Battery Pack Design Criteria and Selection 47 requirement. There are some relatively simple things you can assume here, for instance to deliver 15 MW of power for 15 min likely means that you need a battery that is about 60 MWh of capacity in order to deliver that much power for that period. This of course assumes a 1C discharge rate. However, we also need to understand how many charge–discharge cycles of this type are expected and how often. Perhaps the system only needs to deliver this much power a couple of times per year, or perhaps it needs to deliver this much power on a daily basis.
In each of these two cases we could develop two very different energy storage systems to meet each of these two different cycle requirements. This is a good example of when it is important to conduct a technical review with the customer. For the example of a grid scale application you should also meet with the power electronics company that is providing the inverter and power electronics components for this system (assuming that you as a battery provider are not already providing these components). Meeting with this partner may give you much of the key information that you will need to properly size your battery.
From them you will be able to understand the system voltage requirements, interconnect voltages, communications requirements, and many other important characteristics that you battery will need to meet in order to achieve the customer demands on the system. You may also need to spend some time with the engineering services firm that is doing the overall site design and layout to understand where the battery will be installed, will it be in an air conditioned building or will you need to design a trailer or container in order to house the battery system.
If this is the case, you will also need to understand some of the local building ordinances and requirements and the geographic location for the installation. These may drive safety and fire detection and prevention systems to be installed in the container. Power to Energy Ratios Another topic that we should make sure to discuss is power to energy ratios. The power to energy ratio is a quick number that many customers and system designers use to quickly evaluate the fit of a certain technology for their application.
High power applications, such as a 12 V stop/start type automotive battery, will typically have much higher power in relation to the amount of energy that is stored in the battery pack. In this case we may see a ratio between 15 to 1 (15:1) and 20 to 1 (20:1). This ratio represents the amount of power that the battery is capable of providing compared to the amount of energy that is stored on board. If we continue using this example and base it on the USABC guidelines (see Appendix A) for our approximations, we find that the “ideal” 12 V stop/start battery should be able to provide up to 6 kW of power for 1 s, and it should also provide 360 Wh of energy.
So in this case, 6,000 W/360 Wh (converting to similar units) equals a 16.7:1 power to energy ratio. Table 1 below shows some average power to energy ratio ranges for different automotive applications; however, note that these same estimations can be made for all other energy storage applications.