Common Terms in the Lithium Battery Industry

Datasheets are full of shorthand. These six terms cover most of what you will read about a lithium battery.

1. Cycle Life

One full charge plus one full discharge is one cycle. Cycle life is the number of charge/discharge cycles a battery can go through before its capacity drops below a defined threshold — typically 80% of rated capacity. As a cell is repeatedly charged and discharged, its usable capacity gradually declines.

2. Energy Density

The energy released per unit volume or per unit mass, usually quoted as volumetric energy density (Wh/L) or gravimetric energy density (Wh/kg). Higher density means more energy in the same size or weight.

3. Self-Discharge Rate

Also called charge retention: the ability of a battery sitting in open circuit to hold its stored energy. It is a key performance indicator and is strongly affected by temperature — higher temperature accelerates self-discharge.

4. Depth of Discharge (DoD)

The percentage of rated capacity that has been discharged during use. A battery that has released 40% of its capacity is at 40% DoD.

5. State of Charge (SoC)

The remaining capacity as a ratio of the fully-charged capacity, usually given as a percentage, or as a value between 0 and 1. SoC = 0 means fully discharged; SoC = 1 means fully charged.

6. C-Rate (Charge / Discharge Rate)

The C-rate is the current required to deliver the rated capacity in a stated time, expressed as a multiple of rated capacity. 1C fully discharges the battery in 1 hour; 0.2C (that is, 1/5) takes 5 hours. It describes how fast the battery is charged or discharged.