Glossary

Every battery storage term on this site, defined in one sentence each.

These are the words that decide a specification: what a BESS is, what a BMS does that an EMS does not, and why two datasheets can both say 261 kWh and still describe different products. Each entry below is a definition, not a sales description.

Systems and market segments

The words used to describe what is being bought before any number is quoted.

Battery energy storage system (BESS): A battery energy storage system is an installation that charges a rechargeable battery from the grid, from PV, or from both, and discharges it later. A complete BESS combines battery racks, a battery management system, a power conversion system, thermal management and an energy management system.
Containerized BESS: A containerized BESS is a battery storage system assembled and tested inside a standard freight container or an equivalent enclosure. Racks, cooling, fire detection and often the power conversion equipment are fitted at the factory, so site work is limited to foundations, cabling and grid interconnection.
Commercial and industrial (C&I) storage: C&I storage is a battery system sized for a business site rather than a single home. Its usual duties are reducing peak demand charges, shifting consumption on time-of-use tariffs, and backing up critical loads when the grid fails.
Residential battery storage: A residential battery is a storage system installed at a house, usually alongside rooftop PV. It stores surplus daytime generation and supplies the home at night or during an outage, and it is normally sized in kilowatt-hours against the household load rather than against the PV array.
All-in-one ESS: An all-in-one ESS is a storage product that ships with the battery, inverter, protection and enclosure already integrated in one unit. It reduces the number of interfaces an installer has to commission, at the cost of less freedom to mix components from different vendors.

Cells and chemistry

What sits inside the rack, and why the choice fixes most of the other numbers.

Lithium iron phosphate (LiFePO4 / LFP): Lithium iron phosphate is a lithium-ion chemistry whose cathode is iron phosphate. LFP cells trade some energy density for thermal stability and long cycle life, which is why LFP is common in stationary storage cabinets rather than in vehicles.
Nickel manganese cobalt (NMC): NMC is a lithium-ion chemistry with a cathode of nickel, manganese and cobalt. NMC cells store more energy per kilogram than LFP, but their thermal stability threshold is lower, so they appear more often in vehicles than in stationary battery cabinets.
Battery cell: A battery cell is the smallest electrochemical unit in a battery system, usually built in prismatic, cylindrical or pouch format. Cells are the building block from which modules, racks and cabinets are assembled.
Battery rack: A battery rack is a frame that holds battery modules together with their monitoring electronics. The rack is the unit that is lifted into a cabinet or container, wired to the DC bus and serviced in the field.
Cell balancing: Cell balancing is the process that equalises state of charge across the cells of a series string. Without it the weakest cell decides where the string has to stop charging, so usable capacity falls before the battery is actually empty.
Cell-to-pack and cell-to-rack design: Cell-to-pack design mounts cells directly into a pack or rack structure and skips the separate module housing. It raises the share of the enclosure volume that holds cells, and it changes how a single failed cell is serviced.

Electrical conversion and coupling

The parts that move energy between the battery, the solar array, the grid and the load.

Power conversion system (PCS): A power conversion system is the bidirectional converter between the battery DC bus and the AC grid or load. It sets charge and discharge power, follows grid voltage and frequency, and enforces the current limits the battery can accept.
Hybrid inverter: A hybrid inverter is an inverter that manages both PV input and a battery before supplying AC loads. It differs from a PV-only string inverter because it can charge the battery from solar and keep supplying the house when the grid is absent.
DC coupling: DC coupling is an architecture in which the PV array and the battery share one DC bus and one inverter. Solar energy can flow into the battery without an extra AC conversion step, which usually gives the highest round-trip efficiency for self-consumption.
AC coupling: AC coupling is an architecture in which the PV array keeps its own grid-tie inverter and the battery system uses a separate inverter connected on the AC side. It is the usual way to add storage to a PV installation that is already running.
C-rate: The C-rate is a charge or discharge current expressed as a multiple of the battery capacity. At 1C a battery delivers its full nominal capacity in one hour; at 0.5C the same battery takes two hours.
Round-trip efficiency: Round-trip efficiency is the share of the energy put into a battery that comes back out over a full charge and discharge cycle. It accounts for conversion losses in the inverter, losses inside the cells, and auxiliary loads such as cooling.

Capacity, ageing and performance

The four numbers that decide whether a quoted kilowatt-hour is the one you can actually use.

State of charge (SOC): State of charge is the energy remaining in a battery at a given moment, expressed as a percentage of its capacity. It is the battery equivalent of a fuel gauge, and it is estimated from voltage, current and coulomb counting rather than measured directly.
Depth of discharge (DoD): Depth of discharge is the share of capacity taken out of a battery during a cycle, the inverse of its minimum state of charge. A cycle that runs from 100% down to 20% uses 80% DoD.
State of health (SOH): State of health compares a battery's present capacity or internal resistance with its beginning-of-life value. A battery at 80% SOH has lost roughly a fifth of its original usable capacity.
Cycle life: Cycle life is the number of charge and discharge cycles a battery completes before its capacity falls to an end-of-life threshold stated in the datasheet. Cycle-life figures are only comparable when the C-rate, the ambient temperature and the end-of-life percentage are the same.
Usable capacity: Usable capacity is the part of a battery's nominal capacity that the system allows you to discharge in normal operation. It is lower than the nominal figure because the battery management system keeps a margin at the top and bottom of the voltage range to protect the cells.
Capacity fade: Capacity fade is the gradual loss of usable capacity as a battery ages. It is driven by calendar time, cycle count, temperature and the depth of the cycles, which is why two identical cabinets in different climates do not age at the same rate.

Controls, thermal management and safety

The systems that decide what the battery does, keep it in its temperature window, and contain a failure.

Battery management system (BMS): A battery management system is the electronics that monitor and protect the cells. It measures voltage, current and temperature, balances the cells, estimates state of charge, and opens the contactors when a limit is exceeded.
Energy management system (EMS): An energy management system is the control layer that decides when the battery should charge or discharge. It reads load, tariff, PV forecast and grid signals, then sends power setpoints to the PCS, while the BMS keeps the cells inside their limits.
Thermal management: Thermal management is the system that keeps cells inside their operating temperature window, using air or liquid. Liquid cooling holds a tighter temperature spread across a large cabinet, which matters because cells that drift apart in temperature age at different rates.
IP rating: An IP rating is a two-digit code from IEC 60529 describing how well an enclosure resists dust and water. The first digit covers solids and the second covers water, so IP54 means dust-protected and resistant to splashing water from any direction.
Thermal runaway: Thermal runaway is a self-accelerating exothermic reaction inside a cell, where heat generation outruns heat removal. Once it starts the cell can vent flammable gas or ignite, and a propagation test asks whether that event spreads to its neighbours.
IEC 62619, UL 9540A and UN38.3: IEC 62619, UL 9540A and UN38.3 are the three standards most often quoted for stationary lithium storage. IEC 62619 sets safety requirements for industrial lithium cells and batteries, UL 9540A is a test method for evaluating thermal runaway fire propagation in a BESS, and UN38.3 is the transport test series a lithium battery must pass before it can be shipped.

Where these terms appear in a real system

The technology page walks through the same layers in the order they are specified — chemistry, conversion, thermal design, controls and safety — with the comparison tables a project brief usually needs.

How the system layers fit together Product platforms by application Solutions by site type

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