. Having a seasoned team of battery and BMS experts will help to reduce BMS development, last minute changes and the possibility of missing a critical area.
The mechanical design of a battery system is both an outside in and an inside out approach as the application’s physical requirements and cell’s physical structure need to be taken into account. A properly designed enclosure will insure the system meets performance, safety and financial requirements. In addition there can be a lot of specific details required in the battery pack design depending on which cell manufacturer’s product is being used. Cell orientation, fusing, cell pressure and mounting techniques are just a few of things that need to be considered when developing a mechanical structure. As with the BMS a seasoned team of experts can dramatically reduce development time, last minute changes and the possibility for costly mistakes. They can also go a long way to insure transportation and other agency testing are passed on time and on budget.
The chart below shows how various chemistries from high quality manufacturers compare in a hypothetical motive application. It also shows how a lithium Titanate solution could be optimized to take advantage of natural machine downtime for opportunity charging (in white). While the product has the highest cost per watt hour of the various chemistries the lower total energy and long cycle life make it one of the least expensive options in the application.
While an inexpensive cell is fine for some applications high quality cells are necessary for others. For an informed decision to be made system comparisons need to be generated for each cell option based on how its capabilities match up with the project requirements.