Lithium-ion Battery Glossary

The following glossary is provided as an overview of key terms used the industrial lithium ion battery industry. Click on a term to jump to the definition



  • Absorption – (see Bulk, and Float) – With this stage (the remaining 20% left from Bulk Charge) the charger holds the voltage at the charger’s absorption voltage and decreasing the current until the battery is fully charged.


  • Ampere – Is the SI (International Standard) unit of electrical current abbreviated as ‘amps’ or ‘A’. Amps = Watts/Volts or A = W/V.


  • Ampere Hour (Ah) – The capacity of a storage battery is measured in amp-hours (Ah) and is the amount of energy charge in a battery that will allow one ampere of current to flow for one hour. Five amp-hours means a current flow of one ampere for five hours, a current flow of 2 1/2 ampere for 2 hours, or any multiple of current and time that will result in five. Other amp hour ratings may be expressed in similar format.
    Note: A PBES system is designed so that the Ah rating does not change with discharge time.


  • Anode – The negative electrode from which electrons flow out towards the external part of the circuit.


  • Application – The end use of an energy storage system. An application could be a mobile phone, a laptop computer or a hybrid passenger ferry.


  • Boost Charge – Boost charging involves a high current for short period of time to charge the battery.  It is generally used if the battery has been discharged heavily.  Boost charge enables the quick charging of depleted batteries.


  • Bulk Charge – This charging stage involves about 80% of the recharge, wherein the charger current is held constant (in a constant current charger), and voltage increases.


  • Capacity – Battery capacity is a measure (typically in Amp-hr) of the charge stored by the battery, and is determined by the mass of active material contained in the battery. The battery capacity represents the maximum amount of energy that can be extracted from the battery under certain specified conditions. However, the actual energy storage capabilities of the battery can vary significantly from the “nominal” rated capacity, as the battery capacity depends strongly on the age and past history of the battery, the charging or discharging regimes of the battery and the temperature.


  • Cascading Thermal Runaway – Occurs when the heat generated by one cell going into thermal runaway transferring  heat to adjacent cells in a module and adjacent modules in a battery installation, resulting in the other cells also going into thermal runaway. If the gasses generated during the thermal runaway event ignite, the rate of cascade will increase, eventually consuming all combustable material in the battery system.


  • Cathode – the positive electrode from which electrons flow out towards the external part of the circuit.


  • C Rate (or Capacity, Rated) – A designation by the battery manufacturer, which defines the measure of the rate at which a battery is discharged relative to its maximum capacity. A 1C rate means that the discharge current will discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps.


  • Capacity, Residual – The quantity of charge that would be removed from a cell if it were brought from its present state to a fully discharged state.


  • Cell – the combination of the anode, cathode, and electrolyte.


  • Cell Balancing – Refers to techniques that maximize the capacity of a battery pack with multiple cells in series to make all of its energy available for use and increase the battery’s longevity.


  • CellCool™ – PBES proprietary liquid cooling system that ensures safe operation of the PBES module at high performance levels.


  • Charge –  The conversion of electrical energy from an external source, into chemical energy within a cell or battery.



  • Charge Rate – The current expressed in amperes (A) or milli amps (mA) at which current is applied to a cell or battery to restore its capacity.


  • Charge, State of (SOC) – Measure of the amount of capacity remaining and is the equivalent of a fuel gauge for the battery pack in a battery electric vehicle (BEV), hybrid vehicle (HV), or plug-in hybrid electric vehicle (PHEV). The units of SOC are percentage points (0% = empty; 100% = full). An alternate form of the same measure is the depth of discharge (DoD), the inverse of SOC (100% = empty; 0% = full).


  • Charger – Device capable of supplying electrical energy to charge a battery.



  • Charging – The process of converting electrical energy to stored chemical energy. Charging is the opposite of discharging. (See Bulk, Absorption and Float charging descriptions).


  • Charging Efficiency – Measure of how much energy is absorbed by the battery relative to how much energy is delivered.


  • Charge Retention – The fraction of full capacity available from a battery under a specified set of conditions, after the battery has been stored for a given amount of time.


  • Constant Current (CC) Charge – Constant-current charging means that the charger supplies a relatively uniform current regardless of the battery state of charge or temperature.  This charging method should be used sparingly, as battery damage may result if performed on a repetitive basis.


  • Constant Potential (CP) – Constant Voltage (CV) Charge – this method specifies that chargers maintain nearly the same voltage input to the battery throughout the charging process, regardless of the battery’s state of charge.  Constant-voltage chargers provide a high initial current to the battery because of the greater potential difference between the battery and charger.  A constant-voltage charger may return as much as 70% of the previous discharge in the first 30 minutes.  This proves useful in many battery applications involving multiple discharge scenarios.


  • Current – The rate of flow of electricity,  comparable to the flow of a stream of water measure in Amps (ampere).


  • CellCool™ – PBES proprietary liquid cooling system that ensures safe operation of the PBES module at high performance levels.


  • Cut Off Voltage – The voltage at which a battery is considered fully discharged, beyond which further discharge could cause harm. Some electronic devices, such as cell phones, will automatically shut down when the cut-off voltage has been reached.  Also known as End Point Voltage (EPV).


  • Cycle – The term is typically used to specify a battery’s expected life, as the number of charge / discharge cycles affects life more than the mere passage of time.


  • Cycle Life – The cycle life is the number of complete charge/discharge cycles that the battery is able to support before that its capacity falls under 80% of it’s original capacity.


  • Deep Discharge – Withdrawal of more than 80% of the rated capacity.


  • Depth of Discharge – The portion of the capacity taken out during a discharge, expressed as a percent of rated capacity. A battery at 25% depth of discharge has 75% capacity remaining versus what it could give if fully charged.


  • Discharge – The withdrawal of energy of a cell or battery into a load.


  • Duty Cycle (Duty) – The end use of the battery for a specific application. The duty of a mobile phone battery is to provide power to the phone while not connected to charging power. For an hybrid or electric ferry the duty will be to provide propulsion for the ferry and power to other shipboard systems.


  • End of Life – The stage at which the battery fails to deliver acceptable capacity.  This may vary between different battery types.


  • Electrolyte – the material that allows electricity to flow from one plate to another (between positive and negative electrodes).


  • E-Vent™– PBES proprietary integrated gas venting system.


  • EPV – See Cut off Voltage


  • Float Charge – A charge method where a specified voltage (Float voltage) is maintained in the battery after it has been fully charged that will maintain that capacity by compensating for self-discharge of the battery.


  • Gassing – Commonly found in lead acid batteries that are being charged. There is no gassing under normal operation with PBES energy storage systems. They are a completely sealed, solid-state unit.


  • Internal Impedance – See internal resistance.


  • Internal Resistance – Defined as the opposition to the flow of current within the battery.  There are two basic components that impact the internal resistance:  the ionic and electronic resistance of the cell components. The resistance value varies with current, state of charge, temperature, and age.


  • Kilowatt Hour (kWh) – Similar to Ah, kWh is used to express system capacity on large energy storage systems.


  • Megawatt Hour (MWh) – Similar to Ah, MWh is used to express system capacity on very large energy storage systems.


  • Memory – Reduction in usable capacity over time.


  • Open Circuit Voltage – The voltage of a battery when it is not delivering or receiving power, and has been at rest long enough to reach a steady state (normally, at least 4 hours).


  • Overcharge – The continued charging after 100% state of charge is achieved.   This can result in heat generation, the decomposition of water in the electrolyte into hydrogen and oxygen gas, and corrosion of the positive electrode, which may result in fire.


  • Parallel Connection – Voltage of the system stays constant regardless of how many cells or batteries are added to a system. Batteries connected in parallel will result in capacity increases. (See also Series Connection).


  • RMS – RMS voltage is a method of representing an AC voltage waveform in which the RMS value specifies the amount of DC voltage that will produce the same heating effect, or power dissipation, in a circuit, that the AC signal would over its entire cycle. Thus, the RMS value is a very important value when comparing AC signals to DC signals, in that they both produce the same amount of heating in a circuit and cause the same amount of dissipation of power.


  • Self Discharge – The decrease in the state of charge of a cell or a battery over time due to internal electrochemical losses.


  • Series Connection – Voltage of the system is cumulative. Capacity stays the same, regardless of how many cells or batteries are added to a system. (See Parallel Connection).


  • Shelf Life – The period of time (measured from date of manufacture) at a specified storage temperature, after which the cell or battery needs to be boost charged so it does not suffer permanent capacity loss.


  • Specific Energy – Energy per unit mass.


  • Specific Power – Time rate of energy transfer per unit volume.


  • State of Charge– see Charge, State of (SOC)


  • State of Health– A measure of the condition of a battery (or a cell, or a battery pack), compared to its ideal conditions. The units of SOH are percent points (100% = the battery’s conditions match the battery’s specifications).


  • Temperature, Ambient – The average temperature of the battery’s surroundings.


  • Temperature, Cell – The temperature of the battery’s internal components.


  • Thermal Runaway – An exothermic chemical reaction between the electrolyte and the battery electrodes that happens when a lithium-ion cell is abused. Cell abuse may include over and under voltage, over and under temperature, mechanical abuse, and internal cell defects. If the exothermic reaction is not mitigated by heat removal, the cell temperature will increase at a continuing rate until the cell vents gasses. The vented gasses are toxic and flammable.


  • Thermal-Stop™ – PBES proprietary module design that limits thermal propagation across cells within the module.


  • Trickle Charging – Charging a fully charged battery under no-load at a rate equal to its self-discharge rate, thus enabling the battery to remain at its fully charged level. A battery under continuous float voltage charging is said to be under float-charging.


  • Vent Valve – A check valve which allows the controlled escape of gases when the internal pressure exceeds the valve’s rated value.


  • Venting – A safe release of gas to prevent excessive gas build-up inside an energy storage system.  (See E-Vent™).


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