The 4 main requirements to start sizing and selecting your battery - MBF Insights
When you decide to install batteries on board your ship and you’ve figured out how you want to use the batteries, it is time to start your search for the right battery system. This all starts with defining the operational profile of the batteries.
Photo credit - Arksen Marine
The operational profile tells you how the batteries will be used. There are 4 main requirements that need to be determined based on the operational profile, to start sizing and selecting your battery.
The minimum required usable energy at end of life is the starting point of your battery design. This is the energy in kWh that is needed to complete the full operational profile. If this energy is not available anymore, the battery can be considered at the end of its intended (first) life. The total installed capacity will always be more than this, because you need to consider the usable SOC range of the battery, the aging of the battery, and the charge and discharge rates. The total installed capacity will depend on the type of battery you select. Some batteries can deliver the same performance compared to others with a smaller total capacity. Therefore, you should never start by defining a required total installed capacity.
The discharge power, in kW, needs to be determined to define the maximum discharge C-rates on the battery, as well as the discharge power over longer periods of time. The maximum needs to be known to determine if the batteries can handle the required discharge power over the full SOC range of the batteries. Because the voltage of a discharged battery is lower compared to a fully charged battery, you want to make sure that you can use all the required power on board no matter what the SOC is of the batteries. The discharge power over longer periods of time is used to determine the thermal behaviour of the batteries and the required cooling power. Temperature is an important factor in the aging of batteries and therefore rising temperatures need to be minimized.
The charge power, in kW, needs to be determined for the same reasons compared to the discharge power, both the maximum and over longer periods of time. However, batteries can have different characteristics when it comes to charging or discharging power. Therefore, charge, and discharge power need to be defined separately. The charge power is also important to determine the requirements for the charging system and charging schedules.
The number of cycles, or energy throughput, is the final piece of the puzzle. It can be complicated to define the number of cycles. Cycles can have different sizes, they can be interrupted by partial charges in between, and they can be made at different SOC ranges of the battery. One solution for this is to determine the energy throughput. The size of the cycles, the number of cycles, and the SOC range of the cycles, have an impact on the aging of the batteries. The battery manufacturer needs to know the specifics of the cycles that will be made for an accurate calculation of the expected battery lifetime. Therefore, it is always useful to determine the number of cycles as well as the size of the cycles in energy throughput.
You can start your search for the right battery when you have determined your minimum required energy at end of life, discharge power, charge power, and the number and size of cycles based on your operational profile.
These four requirements are needed to select the right battery type and sizing strategy, and as a starting point to calculate the costs, weight, volume, and expected lifetime of the batteries. Always give the same starting requirements to different battery manufacturers because different types of batteries will require different sizing strategies.