Sodium-ion battery glossary

This glossary explains, in one or two sentences, the terms commonly used when discussing sodium-ion batteries, photovoltaic energy storage and hybrid inverters. The definitions are general; where the term relates directly to Heiwit products, the specific information is provided along with a link to the relevant page.

Sodium-ion battery

Also known as: sodium-ion battery, Na-ion, sodium battery

A rechargeable battery in which the charge carrier is the sodium ion (Na⁺) rather than the lithium ion. The operating principle is the same as that of lithium batteries; what changes are the electrode materials, which contain no lithium. Heiwit’s cells contain no cobalt either. Read more.

Salt battery

Also known as: salt batteries, salt battery

A common name used in Italy to refer to sodium-ion batteries, as the sodium is derived from sodium chloride, or table salt. The term is also used for other technologies, such as sodium-nickel-chloride (ZEBRA) batteries and saltwater batteries: Heiwit manufactures sodium-ion batteries exclusively. Read more.

NFPP

Also known as: Na₄Fe₃(PO₄)₂P₂O₇, sodium iron phosphate-pyrophosphate, polyanionic cells

A polyanionic cathode material based on sodium, iron, and phosphate and pyrophosphate groups. The sodium moves within a rigid three-dimensional lattice, which provides stability over time and at extreme temperatures, albeit at the cost of a lower energy density compared to layered oxides. This is the cell chemistry used in the 2027 version of the Heiwit systems, with a cycle life of 9,000 cycles; the 2025–2026 systems use layered oxide cells. Read more.

Layered oxides

Also known as: layered oxide, NaxTMO2

A family of cathode materials for sodium-ion batteries in which ions are intercalated between layers of transition-metal oxide. They offer more energy per kilogram than polyanionic cathodes, but the layers expand and contract with every cycle.

Cathode

Also known as: positive electrode

Positive electrode of the cell: during discharge, it receives the sodium ions coming from the anode. The cathode material (NFPP, layered oxides, or LFP in lithium cells) largely determines the battery’s performance and lifespan.

Anode

Also known as: negative electrode, hard carbon

The negative electrode of the cell, which accepts sodium ions during charging. In sodium-ion batteries, hard carbon is typically used, as the graphite in lithium cells does not accommodate sodium well.

Electrolyte

A medium – whether liquid or solid – that separates the anode and cathode and allows ions to pass through but not electrons. The electrons flow through the external circuit: it is this flow that powers the load.

Current collector

Also known as: current collector

A metal foil onto which the active material of each electrode is deposited. In sodium-ion cells, both current collectors can be made of aluminium. Lithium-ion cells need copper at the anode. The aluminium at the anode is what allows storage at zero volts.

Storage at 0 V

Also known as: zero-volt storage, 0 V transport

It is possible to fully discharge a sodium-ion cell for storage and transport without any residual energy, thanks to the aluminium current collectors. Lithium cells cannot withstand zero volts because the copper current collector degrades.

BMS

Also known as: Battery Management System

An electronic system that monitors the voltage, current and temperature of each cell, balances the battery pack, calculates the state of charge and state of health, and interrupts charging and discharging if these fall outside safety limits. In Heiwit batteries, this system is integrated and communicates with the inverter via CAN Bus.

SoC (state of charge)

Also known as: State of Charge

The percentage of energy available in the battery relative to its full capacity. It is an estimate calculated by the BMS, not a direct measurement, so it can jump in steps after a recalibration. Read more.

SoH (state of health)

Also known as: State of Health

The ratio of the battery’s current capacity to its initial capacity, expressed as a percentage. It describes ageing: a battery with an SoH of 90 per cent stores 90 per cent of the energy it stored when new.

DoD (depth of discharge)

Also known as: Depth of Discharge

The proportion of capacity actually drawn down in a cycle. The stated cycle life figures must always be considered in conjunction with the test DoD: 6,500 cycles at 80 per cent DoD are not comparable with cycles measured at 50 per cent.

Charge and discharge cycle

Also known as: cycle

A full charge followed by a full discharge, or the equivalent when partial cycles are added together. It is the unit used to measure a battery’s service life.

Cycle life

Also known as: number of cycles, cycle count

The number of cycles a cell can withstand before its capacity falls below a threshold – usually 80 per cent of its initial capacity – under the specified test conditions. This does not represent a lifespan in years: it depends on temperature, the average state of charge and the actual depth of discharge. Read more.

C-rate

Also known as: charge rate, C-rate

Ratio of current to rated capacity: 1C charges or discharges the battery in one hour, 0.5C in two hours. A lower C-rate extends the life of the cell.

Self-discharge

Also known as: self-discharge

The loss of charge in a battery left at rest, with no loads connected. On several Heiwit batteries it has been measured at 0.35 per cent per month, equal to 6 per cent over 539 days, at ambient temperature and at the latitude of Milan. Read more.

Energy density

Also known as: Wh/kg, Wh/l, energy density

Energy stored per unit of mass (Wh/kg) or volume (Wh/l). Sodium-ion cells have a lower energy density than lithium-ion cells: this is a key factor in mobile applications, but much less so in stationary storage, where weight and volume are only a consideration on the day of installation. Read more.

Thermal runaway

Also known as: thermal runaway

A chain reaction in which the heat generated by a cell accelerates reactions that produce further heat, ultimately leading to a fire. The stability of polyanionic materials reduces the likelihood of NFPP cells triggering such a reaction; however, safety remains a property of the complete system, not of the cell alone. Read more.

Hybrid inverter

Also known as: hybrid inverter, inverter with storage

An inverter that manages the photovoltaic system, battery, grid and loads together, deciding when to charge, discharge, export to the grid or import from it. The Virgo (single-phase) and Lybra (three-phase) hybrid inverters by Heiwit are designed for 48 V sodium-ion batteries. Read more.

MPPT

Also known as: Maximum Power Point Tracking

An inverter circuit that regulates the voltage and current of the photovoltaic strings to extract maximum power as irradiance and temperature vary. Multiple MPPT units allow for strings with different orientations.

UPS (backup) function

Also known as: EPS, emergency power supply, switchover time

The hybrid inverter’s ability to power loads from the battery when the mains supply is lost. The switchover time is the interval between the loss of the mains supply and the restoration of power: 10 ms on the Virgo, less than 10 ms on the Lybra.

Stationary storage

Also known as: stationary energy storage, BESS

A storage system installed in a fixed location, such as a home, a business or an electrical substation, as opposed to batteries used for mobility. Here, service life, safety, thermal behaviour and cost per kWh cycled matter more than energy density.

Storage retrofit

Also known as: revamping, adding a battery to an existing system

Adding a battery to a solar PV system that is already in operation. With Heiwit batteries, a Heiwit hybrid inverter is installed — the single-phase Virgo or the three-phase Lybra — connected on the AC line: the existing PV inverter stays in place. Inverters from other brands do not support the voltage range of sodium batteries. Read more.

IP rating

Also known as: IP65, IP66, Ingress Protection

A two-digit code from the IEC 60529 standard indicating protection against dust (first digit) and water (second). IP66 means fully protected against dust and powerful water jets; IP65 means protected against water jets.

CEI 0-21

Also known as: Standard CEI 0-21, technical regulation for low-voltage connections

The Italian technical regulation for the connection of active and passive users to low-voltage networks. Inverters with storage must comply with it in order to be connected to the grid: the Heiwit Virgo and Lybra inverters are CEI 0-21 certified. For the Virgo with the 48210 battery, certification is issued by TÜV SÜD on the basis of the type tests. For the Lybra with the HEIWIT-NA-48V210Ah battery, certification is issued by TÜV Rheinland (certificate no. A3 50741827 0001). Read more.

Derating

Also known as: power reduction

Automatic reduction of output power when the temperature, altitude or other conditions exceed a threshold, to protect the electronics. The Virgo reduces power above 40 °C, and the Lybra above 45 °C.

LFP

Also known as: lithium iron phosphate, LiFePO₄

Lithium chemistry with a lithium iron phosphate cathode, now the industry standard in residential energy storage for its stability and longevity. It is the appropriate benchmark for sodium-ion batteries in stationary applications. Read more.

NMC

Also known as: nickel-manganese-cobalt, lithium NMC

Lithium chemistry with a cathode based on nickel, manganese and cobalt, offering high energy density and used mainly in electric vehicles. It contains cobalt and nickel. Heiwit’s sodium-ion cells contain no cobalt. The 2027 version, with an NFPP polyanionic cathode, contains no nickel either.

kWh and kW

Also known as: kilowatt-hour, kilowatt, capacity and power

A kWh measures the energy stored (the battery’s capacity), whilst a kW measures the power at which it is delivered or consumed. A 10 kWh battery with a rated power of 5 kW can power a 5 kW load for approximately two hours.

Round-trip efficiency

Also known as: round-trip efficiency, cycle efficiency

The ratio of the energy returned by the battery to the energy used to charge it, including inverter and BMS losses. The higher it is, the less energy is lost on its way through the storage system.

Batteries in parallel

Also known as: modules in parallel, storage expansion

Connecting multiple battery modules to the same inverter to combine their capacity whilst maintaining the same voltage. The Heiwit inverters can accommodate up to three 10 kWh modules, for a total of 30 kWh.

LCOS

Also known as: Levelised Cost of Storage

The cost per kWh that passes through the battery over its lifetime, calculated by dividing the total cost of the system by the total energy cycled. This is the key figure for comparing technologies, and does not correspond to the list price. Read more.

Peak shaving

Also known as: peak lopping

Use of the battery to cover peaks in grid import, reducing peak power demand and the related charges. A typical application for commercial and industrial energy storage.

Self-consumption

Also known as: photovoltaic self-consumption, self-consumption rate

The proportion of the energy generated by the photovoltaic system that is used directly on site rather than being fed into the grid. A battery increases it by shifting energy generated during the day to the evening hours. Read more.