Glossary of Technicals Terms

Glossary & Technicals Terms

Find all the words and the most commonly used definitions in the energy storage market and batteries through our Glossary.


Accumulator – A rechargeable battery or cell (see also Secondary battery).

Anode – The negative electrode of the cell. The anode gives up electrons during discharge.

Aqueous Batteries – Cells with water-based electrolytes. The electrolyte may not appear to be liquid since it can be absorbed by the battery’s separator.

Battery – An electrochemical device used to store energy. The term is usually applied to a group of two or more electric cells connected together electrically. In common usage, the term “battery” is also applied to a single cell, such as a AA battery.

Battery Charger – A device capable of supplying electrical energy to a battery.

Battery Types – There are, in general, two types of batteries: primary batteries, and secondary storage or accumulator batteries. Primary types, although sometimes consisting of the same active materials as secondary types, are constructed so that only one continuous or intermittent discharge can be obtained. Secondary types are constructed so that they may be recharged, following a partial or complete discharge, by the flow of direct current through them in a direction opposite to the current flow on discharge.

BMS (Battery Management Systems) – Electronic system that monitors a battery pack to maximise the lifetime of the battery pack and guarantees the safety for a specific use, by monitoring the distribution of the energy in the pack and following some key parameters. It also allows communication with other systems like the charger or the user.

Cathode – The positive electrode of a cell. The cathode absorbs electrons during discharge.

Cell – An electrochemical device composed of positive and negative plates and electrolyte, which is capable of storing electrical energy. It is the basic “building block” of a battery pack.

CID (Current Interrupt Device) – Safety device, usually in Lithium-ion cells, that protects against over-pressure that breaks the internal electrical connection when internal pressure reaches a set value.

Dry Cell  A primary cell in which the electrolyte is absorbed in a porous medium, or is otherwise restrained from flowing. Common practice limits the term “dry cell” to the Leclanché cell, which is the common commercial type.

Electrochemical Couple – The system of active materials within a cell that provides electrical energy storage through an electrochemical reaction.

Electrode – An electrical conductor through which an electric current enters or leaves a conducting medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum. For electrolytic solutions, many solids, and molten masses, an electrode is an electrical conductor at the surface of which a change occurs from conduction by electrons to conduction by ions. For gases and vacuum, the electrodes merely serve to conduct electricity to and from the medium.

Electrolyte – A chemical compound which, when fused or dissolved in certain solvents, usually water, will conduct an electric current. All electrolytes in the fused state or in solution give rise to ions which conduct the electric current.

EMS (Energy Management System) – Electronic system that manages battery, customer application and charger in order to optimise performances and cost.

Gassing – The evolution of gas from one or both of the electrodes in a cell. Gassing commonly results from self-discharge or from the electrolysis of water in the electrolyte during charging.

Nonaqueous Batteries – Cells that do not contain water, such as those with molten salts or organic electrolytes.

Ohm’s Law – The formula that describes the amount of current flowing through a circuit. Ohm’s Law – In a given electrical circuit, the amount of current in amperes (I) is equal to the voltage (U) in volts (V) divided by the resistance, in ohms (R). Ohm’s law can be shown by three different formulas:

  • The most famous : U = R x I
  • To find Current I = U/R
  • To find Resistance R = U/I

Parallel Connection – The arrangement of cells in a battery made by connecting all positive terminals together and all negative terminals together. The voltage of the group remains the same as the voltage of the individual cell. The capacity is increased in proportion to the number of cells.

PCM (Protective circuit Management) – see BMS; usually a simpler system than BMS, which focus on the basic and safety features of the battery pack. Compulsory in Lithium ion systems, even simple ones.

Polarity – Refers to terminals of a battery (positive or negative).

Primary Battery – A battery made up of primary cells. See Primary Cell.

Primary Cell – A cell designed to produce electric current through an electrochemical reaction that is not efficiently reversible. The cell, when discharged, cannot be efficiently recharged by an electric current. Alkaline, lithium, and zinc air are common types of primary cells.

PTC – Positive Temperature Coefficient (of expansion) : generally disks used in Lithium-ion cell headers to limit current when it is too high.

Rechargeable – Capable of being recharged; refers to secondary cells or batteries.

Secondary Battery – A battery made up of secondary cells. See Storage Battery; Storage Cell.

Separator – The permeable component that allows the passage of ions, but prevents electrical contact between the anode and the cathode (which can lead to short circuit).

Series Connection – The arrangement of cells in a battery configured by connecting the positive terminal of each successive cell to the negative terminal of the next adjacent cell so that their voltages are cumulative. See Parallel Connection.

Shutdown separator – A special type of separator that stops the ionic conduction between anode and cathode if temperature in the cell reaches a certain temperature. This feature limits the risk of thermal runaway.

Terminals – The parts of a battery to which the external electric circuit is connected (positive and negative).

Thermal Runaway – A condition whereby a cell on charge or discharge will destroy itself through internal heat generation caused by high overcharge or high rate of discharge or other abusive conditions. This phenomenon is divergent.

Vent – A normally sealed mechanism that allows for the controlled escape of gases from within a cell.

Wet Cell – A cell, the electrolyte of which is in liquid form and free to flow and move.


Ampere – An Ampere is a unit of measurement for an electrical current. One ampere is the amount of current produced by an electromotive force of one volt acting through the resistance of one ohm. Named for the French physicist Andre Marie Ampere. The abbreviation for Ampere is A but its mathematical symbol is “I”. Small currents are measured in milli-Amperes (mA), sometimes in mirco-amperes (µA).

Ampere-Hour – A unit of measurement of a battery’s electrical storage capacity. Current multiplied by time in hours equals ampere-hours (Ah). One amp hour is equal to a current of one ampere flowing for one hour. Also, 1 Ah is equal to 1,000 mAh.

Watt Hours – Energy available, expressed as capacity times voltage. The abbreviation is Wh.

Volt – The unit of measurement of electromotive force, or difference of potential, which will cause a current of one ampere to flow through a resistance of one ohm. Named for Italian physicist Alessandro Volt. Its abbreviation is V.

Watt – A measurement of total power. It is amperes multiplied by volts. Its abbreviation is W. Example : 120 V @ 1 A = 120W = 12 V @ 10 A


Alternating current (AC) – electrical current, in which the direction of the flow of electrons switches back and forth at regular intervals or cycles, as current flowing in power lines.

C – Used to signify a charge or discharge rate equal to the capacity of a battery divided by 1 hour. Thus C for a 1600 mAh battery would be 1.6 A, C/5 for the same battery would be 320 mA and C/10 would be 160 mA. Because C is dependent on the capacity of a battery, the C rate for batteries of different capacities must also be different.

Capacity – The number of ampere-hours (Ah) which can be delivered by a battery on a single discharge. The ampere-hour capacity of a battery on discharge is determined by a number of factors, of which the following are the most important: cut off voltage; discharge rate; type and density of electrolyte; design of separators; temperature, age, and life history of the battery; and number, design, and dimensions of electrodes.

  • Rated : quantity of electricity C5 Ah (ampere-hours) declared by the manufacturer which a single cell or a battery can deliver during a 5 h period when charging, storing and discharging under the conditions specified in associated standards.
  • Nominal : the total Amp-hours available when the battery is discharged at a certain discharge current from 100 percent state-of-charge to the cut-off voltage under given conditions by manufacturer. This could be Rated capacity.
  • Minimum : the total Amp-hours available guaranteed when the battery is discharged at a certain discharge current from 100 percent state-of-charge to the cut-off voltage under given conditions by manufacturer.

Charge – The conversion of electric energy, provided in the form of a current, into chemical energy within the cell or battery.

  • Standard charge : The charge conditions defined by manufacturer.
  • Topping/ balancing charge : step of charging in which a battery of several cells in serie is balanced, to compensate self discharge difference between cells.
  • Float Charge/ Trickle charge : Step of charging in which a secondary cell is maintained in fully charged condition.

Charge Rate – The current expressed in amperes (A) or milli-amperes (mA) at which a battery is charged. This rate is commonly expressed as a fraction of the capacity of the battery. For example the C/2 or C/5.

Constant-Current Charge – A charging process in which the current applied to the battery is maintained at a constant value.

Constant-Voltage Charge – A charging process in which the voltage applied to a battery is held at a constant value.

Cutoff Voltage – The prescribed lower-limit voltage at which battery discharge is considered complete. The cutoff voltage is usually chosen so that the maximum useful capacity of the battery is realised. The cutoff voltage varies with the type of battery and the kind of service in which the battery is used. When testing the capacity of a Ni-MH or Ni-Cd battery a cutoff voltage of 1.0 V is normally used. 0.9V is normally used as the cutoff voltage of an alkaline cell. A device that is designed with too high a cutoff voltage may stop operating while the battery still has significant capacity remaining.

Cycle – One sequence of charge and discharge.

  • Deep Cycle – A cycle in which the discharge is continued until the battery reaches its cut-off voltage, usually 80% of discharge.
  • Shallow Cycling – Charge and discharge cycles which do not allow the battery to approach its cutoff voltage. Shallow cycling of Ni-Cd cells lead to “memory effect”. Shallow cycling is not detrimental to Ni-MH cells and it is the most beneficial for lead acid batteries.

Cycle Life – For rechargeable batteries, the total number of charge/discharge cycles the cell can sustain before its capacity is significantly reduced. End of life is usually considered to be reached when the cell or battery delivers only 80% of rated ampere- hour capacity. Depending on the electrochemistry, conception, use, the number of cycles ranges from 300 to several thousand cycles. The cycle of a battery is greatly influenced by the type depth of the cycle (deep or shallow) and the method of recharging, the temperature… Improper charge cycle cutoff can greatly reduce the cycle life of a battery.

Depth of Discharge – The amount of energy that has been removed from a battery (or battery pack). Usually expressed as a percentage of the battery capacity. For example, 50% depth of discharge means that half of the energy in the battery has been used. 80% DOD means that eighty percent of the energy has been discharged, so the battery now holds only 20% of its initial capacity.

Direct Current (DC) – Electrical current which flows consistently in one direction. Typically the type of electrical current that a battery can  supply. One terminal is always positive and another is always negative.

Discharge – The conversion of the chemical energy of the battery into electric energy.

  • Deep Discharge – Withdrawal of all electrical energy to the cut off voltage before the cell or battery is recharged.
  • High-rateDischarge – Withdrawal of large currents for short intervals of time, usually at a rate that would completely discharge a cell or battery in less than one hour.
  • Low-rateDischarge – Withdrawal of small currents for long periods of time, usually longer than one hour.

Energy Density – Ratio of cell energy to weight or volume (watt-hours per kg Wh/Kg, or watt-hours per liter Wh/l).

Internal Resistance – The resistance to the flow of an electric current within the cell or battery.

Memory Effect – A phenomenon in which a cell, operated in successive cycles to less than full depth of discharge, temporarily loses the remainder of its capacity at normal voltage levels (usually applies only to Ni-Cd cells). Note, memory effect can be induced in Ni-Cd cells even if the level of discharge is not the same during each cycle. Memory effect is reversible.

Nominal voltage – The reported or reference voltage of the cell or battery, also sometimes thought of as the “normal” voltage of the battery (voltage around 50% SOC).

Open Circuit – Condition of a battery which is neither on charge nor on discharge (i.e., disconnected from a circuit).

Open-Circuit Voltage – The difference in potential between the terminals of a cell when the circuit is open (i.e., a no-load condition).

Self Discharge – Discharge that takes place while the battery is in an open-circuit condition..

Shelf Life – For a dry cell, the period of time (measured from date of manufacture), at a storage temperature of 21 degrees C (69 degrees F), after which the cell retains a specified percentage (usually 90%) of its original energy content.

Short-Circuit – A condition that occurs when a short electrical path is unintentionally created. Batteries can supply hundreds of amps if short-circuited, potentially melting the terminals and creating sparks.

Short-Circuit Current – That current delivered when a cell is short-circuited (i.e., the positive and negative terminals are directly connected with a very low-resistance conductor).

State of Charge (SOC) – Remaining capacity in the cell/ battery as a percentage of initial capacity.

Stationary Battery – A secondary battery designed for use in a fixed location.

State of Health (SOH) – Remaining life time of the cell/ battery as a percentage of initial life time.

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