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Battery Glossary

The Battery-Kutter 'Battery Glossary' is not intended to be fully comprehensive. Rather, it exists simply to provide explanations for a few important terms in our business. We would be happy to hear from you if you have any thoughts on how we can improve it.

  • Acid density

    The acid density contains information about the state of charge of a lead-acid battery. It is measured with an acid siphon. A fully-charged battery has an acid density of 1.28 kg/l and a discharged battery ca. 1.1 kg/l.

  • Acid stratification

    When charging a battery, the aim is to achieve a high acid density. The acid sinks to the lower part of the battery container. After repeated charges, variable densities form in the battery acid – higher densities below and lower densities above. It's important not to maintain the battery in this state for any length of time because there is a risk of it being destroyed. The major battery brands offer automatic acid circulation devices that ensure that the acid density remains constant.

  • Accumulator / Rechargeable battery

    An accumulator or rechargeable battery is a rechargeable reservoir of electrical energy that works electro-chemically. It stores electrical energy by converting it into chemical energy, and vice versa.

  • AGM

    Safe, powerful and durable!

    Powerful modern batteries are based on AGM (Absorbent Glass Mat) technology. Special micro glass-fibre mats lie in between the battery’s lead plates and absorb all the battery acid. The glass fibre mats is designed to absorb and hold all the acid, but without fully saturating the mat. The sealed system is equipped with a VRLA (Valve Regulated Lead Acid), a pressure-relief valve that safely disposes of any gases.

    The technology offers many advantages:

    The battery remains dry and leak-proof. In the case of frost, the expanding liquid can’t cause any damage. Because of the micro glass-fibre mats, there is scarcely any plate movement any more, which means that the battery is unaffected by jolts and vibrations. Thus, AGM batteries can even be assembled on their sides and operated in any environment. In addition, because the acid is completely trapped in the mat, maintenance considerations such as water refills and monitoring the electrolyte are unnecessary! The way they are constructed means that AGM batteries have an extremely low internal resistance. Reactions between the acid and plate material are faster, and so, for example, in a challenging situation like charging in the extreme cold, more energy can be transferred! Charging AGM batteries is designed to be as simple as possible. A conventional car battery charger will work, you don’t need specialist chargers or adapters!

  • Alkaline:

    The galvanic alkaline battery is one of the most important electro-chemical energy storage systems. It has supplanted the zinc-carbon for many uses because of its higher capacity, better load capacity and longer storage capacity. Alkaline batteries are primary batteries, so they are non-rechargeable.

    The main alkaline battery types are cylindrical round-cell (e.g. LR6 = Alkaline AA or Mignon) and button-cell.

  • Ampère [A]

    The unit of electrical current, shortened to: A.

  • Ampere hours [Ah]

    Ampere hours are the unit of electrical charge. It is the product of the current (measured in Amperes) and time (measured in hours). Ampere hours are typically the unit that the quantity of electric charge (capacity) of a battery or cell is given in. It is shortened to Ah.

  • Anode

    The anode is the counterpart to the cathode, and is where oxidation in a liquid solution occurs and electrons are released. During this process, anions (negatively charged electrons) are discharged. Both electrodes can function as the anode, depending on the direction of the electrical current in the secondary cells. During discharge, the negative electrode becomes the anode.

  • Back-up battery

    A replacement / auxiliary power supply that is switched constantly at float voltage.

  • Battery

    The battery is an electro-chemical energy reservoir. It is usually made up of a combination of electro-chemical cells – the so-called galvanic elements. The cells contain two electrodes that are kept separated by an ion-conducting fluid or solid electrolyte. Different battery types will have different voltages and energy densities depending on which chemical system they use. The material that the electrodes are made from defines how high the nominal voltage is, whereas the amount of energy that can be stored depends on the composition and quantity of the battery material. When the battery is discharged, chemically-stored energy is converted to electrical energy through an electro-chemical reaction. The main parameters here are: hold-up time, apparent power, cos.Phi of the power user, inverter efficiency rate, link voltage, discharge voltage and charging voltage.

    Battery types:

    • Sealed battery
    • Vented battery
    • Open battery
  • Battery Capacity

    A battery’s nominal capacity is the capacity that it has when discharging over a specific discharge period (nominal discharge time tN), at a nominal temperature, nominal density and nominal electrolyte condition without it falling below the cutoff voltage (COV). Capacity values typically apply to a 10 or 20 hour discharge period for lead batteries and a 5 hour discharge for nickel-cadmium batteries. When used in UPS installations, short discharge times mean the batteries' working capacity is much smaller than the nominal capacity. Therefore, to get an accurate calculation you need tables or curves showing how discharging performance relates to discharge time. In addition, it is worth noting that some batteries reach their full nominal capacity after several charging cycles – i.e. indicating a smaller capacity than expected at the beginning may not be a sign of a defective battery.

  • Battery interface cabinet

    A unit housing battery connections in a cabinet to accommodate the DC battery switch and cabling.

  • Battery pack

    A battery pack refers to a package of inter-connected rechargeable battery cells that are housed in a container for pratical reasons - often to shield it from environmental factors. Battery packs are designed to be replaceable / removable, and have removable electrical contacts such as plug connectors.

    To ensure that voltage, capacity and load capacity are as uniform as possible, the cells should all be the same type and made by the same manufacturer. Inside a battery pack, blocks containing several cells can be connected in parallel, and likewise, many blocks in a row.

  • Battery room

    For particularly large battery installations, a dedicated room is often constructed in the building. The specifications for equipping a battery room are set out in DIN VDE 510 Part 2. These cover aspects such as ventliation and extraction, and adequate spacing for combustible and spark-generating elements.

  • Capacity

    Describes the available quantity of storage or power of a battery or cell (measured in Ampere hours). Battery temperature and discharge current are integral to calculating capacity, therefore that data must necessarily be provided. (e.g. cold starting capacity measured in seconds, plus cold cranking current and temperature -18°C)

  • Capacity (Nominal)

    This describes capacity under normal conditions and depends on battery type (e.g. 20 hour capacity in Ah, reserve capacity during discharge: 25 A in min.)

  • Capacity (Residual)

    The residual capacity describes the useable capacity of a battery with an unspecified state of charge when being discharged at a nominal current (e.g. after a prolonged period of downtime).

  • Cathode

    Describes the negative electrode where the process of reduction happens in a fluid solution - i.e. the cations (positively charged ions) are deposited (gaining an electron). In secondary (rechargeable) cells, either electrode can become the cathode depending on the direction of the current . The cathode is the positive electrode during discharge.

  • Cell

    The smallest unit of a battery consisting of positive and negative electrodes, a separator and the electrolyte. It is the primary building block of a battery. The cell stores electrical energy, with the size of the cell being defined by its capacity.

  • Charge efficiency

    The charge efficiency describes the ratio between used capacity and chargeable capacity, which is 0.85 in the case of lead batteries.

  • Charge factor

    The charge factor describes the ratio between discharged battery capacity and capacity needed to recharge the battery. Because of their energy efficiencies, lead batteries need to be charged ca. 20% more and NiCd batteries ca. 40%.

  • Charging curves

    The following curves are used to charge batteries:

    • I – curve
    • IU – curve
    • U – curve
    • W – curve
    • Wa – curve

    Which charging curve should be used depends on the battery type. You can find out which one to use by referring to the manufacturer’s user manual.

    I – curve
    This means charging with a constant current. Here charging voltage is not controlled, which can lead to the battery temperature rising, the electrolyte boiling and water loss. It may also cause a chemical reaction where hydrogen is released, raising the risk of an explosion. This charging method is only suitable for low charging loads of a couple of milliamps.

    IU – curve
    An empty battery is charged with a constant current (I curve). The charge voltage is set at the float voltage. As the charge capacity of the battery increases, the charging current gradually decreases, and the charge voltage rises until it reaches the intended float voltage (2.26V/cell – 2.29V/cell, depending on the type of lead battery). At this point the charger switches to the U curve.

    U – curve
    Charging at a constant voltage. If a high voltage charge of 2.4V/cell is reached, the current is automatically reduced to prevent the battery temperature from rising too much. (An overly high temperature can destroy the battery.)

    W – curve
    Charging follows a resistance curve (W). As the charge voltage rises the charge current is reduced. Similarly to the U-curve above, a rise in temperature caused by a high current must be prevented through an increase in charging voltage. Wa – curve
    Charging that follows the W-curve, although this time charging stops after a specific amount of time.

  • Constant current tables

    Manufacturers' constant current tables display which max. cut-off voltage the battery can supply a constant current per cell (or per block of cells) relative to time.

  • Cut-off voltage

    Describes the voltage measured between a battery’s terminals after discharge. The value for each specific battery will be provided by the manufacturer. Falling below the cut-off voltage can cause the battery to be destroyed. The cut-off voltage also depends on the battery’s load capacity. For very small loads, it is wise to maintain higher cut-off voltages in order to avoid damaging the battery.

  • Cycle

    A cycle is a regular, repeated process during battery charging and battery discharge.

  • Cycle-resistance

    Cycle-resistance describes how often a battery can be charged and discharged before it reaches the end of its life.

  • Deep discharge

    Describes the condition of the battery after total it has been totally discharged at a low current, and has dropped below the cut-off voltage. Deep-discharged AGM batteries must be recharged within 12 hours or they will be irreparably damaged. AGM GEL batteries must be recharged within 5 – 7 days.

  • Diffusion

    Diffusion describes the physical process that creates a total mixture of two or more materials.

  • Direct Current

    Direct Current describes electricity that does not vary in terms of power or direction over time (e.g. electricity from an electro-chemical energy source)

  • Electrode

    Consisting of an active material and a conductor, this conductive mass is where electro-chemical reactions happen inside the cell.

  • Electrolyte

    Electrolyte is a solid, liquid or dissolved chemical compound that dissociates into ions, and that is moved in single direction under the influence of an electric field.

  • Electrons

    Electrons are negatively charged elementary particles.

  • Energy

    Describes work that is measured in watt-hours, and which is defined by voltage and capacity. The more energy a battery stores and can supply, the more work can be achieved with it.

  • Environmental factors

    Describes factors that can significantly reduce the battery's working life when adjustments are not made according to the manufacturer's guidelines. In general, these are: vibration, shocks, ambient temperature, humidity and height...

  • Equalisation charge

    Equalisation charges are necessary in some open battery types in order to equalise decreasing charge capacity. Equalisation charges are required at relatively long intervals (6 – 9 months), depending on the battery type and manufacturer guidelines. The appropriate voltage and current levels can be taken from manufacturers’ data sheets.

  • ETN numbers

    Describes code numbers for standard lead (starter) batteries.

    The “European Type Number” is the (almost) universally-adopted successor to DIN specifications (DIN = German Industrial Standard). The numbers in the code contain all important battery information (voltage, capacity, product category and cold cranking current). The ETN number is comprised of 3 groups of numbers with 3 numbers each. These encode the following information:

    ETN 536 046 030;

    • 1. Group 5 3 6 = group A:
      • The first figure denotes voltage, with numbers 1 – 4 being reserved for 6 volt batteries
        • the number 5 for 12 volt batteries under 100Ah
        • the number 6 for 12 volt batteries above 100Ah
        • the number 7 for 12 volt batteries above 200Ah
        • 8 for specialist starter batteries
        • 9 for drive, lighting and solar batteries
      • The second and third figures describe the capacity of batteries in Ah, so:
        • 5 3 6 = 12 volt battery with a capacity of 36Ah (below 100Ah)
        • 6 3 6 is a 12 volt battery with 136Ah (over 100Ah)
        • 7 3 6 is a 12 volt battery at 236 Ah (over 200Ah)
        • the number 8 for specialist starter batteries
        • the number 9 for drive, lighting and solar batteries
      • The second group 0 4 6 = group B:
        This group of figures encodes information about measurements and special features (for of the container base, type of container lid, base plate, etc.)
      • The third group 0 3 0 = group C:
        These 3 figures describe the cold cranking current x 10. For example: 030 x 10 = 300A, 115 = 1150A.

    The previous DIN codes, e.g. 5 36 46, are constructed from the same information but lack cold cranking current specifications. However, in normal speech people would ask for a battery, 12 volt – 36Ah, or battery, 12 Volt – 88Ah. Here, it simply has to be established first that we are referring to standard batteries available as typical commercial stock.

  • Eurobat

    EUROBAT is an association that promotes the interests of the European battery industry. With 34 members who make up more than 85% of the battery industry in Europe, EUROBAT works to develop new battery solutions and renewable energy storage methods. In addition, it can offer recommendations about the definition of technical battery terms such as working life.

  • Explosive (hydrogen) gas

    Hydrogen gas is produced when the battery is overcharged. A mixture of hydrogen and oxygen forms via a chemical reaction in the electrolyte. This mixture is highly explosive.

  • Final charge voltage

    When a battery is completely charged, the float voltage that offsets the battery's self-discharge is the only remaining energy flow. Charging voltage is set between 2.26V/cell and 2.29V/cell depending on the battery type.

  • Float voltage

    This is a constant voltage is applied to stop the battery self-discharging. The charging voltage is set at 2.26V/cell and 2.29V/cell, depending on battery type.

  • Float Charging

    The voltage required to keep batteries in a fully-charged state is called the float charge. Standard values at 20°C: lead batteries – 2.23V – 2.27V, 1% per cell; NiCd batteries – 1.40V per cell. It is imperative to follow the manufacturers’ guidelines. If temperatures deviate from the norm where the battery is kept, either permanently or consistently, then the above values will change according to the manufacturer’s specifications.

  • Gases

    When the battery is overcharged, it can lead to gassing - which means hydrogen coming out of the battery, and the corresponding risk of explosion.

  • Gassing voltage

    The voltage at which the electrolyte in a battery is coverted into a gaseous state and escapes. The voltage should not stay at the gassing voltage too long, as there would be a significant amount of electrolyte lost and it could destroy the battery. Typically, the gassing voltage is around 2.4V/cell in lead batteries and about 1.55 volts/cell in nickel-cadmium batteries.

  • Gel battery

    With its frequent discharge cycles and starting ability in all weather conditions, a gel battery has an ability to power a range of electrical devices. Motorbikes are one good example; driving on vibration-heavy dirt track, racing round corners, installing the battery in different positions, seasonal use with long periods of downtime and irregular charging are all instances where a battery using specialised GEL technology is required.

    With the electrolyte bound by the gel, state-of-the-art GEL technology offers outstanding vibration-resistance. With the electrodes and liquid acid securely preserved in the multi-component gel, this set-up guarantees excellent cycle-resistance in a wide range of discharge situations, as well as the best leak-proofing. An internal gas recombination process prevents gas build-ups and allows the battery to be sealed. A pressure-regulated valve ensures that it meets the highest safety standards, even when overcharged.

  • GroE batteries

    GroE batteries are closed, stationary lead batteries with liquid electrolyte (containing dilute sulphuric acid). Their construction is distinctive - they have completely casted plates with a lamellar structure. Their lead technology, high electrode strength and low acid density of 1.22 kg/l mean that they have an exceptional working lifespan of at least 20 years.

    GroE batteries are manufactured with a range of capacities from 75Ah to 2600Ah. Their positive electrode is shaped like a large plate. The electrolyte is in liquid form and needs to be monitored whilst the battery is operating.

    The number of cycles is over 200. The GroE battery range has the very best reliability, and they are extraordinarily safe to operate. The batteries also manage high voltage at a high discharge current, as well as a high level of consistency in their electrical properties throughout their working life.

    The GroE has been used for more than 100 years and is therefore one of the most developed and secure types of battery system.

  • GRS

    stands for Gemeinsames Rücknahmesystem Deutschland (the German National Battery-Collection Foundation).

    The GRS foundation regulates the proper disposal of used batteries throughout Germany. Whether you're a industrial end-user or a public waste disposal carrier, GRS provides services that cover the pick-up, sorting and evaluation of old batteries.

  • High current charging

    Charging where the current is at or above 1 C.

  • High current discharge

    Discharging where the current is above 5 C.

  • Industrial battery

    Rechargeable batteries that are exclusively for industrial, commercial and agricultural uses (e.g. for fork-lift trucks, hybrid vehicles).

  • Inner resistance

    Ohmic resistance of a battery

  • I/V Curve

    The I/V curve is tool to enable careful charging. First, the battery is charged at a constant current until the float (trickle) voltage is reached. After this, they are charged at a constant voltage.

  • Lead battery

    A rechargeable battery that uses weak sulphuric acid as an electrolyte. The electrodes are made from lead.

  • Lead sulphate (PbSO4)

    A chemical compound that forms at the plus and minus plates of a lead battery during discharge. It is formed by the chemical reaction between sulphuric acid and lead oxide at the positive electrode, and lead metal at the negative electrode.

  • LIfe (battery life)

    Battery life depends on a number of factors, but the battery must be maintained according to user manual guidelines to achieve maximum longevity. The battery life is directly dependent on the following factors: the charging of the battery, the temperature of the surrounding environment, the number of charging cycles and deep discharges.

  • Lithium ions

    The battery cell contains lithium ions in the postive electrode, the negative electrode and the electrolyte.

  • Low battery

    A warning signal about an upcoming deep-discharge curve. It is used on some UPS installations as an advance warning at the interface or via the relay board.

  • Maintenance free batteries

    A battery where the electrolyte is in gel form or in a micro glass-fibre mat (AGM). The battery is sealed, and does not need to be filled with distilled water.

  • Memory effect

    The memory effect describes a loss of capacity in the battery where it remembers the input-energy needed after a previous discharge, rather than the actual one. If the voltage of the cells falls below the minimum level, the battery will become unusable even though it still has electrical energy.

  • Nickel-cadmium

    In a nickel-cadmium battery, one electrode is made of nickel and the other from cadmium. These rechargeable batteries are highly resistant to deep discharge and over-charging, and the cells are also cycle- and temperature resistant. NiCd batteries are banned for most uses, with battery regulations restricting their usage to bicycles, emergency lights, alarm systems and medical devices.

  • Nickel-metal hydride

    Consisting of a positive electrode of nickel hydroxide and a negative electrode of metal hydride, NiMH batteries offer about twice the energy density as NiCd batteries at the same voltage.

  • Nominal capacity

    The usable capacity of a battery under the conditions given by the manufacturer.

  • OGi battery

    The OGi battery is a low maintenance, wet-cell battery with a liquid electrolyte. The design life of the battery is up to 15 years with an ambient temperature or 20-25°C. The maximum ambient temperature is -20°C to +50°C. These batteries are constructed with long-lasting positive grid plates, fine-grain pasted negative grid plates, micro-porous separators and containers made from a stable, flame-retardant, crystal clear SAN material.

    The batteries are assembled as individual blocks consisting of several cells, or as single cells. Their rates of charge range from 25Ah – 900Ah depending on the model and size. The round mesh construction of the plate grid lowers the internal resistance of the battery, which means that the OGi can supply a very high current in a very short space of time. The cycle number of the individual charges/discharges is > 1000.

  • OPzS batteries

    OPzS batteries are wet-cell lead batteries with a liquid electrolyte (diluted sulphuric acid). The tubular-plated construction of OPzS batteries offers a very high cycle life at up to 1500 cycles at 80% depth of discharge. Hence tubular-plated batteries are ideal for applications where a high charge/discharge capacity is needed - e.g. in solar applications or where there are long bridging times, such as in IT/telecoms and emergency lighting. Their typical working lifespan is around 20 years.


    • IT / Telecomms
    • Emergency lighting
    • Solar and UPS
    • Back-up batteries, BEV technology
    • Wind installations


    • High charge and discharge capacity
    • High cycle resistance and longevity
    • Exceptional reliability
    • Very suitable for long bridging times
  • OPzV batteries

    OPzV batteries are low maintenance, sealed indivdual cells that are usually made with plastic containers. The positive electrode is a tubular plate. Smaller capacity batteries up to ca. 300Ah are available in 12V models. The batteries’ working lifespan is typically over 15 years. The batteries are produced with capacities of up to 3200 Ah. The electrolyte is in gel form and does not need to be monitored during the battery’s life. They are best employed for uses where a bridging time of more than half an hour is necessary. As well as a long working life in parallel standby mode, they also have a cycle-resistance that is almost as high as in OPzS batteries.

    Therefore tubular-plate OPzV batteries are ideal for usage in applications where a high charge/discharge capacity is needed, e.g. IT/telecoms, emergency lighting, UPS, BEV and wind-power installations.


    • High current capability
    • Horizontal installation option
    • High cycle-resistance and working life
    • Minimal maintenance
    • Excellent reliability
  • Overcharging

    A battery is overcharged when the chosen charging voltage is too high. Initially, the lead sulphate is converted back to lead and lead oxide, but because the charging current continues to flow, the lead on the grid also starts to be affected. This causes the grid to expand and weakens the press-fitted materials.

  • Parallel connection

    Several battery blocks or wires are connected with each other to increase capacity. See also: Parallel switched battery cables.

  • Parallel switched battery cables

    When connecting several batteries in parallel, it is important to make sure that the cabling between individual battery wires is symmetrical. It’s best to make them all the same length, so that the charging voltage of individual wires is the same, and no battery receives a charging voltage that is too low or too high.

  • Positive terminal

    Positive connection on a cell or battery.

  • Polarity

    This term describes the relative charge or voltage (opposing) between two electrodes

  • Primary cell

    A non-rechargeable cell.

  • Residual capacity

    The amount of capacity left in the battery after it has been discharged. The faster a battery is discharged, and the higher the discharge current, the higher the residual capacity in the battery will be.

  • Residual charge

    A full charge from an undefined state of charge.

  • Round cell

    A round cell, in contrast to a prismatic cell.

  • Sealed batteries / maintenance free

    These are batteries with a gas recombination rate of at least 95%. I.e. it is not necessary to refill them with water throughout their entire working life. The batteries are generally described as “maintenance free”.

  • Secondary battery

    A rechargeable battery

  • Self-discharge

    Describes the battery's own discharge that is independent of connection to an electrical device. Self-discharge is a constant process of chemical reactions that depends on the temerature.

  • Separation

    A separator ensures that the positive and negative electrodes (cathode and anode) remain isolated from one another.

  • Serial connection

    Connecting the negative terminal of a battery to the positive terminal of another battery. The combined pair has double the voltage at a constant capacity.

  • Short circuit current

    This describes the current that flows through the terminal of the battery during a short circuit. It is only limited by the internal resistance of the battery. Short circuit currents should never be underestimated, and they can increase to several thousands of amps in large batteries. (For example: a battery at 65Ah can supply a short circuit current of up to 1500A). WIth short circuits there is always the possibility of burns being caused or objects being set on fire. A total short circuit at the battery terminals can cause the battery to explode, which is why every battery must be fitted with a suitable fuse.

  • Standby

    In a standby set-up, the battery is usually connected to an electric appliances in parallel, and it is only discharged when the normal power source fails and the emergency power system is initiated.

  • Sulfation

    When a lead battery is in a state of deep discharge, a process of recrystallisation can lead to the formation of coarse-grained lead sulphate at the electrodes. This reduces reactivity at the electrode's surfaces and can lead to short circuits when the battery is jolted.

  • Terminal type

    The terminal type describes how the battery can be connected.

  • Traction battery

    The traction battery is a drive battery.

  • UPS installation

    UPS= Uninterruptible Power Supply. These installations are switched on during disruptions to the electricity supply so that electricity continues to be supplied without any outage.

  • Vented batteries / low maintenance

    The electrolyte liquid in the individual battery cells can be topped up through openings in the battery case. This battery type is also known as an “open” battery.

  • Volt

    The unit for electrical voltage, shortened to “V“. The term was coined in 1897 after the Italian physicist and medic, Duke Alessandro Volta.

  • Voltage delay

    This describes a drop in voltage in lead batteries at the beginning of a discharge. This effect is highly dependent on how high the required current is. High load capacity batteries should therefore also be used. HR (high rate) type batteries are particularly useful in this regard.

  • Wet-cell batteries

    Conventional wet-cell consumer batteries have an energy supply that is just as reliable as that of AGM batteries. However, wet batteries do need maintenance, with regular checks of the battery liquid and top-ups with distilled water needed to guarantee battery longevity.

  • Working life (battery)

    A period of time that describes how much capacity a battery has to fulfil a task, once loss of capacity due to storage-time and temperature-related effects.