18650 Battery Capacity Calculator

When you build a battery pack using 18650 cells, you have to consider that the total capacity of that battery is not going to be the same than the total capacity of the individual cells. Many people use calculations to calculate the total capacity of a battery pack by simply multiplying the capacity of each individual 18650 cell by the total number of cells. However, this calculation is often incorrect because the cells dont deliver all of the energy that they are capable of delivering.

Instead, the total capacity of the battery will be less than calculated due to efficiency losses, decay of the cells over time, and the limit of the cells discharge rates. Battery packs has to be built to meet three needs of the battery pack: the runtime of the device that uses the battery, the peak current that the load of the device draws, and the space in which the battery pack will fit. Cells that contain high amount of energy often cannot provide the high amount of current that a motor requires, for example.

What to check when building an 18650 battery pack

Conversely, cells that can provide high peaks of current can often contain less total energy then the cells that provide energy at more steady rates. Thus, individuals has to decide which of these three feature are the most important to the battery pack that they are building. The nominal voltage of the battery pack will determine the number of cells that are used in series to create that battery pack.

For example, if a 12 volt router is to be powered, a 3S battery pack can be used. However, the voltage of the battery will range from 12.6 volts when the battery is fully charged to 9 volts when the battery is depleted. If the router is sensitive to these voltage drop, a 4S battery pack and an voltage regulator would be utilized to ensure proper operation of the device.

The voltage requirements of the device will help to determine the number of cells that is required for the battery pack. Beyond the voltage of the battery pack, it is also important to understand the difference between the theoretical and the usable capacity of those cells. The theoretical capacity of a lithium cell is energy that can be used, but the device that utilizes the battery pack should not use 100% of that energy.

Over time, using 100% of the energy of the cells can degrade the cells and limit their life span. Thus, if an individual desires to use 80% of the cells capacity, the battery pack must be built to 20% larger than calculated to allow for these cells to be depleted to only 80% of their calculated capacity. If the pack is built to the calculated amount of required wattage, the battery will fail before the calculations indicates that it should.

The energy that passes through buck and inverter converters into the devices that are to be powered is often lost as heat. Thus, a portion of the energy that was stored within the battery has dissipated into the surrounding environment. You can account for these losses when building the battery pack; however, other issues, like the aging of the cells or the use of the batteries in cold weather, can also reduce the capacity of the batteries to deliver the amount of energy that is required of the batteries.

Therefore, the gross capacity of the battery must be larger than the calculations made for the devices energy requirement. The batteries that are commonly used in these types of battery packs are either energy cells or power cells. Energy cells are used in applications that require low and steady rates of energy, such as network bridges.

Power cells are used in applications that require high rates of current, such as power tools. To calculate the number of cells that will be used in the battery pack, the individual must ensure that there is enough cells in parallel to meet the energy requirements of the device and enough in parallel to meet the current requirements of the device. A Battery Management System (BMS) is required for battery packs that contain more than two cells.

The BMS is responsible for the protection of the cells and the safety of the battery pack. You must size the BMS according to the peak current of the load that the battery pack is to supply. For instance, if the load, such as an NVR, requires the BMS to supply a peak amount of current to the hard drives when they are spinning up, if the BMS is sized according to the average load current, the BMS may shut off the system.

Thus, the BMS must be sized appropriately to allow for headroom in case of these high currents, or the system may shut off. Finally, the weight of the battery pack has to be considered. Battery packs that contain a high number of cells, such as 13S12P batteries, have a high mass.

You have to know the mass of each of the cells prior to building the battery pack; otherwise, the battery pack may fail due to insufficient strength of the case that contains the battery pack. If the battery pack is too heavy for the case, the case may flex or crack due to the weight. Thus, it is important to consider these factors prior to building the battery pack.

By considering each of these factors, an individual can build a battery pack that will function as a reliabel infrastructure.