Battery Encyclopedia.

Reliable technology with high power density

AGM (Absorbent Glass Mat) batteries use a glass-fibre separator placed between the lead plates, which fully absorbs the electrolyte. This design prevents leakage and enables stable operation in any mounting position. An integrated pressure relief valve (PRV) provides additional protection against gas build-up.

Benefits of AGM technology

This construction makes the battery highly resistant to vibration and temperature fluctuations. It is particularly suitable for demanding operating conditions, as no water top-up is required and the batteries can be operated maintenance-free. Thanks to their low internal resistance, AGM batteries respond quickly to high current demands—ideal for cold environments or high-load applications.

AGM batteries can be charged with standard chargers; specialised equipment is not required.

Alkaline manganese batteries are among the most important non-rechargeable power sources. Compared with zinc-carbon cells, they offer higher energy density, better load capability, and longer shelf life. Typical formats include AA cells and button cells.

The ampere (A) is the international unit of electric current. It defines how much electric charge flows through a conductor per second.

The ampere-hour (Ah) is a common unit used to specify the amount of electrical charge a battery can deliver. It is the product of current (amperes) and the time (hours) over which the current flows. The higher the ampere-hour rating, the longer a device can typically be operated.

The anode is the electrode at which oxidation occurs, i.e., electrons are released. In electrochemical cells, this means negatively charged ions (anions) give up electrons. In rechargeable batteries (secondary cells), the anode’s function can reverse depending on whether the cell is charging or discharging.

A battery is an electrochemical energy storage device that provides electrical energy through controlled chemical reactions. It consists of multiple cells (galvanic elements), each containing two electrodes—anode and cathode—and an electrolyte. The battery’s voltage and capacity depend largely on the materials used and the cell design.

During discharge, chemically stored energy is converted into electrical energy. Key selection criteria include discharge duration, nominal voltage, efficiency, as well as autonomy and charging parameters.

Battery capacity indicates how much energy can be extracted under defined conditions. It is measured in ampere-hours (Ah) and depends on temperature, discharge time, and other influencing factors.

Battery lifetime depends on several factors. In particular, maintenance must be performed in accordance with the operating instructions to achieve maximum service life. Key influencing factors include charging, ambient temperature, charge cycles, and depth of discharge.

A battery pack consists of multiple interconnected individual cells housed in a common enclosure. This design protects the cells from external influences and enables straightforward replacement via standardised connectors.

For reliable operation, all cells within the pack should be of the same type and have identical electrical characteristics (voltage, capacity, permissible load). Combinations of parallel and series-connected cell groups are common.

A dedicated room for installing larger battery systems. Ventilation and safety requirements are governed by DIN VDE 0510, Part 2.

The period during which a battery can still perform its function reliably despite natural ageing. Also referred to as the nominal service life.

A basic distinction is made between vented, sealed, and valve-regulated batteries. The design influences maintenance requirements, safety, and typical applications.

The Battery Connection Unit (BCU) is a switchgear cabinet module used to integrate battery systems into DC networks. It serves as the central interface for connecting multiple battery strings and typically includes the DC main disconnect switch.

Capacity indicates the available storage or deliverable output of a battery or cell (measured in ampere-hours). Battery temperature and discharge current also influence capacity determination and must be specified (e.g., cold cranking capacity in seconds at cold test current and -18 °C).

The negative electrode in liquid solution at which reduction processes occur, i.e., cations (positively charged ions) are deposited (electron gain). In secondary cells, either electrode can act as the cathode depending on the direction of current flow. During discharge, the positive electrode is the cathode.

A cell is the smallest unit of a battery, consisting of positive and negative electrodes, a separator, and an electrolyte. It forms the basic building block of a battery. The cell stores electrical energy, and its size is determined by its capacity.

The charge factor describes the ratio between the capacity removed and the capacity that must be recharged. Due to efficiency losses, lead-acid batteries typically require approx. 20% more charge input; for NiCd batteries the value is approx. 40%.

The following characteristics are distinguished for battery charging:

• I characteristic
• IU characteristic
• U characteristic
• W characteristic
• Wa characteristic

Depending on the battery type, one of the above charging characteristics must be used. The applicable method is specified in the battery manufacturer’s operating instructions.

I characteristic:

Charging with constant current. The charging voltage can rise uncontrollably, which may lead to a significant temperature increase in the battery. The electrolyte could boil, resulting in water loss. In addition, oxyhydrogen gas may form due to the chemical reaction, creating an explosion hazard in the surrounding area. This charging method is only suitable for very small charging outputs in the milliampere range.

IU characteristic:

When the battery is discharged, charging starts with a constant current (I characteristic). The charger is set to the float voltage. As the state of charge increases, the charging current gradually decreases and the voltage rises to the set float voltage (for lead-acid batteries typically 2.26–2.29 V per cell). The charger has then switched to the U characteristic.

U characteristic:

Charging with constant voltage. When the boost charge voltage of 2.4 V per cell is reached, the current is automatically reduced to prevent excessive temperature rise in the battery. Excessive battery temperature can destroy the battery.

W characteristic:

Charging according to a resistor characteristic (W). As the charging voltage increases, the charging current is reduced. As with the U characteristic, temperature rise due to excessive current at high charging voltage must be prevented.

Wa characteristic:

The charging behaviour corresponds to the W characteristic; however, the charging process is switched off after a defined time.

These batteries allow water top-up via dedicated openings and are considered low-maintenance. They are also commonly referred to as “vented batteries”.

When connecting multiple battery strings in parallel, equal cable lengths must be ensured to achieve uniform voltage distribution.

Manufacturers’ constant-current tables indicate the maximum end-of-discharge voltage at which the battery can supply a constant current per cell (or per block) as a function of time.

A cycle is a regularly recurring process of battery discharge followed by battery recharge.

Cycle life indicates how many times a rechargeable battery can be charged and discharged before reaching the end of its service life.

A cylindrical cell has a round, cylindrical form factor, in contrast to a prismatic cell with a rectangular form factor.

Deep discharge refers to the condition of a battery after complete discharge at a low current, caused by falling below the end-of-discharge voltage. Deep-discharged AGM fleece batteries must be recharged within 12 hours after deep discharge; otherwise, the battery will be irreparably damaged. AGM gel batteries must be recharged within 5 to 7 days.

Diffusion describes the physical process in which two or more substances mix completely.

Direct current (DC) is an electric current whose magnitude and direction do not change over time (e.g., current supplied by an electrochemical power source).

Efficiency describes the ratio of discharged capacity to charged capacity and is approx. 0.85 for lead-acid batteries.

Consisting of the active material and the current collector, this conductive structure is where electrochemical reactions take place within the cell.

An electrolyte is a chemical compound that dissociates into ions in the solid, liquid, or dissolved state and moves directionally under the influence of an electric field.

Electrons are negatively charged elementary particles.

A backup/auxiliary power source that is continuously maintained on float charge.

When a battery is fully charged, only the float current flows to counteract self-discharge. The charging voltage is set between 2.26 V per cell and 2.29 V per cell, depending on the battery type.

The voltage measured at the battery terminals at the end of a discharge. This value is specified by the manufacturer depending on the battery type. Falling below the end-of-discharge voltage can damage the battery permanently. The end-of-discharge voltage also depends on the applied load: with very low loads, higher cut-off voltages are typically required to avoid battery damage.

Describes the work expressed in watt-hours that results from voltage and capacity. The more energy a battery can store and deliver, the more work it can perform.

Refers to factors that can significantly reduce battery service life if they are not maintained in accordance with manufacturer specifications. Typical factors include vibration, shock, ambient temperature, humidity, installation altitude, and similar conditions.

For certain vented battery types, periodic equalisation charging is required. This process helps restore capacity reduced by natural losses. The exact charging parameters (current and voltage) are defined by the manufacturer and are typically applied every 6 to 9 months.

Key numbers for standard lead (starter) batteries (accumulators).

There is a “European Type Number” used today (almost) everywhere as the successor to the former DIN type series. Its numeric code contains all key data (voltage, capacity, case group, and cold cranking current). The ETN number consists of three groups of three digits each, with the following meaning:

ETN 536 046 030;

• The 1st group “5 3 6” = Group A:
  • The 1st digit indicates the voltage; digits 1–4 are reserved for 6 V batteries
    • 5 for 12 V batteries under 100 Ah
    • 6 for 12 V batteries over 100 Ah
    • 7 for 12 V batteries over 200 Ah
    • 8 for special starter batteries
    • 9 for traction, lighting and solar batteries
• The 2nd and 3rd digits indicate the battery capacity in Ah; therefore:
  • 5 3 6 = 12 V battery with a capacity of 36 Ah (under 100 Ah)
  • 6 3 6 therefore indicates a 12 V battery with 136 Ah (over 100 Ah)
  • 7 3 6 corresponds to a 12 V battery with 236 Ah (over 200 Ah)
  • 7 for 12 V batteries over 200 Ah
  • 8 for special starter batteries
  • 9 for traction, lighting and solar batteries
• The 2nd group “0 4 6” = Group B:
  This group provides information on dimensions and special features (base shape, cover type, floor ledges, etc.).
• The 3rd group “0 3 0” = Group C:
  These three digits specify the cold test current × 10; example: 030 × 10 = 300 A, 115 = 1150 A.

The previous DIN type numbers, e.g. 5 36 46, can be applied analogously to the above explanations; however, they do not include the statement regarding cold test current. In everyday practice, customers typically ask simply for a “12 V – 36 Ah battery” or “12 V – 88 Ah battery” (or similar). In such cases, it is generally sufficient to know that these are standard accumulators (batteries) that can be supplied from normal commercial ranges.

EUROBAT is an association that represents the interests of specialised European battery industries. With 34 members within the EU covering more than 85% of the battery industry in Europe, EUROBAT works on the development of new battery solutions and new forms of renewable energy storage. It also issues recommendations on definitions of technical battery parameters, such as service life.

The voltage required to keep batteries fully charged is referred to as float charging. Standard values at 20 °C are typically: lead-acid batteries 2.23–2.27 V per cell (±1%); NiCd batteries 1.40 V per cell. The manufacturer’s specifications must be followed. If the ambient temperature at the installation site deviates continuously or predominantly from the standard value, these parameters should be adjusted in accordance with the manufacturer’s guidance to optimise service life.

The float voltage is typically set between 2.26 V per cell and 2.29 V per cell, depending on the battery type. This constant voltage compensates for the battery’s self-discharge.

Conventional “flooded” consumer batteries can provide power as reliably as AGM batteries. However, they require maintenance: regular checks of the electrolyte level and, if necessary, topping up with distilled water are essential to achieve a long service life.

Battery overcharging can cause gassing, meaning hydrogen is released from the battery and an explosion hazard may occur.

The voltage at which a battery’s electrolyte transitions into a gaseous state and escapes. The gassing voltage should not be maintained for extended periods, as significant electrolyte loss may result and battery damage cannot be ruled out. Typical gassing voltages are: 2.4 V per cell for lead-acid batteries and 1.55 V per cell for nickel-cadmium batteries.

For various electrical loads, frequent discharge cycles, and reliable starting performance in all weather conditions—for example in motorcycles—a gel battery is often required. Riding on unpaved ground creates strong vibrations. Sporty cornering, installation in different positions, seasonal operation with extended standstill periods, and irregular recharging: all of these conditions require a battery using GEL technology.

The latest GEL technology provides exceptional vibration resistance through an electrolyte bound in gel form. The electrodes are embedded in a multi-component gel, and all liquid acid is immobilised within the gel. This ensures excellent cycle stability across a wide range of discharge scenarios and provides superior leak protection. An internal gas recombination process prevents gas release and enables the battery to be sealed. A pressure-regulated valve ensures a high level of safety even in the event of overcharge.

GroE stands for “large-surface electrode”. GroE batteries are closed, stationary lead-acid batteries with liquid electrolyte (diluted sulphuric acid). They are characterised by a design with fully cast plates and a lamellar structure. The use of pure-lead technology, high electrode thickness, and a low acid density of 1.22 kg/l results in an excellent expected service life of at least 20 years.

GroE batteries are manufactured in a capacity range from 75 Ah up to 2600 Ah. The positive electrode is designed as a large-surface plate. The electrolyte is present in liquid form and must be checked during the service life.

The cycle count exceeds 200. GroE batteries are known for maximum reliability and exceptional operational safety. A high voltage level under high-rate discharge and a largely consistent electrical performance over the service life are additional characteristics of this battery type.

GroE technology has been used for more than 100 years and is therefore among the most mature and safest battery systems available.

This foundation regulates the nationwide, compliant disposal of used portable batteries in Germany. Whether commercial end users or public waste management authorities, GRS services include collection, sorting, and recycling of used batteries.

Charging at a current rate of 1 C or higher.

Discharge at a current rate above 5 C.

Rechargeable batteries intended exclusively for industrial, commercial, and agricultural use (e.g., for forklifts and hybrid vehicles).

The ohmic resistance of a battery.

The I/U characteristic is a gentle charging method for batteries. Charging starts with a constant current until the float voltage is reached. The battery is then charged further at constant voltage.

A rechargeable battery using lead plates as electrodes and diluted sulphuric acid as the electrolyte.

Formed during the discharge of lead-acid batteries at the electrodes through the reaction of lead or lead dioxide with sulphuric acid.

The battery cell contains lithium ions in both the positive and negative electrodes as well as in the electrolyte.

A battery in which the electrolyte is immobilised in a gel or in a micro glass fleece (AGM), and no distilled water needs to be topped up. The battery is sealed.

A battery in which the electrolyte is immobilised in a gel or in a micro glass fleece (AGM), and no distilled water needs to be topped up. The battery is sealed.

The memory effect describes a loss of usable capacity caused by the battery “remembering” the most recent discharge demand rather than the actual energy requirement. If cell voltage drops below the minimum requirement, the battery can no longer be used, even though it may still be able to deliver electrical energy.

In nickel-cadmium (NiCd) batteries, one electrode is made of nickel and the other of cadmium. These batteries are significantly more robust against deep discharge and overcharge. The cells offer high cycle stability and are largely insensitive to temperature. NiCd batteries are largely prohibited and, under the applicable battery legislation, may only be used in bicycles, emergency lighting, alarm systems, and medical devices.

Nickel-metal hydride (NiMH) batteries use a positive electrode made of nickel hydroxide and a negative electrode made of metal hydride. Compared with NiCd batteries at the same voltage, NiMH batteries offer approximately twice the energy density.

This describes capacity determined under standardised, characteristic conditions (e.g., 20-hour capacity in Ah, reserve capacity at end-of-discharge with 25 A in minutes).

The OGi battery is a low-maintenance, closed lead-acid battery with liquid electrolyte. The design life is up to 15 years at an ambient temperature of 20 °C. The float voltage is 2.23–2.30 V per cell at 20–25 °C. The permissible ambient temperature range is -20 °C to +50 °C. These batteries are manufactured with durable positive grid plates, fine-grained pasted negative grid plates, microporous separators, and housings made of robust, flame-retardant, transparent SAN material.

These batteries are produced either as multi-cell blocks or as individual cells. Depending on the version, sizes from 25 Ah to 900 Ah are available. A special round-grid design helps reduce internal resistance, enabling the OGi battery to deliver very high current within a very short time. The cycle life for individual charge/discharge cycles is > 1000.

OPzS batteries are closed lead-acid batteries with liquid electrolyte (diluted sulphuric acid). Their cell design with positive tubular plates provides an exceptionally long service life of up to 1,500 cycles at 80% depth of discharge. This makes tubular-plate batteries well suited for applications with high charge and discharge demands, such as solar systems, or for long autonomy times in IT/telecommunications and emergency lighting. Typical service life is around 20 years.

Applications:

• IT and telecommunications
• Emergency lighting
• Solar and UPS
• Central battery systems (CPS) and BEV technology
• Wind turbines

Key benefits:

• High charge and discharge capability
• High cycle stability and long service life
• Maximum reliability
• Well suited for very long autonomy times

OPzV batteries are low-maintenance, sealed individual cells, primarily manufactured in plastic containers. The positive electrode is designed as a tubular plate. Smaller capacities up to approx. 300 Ah are also available as 12 V compact batteries. Typical service life is more than 15 years. These batteries are manufactured up to a capacity of 3,200 Ah. The electrolyte is present in gel form and does not need to be checked during the service life. They are preferably used where autonomy times of more than half an hour are required. In addition to long service life in float operation, the OPzV series also offers cycle stability nearly comparable to OPzS batteries.

OPzV tubular-plate batteries are therefore well suited for applications with high charge and discharge demands, such as solar systems, or for long autonomy times in IT/telecommunications, emergency lighting, UPS, BEV, and wind energy systems.

Benefits:

• High-rate capability
• Horizontal installation possible
• High cycle stability and long service life
• Minimal maintenance requirements
• Maximum reliability

Battery overcharge occurs when the charging voltage is set too high. During charging, lead sulphate is first converted back into lead and lead dioxide; however, if charging current continues to flow, the lead grid is also attacked. The grid expands and the strength of the pressed active materials decreases.

Oxyhydrogen gas can form as a result of battery overcharging. A chemical reaction produces a mixture of hydrogen and oxygen from the electrolyte, which is highly explosive.

Connection of multiple battery blocks or battery strings to increase capacity. See also “Connecting Battery Strings in Parallel”.

This term describes the relative charge and voltage relationship (opposition) between two electrodes.

Positive connection terminal on a cell or battery.

A primary cell is a non-rechargeable cell.

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

A rechargeable battery (also referred to as an accumulator) is a reusable energy storage device that stores electrical energy in chemical form and converts it back into electrical energy when required. It is based on electrochemical reactions and is widely used in mobile and stationary applications.

The capacity that can be extracted when discharging at nominal current from a battery with an undefined state of charge (e.g., after extended storage).

Residual charge refers to a full charge from an undefined state of charge.

Maintenance-free batteries are sealed and feature near-complete gas recombination. No water top-up is required throughout their service life.

Rechargeable battery.

Describes the battery’s internal discharge process that is independent of an external load. Self-discharge is a continuous, temperature-dependent chemical reaction process.

A separator provides insulation between the positive and negative electrodes (cathode and anode).

The negative terminal of one battery is connected to the positive terminal of the next cell. The total voltage doubles while capacity remains constant.

When a lead-acid battery is deeply discharged, recrystallisation can cause coarse-grained lead sulphate to form on the electrodes. This reduces the reactivity of the electrode surface and can lead to short circuits if the battery is subjected to vibration.

The terminal type specifies the design of the battery connection.

A traction battery is a rechargeable battery designed to supply energy to electric motors over an extended period of time..

The abbreviation UPS stands for Uninterruptible Power Supply. These systems are used as an emergency power solution in the event of disturbances in the power grid to ensure uninterrupted energy supply.

The unit of electrical voltage is abbreviated as V. The unit was named in 1897 after the Italian physicist and physician Count Alessandro Volta.

In lead-acid batteries, voltage sag refers to the initial drop in voltage at the start of discharge. This effect strongly depends on the level of current drawn. For high-rate discharges, high-rate capable batteries should therefore be used, in particular so-called HR types.