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Cell, Modules, and Packs:

The Cell is the smallest unit in the battery pack. A single unit Cell is specified by its Voltage, Capacity, battery chemistry and form factor. For example, the LFP 18650 cell will have 3.2V, a certain capacity (2200mAh, 2600mAh, 2900mAh), Lithium Ferro-phosphate chemistry and a cylindrical form factor. The cells are connected in series and parallel to make a Module of certain Voltage and Capacity. The small modules are assembled together in series and parallel to make the Big Battery Pack.

C-Rate:

Charging and Discharging in batteries is normally defined in terms of C-rate in order to specify the rate against the battery capacity. C-rate is defined as the charging/discharging current relative to the battery maximum capacity. Let's take an example, for a 50Ah battery, if we draw 100A current from the battery that it is said to be discharged at 2C, and 0.5C will mean that we are drawing 25A current from the battery. C-rate also indicates the time/duration for charging/discharging the battery at the specified current. A 1C discharge means that the battery will be charged/discharged in 1hr, 0.5C means it will be discharged in 2hrs, and 2C means it will be discharged in 30minutes.

Primary and Secondary Batteries:

Primary batteries refer to the batteries that have a fixed energy capacity for their Lifetime, which means once drained out they can not be recharged. Secondary batteries can be charged and discharged at regular intervals. They have comparatively lower energy capacity than the Primary cells.

State of Charge (SOC %):

SOC (%) refers to the present battery capacity relative to the maximum battery capacity. In another way, SOC is used to determine the present charge stored in the battery.

Depth of Discharge (DOD %):

DOD (%) refers to the percentage of battery capacity that has been used/discharged mentioned as a percentage of maximum capacity. We get SOC by subtracting DOD (%) from a total of 100 (%).

Terminal Voltage (V):

The voltage measured between battery terminals with the load applied. The terminal voltage varies with SOC and also charge/discharge current.

Open-circuit voltage (V):

The voltage measured between the battery terminals with no load applied. The open-circuit voltage depends on the battery state of charge and it increases with the state of charge.

Nominal Voltage (V):

The reported or reference voltage of the battery, also sometimes thought of as the “normal” voltage of the battery.

Cut-off Voltage:

There are two types of cut-off voltage, the Upper Cut-off Voltage (charge cut-off) and Lower Cut-off Voltage (discharge cut-off). Cut-off voltages are defined to terminate the use of battery above the limits for safer operation of batteries.

Internal Resistance:

The Internal resistance of the battery refers to the internal losses occurring within battery components while charging and discharging. Internal resistances are different for charging and discharging and it also depends on the battery SOC. Battery efficiency depends on the internal resistance of the battery.

Capacity (Ah, for a specific C-rate):

The capacity of the battery refers to the Coulombic capacity/ charge storing capacity of the battery. It is usually defined by the Ampere Hour (Ah) term, which is the multiplication of the discharge Current by total discharge time. Capacity available for use depends on the discharge rate as on higher C rates the internal losses occur in the battery.

Cycle Life (specific for DOD %):

Cycle life refers to the number of charge-discharge cycles the battery can provide before it fails to meet specific performance criteria. Cycle life is estimated for specified charge and discharge conditions. The Cycle Life of the battery depends on operating conditions, the C-rate and DOD.

Specific Energy (Wh/kg):

The battery energy per unit mass sometimes referred to as the Gravimetric Energy Density. Specific Energy characterizes the battery chemistry and packaging. This becomes an important parameter for Electric Vehicles as it determines the dead weight added to the vehicle and helps in calculating the range of the vehicle.

Specific Power (W/kg):

It defines the maximum available power per unit mass. Specific Power is a characteristic of the battery chemistry and packaging. It determines the battery weight required to achieve a specific power performance.

Energy Density (Wh/L):

It defines the nominal battery energy per unit volume, also referred to as the Volumetric Energy Density. Specific energy is a characteristic of the battery chemistry and packaging in terms of volume/space required.

Power Density (W/L):

It defines the maximum available power per unit volume. Specific power characterizes the battery chemistry and packaging. It determines the battery size required to achieve a given performance target.

Maximum Continuous Discharge Current:

It is the maximum current that can be drawn from the battery continuously. This limit is usually defined by the battery manufacturer in order to prevent excessive discharge rates that would damage the battery or reduce its capacity.

Pulse Discharge Current:

It refers to a pulse of a very high current that can be withdrawn from the battery for a very short time. Pulse discharge current is specified by the manufacturer to prevent any harm to the battery by high discharge current.

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