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Glossary · Energy industry & energy transition

Depth of Discharge (DoD)

Depth of discharge depth of discharge Battery storage storage Battery management system cycle stability power system

Depth of discharge, DoD for short, is the discharge depth of a battery storage system. The value indicates what share of a defined capacity is withdrawn during a discharge. A DoD of 80 percent therefore means that 80 percent of the capacity under consideration is used. The reference point is always important. Some specifications refer to the nominal gross or rated capacity, others to the usable net capacity released by the manufacturer. Without this reference, a DoD figure can hardly be compared.

Discharge depth is stated in percent. Behind it is a quantity of energy, usually in kilowatt-hours. An example: a storage system with 10 kWh of usable capacity is discharged by 6 kWh. Relative to the net capacity, that is 60 percent DoD. If the same storage system has a gross capacity of 12 kWh, the same 6 kWh corresponds to only 50 percent DoD. For data sheets, warranties and economic viability calculations, this difference matters, because gross capacity, net capacity and guaranteed residual capacity each describe something different.

Relationship to state of charge

DoD is closely related to the state of charge (SoC), but it is not the same thing. The SoC describes the current charge level. The DoD describes how far a discharge extends. In ageing research, the discharge depth of a cycle is usually defined as the difference between the highest and the lowest state of charge: DoD = SoC max minus SoC min. The frequently cited rule of thumb that DoD and SoC add up to 100 percent therefore applies only if the discharge starts from full charge. A cycle from 100 to 60 percent and a cycle from 80 to 40 percent both have a discharge depth of 40 percent. At the end of the second cycle, however, the SoC is 40 and not 60 percent. For ageing, in addition to the depth, the position of this window also plays a role, that is, the average state of charge.

Ageing also affects the DoD. If the capacity actually available decreases, the same amount of energy withdrawn corresponds to a greater discharge depth, measured against the remaining capacity. If, on the other hand, an operator keeps to fixed SoC limits, each cycle delivers less energy as the battery gets older. The state of ageing is described by the state of health.

Gross capacity, usable capacity and protective reserve

Batteries are rarely used across their entire physically possible capacity. The battery management system sets limits for charging and discharging so that the cells do not enter harmful states. Excessive discharge is not only a question of service life. In lithium-ion cells, deep discharge can also endanger safety. The EU Batteries Regulation describes possible consequences such as polarity reversal and dendrite formation up to short circuit. A storage system can therefore have 100 kWh gross and still release only 90 kWh net. The remainder is not a reserve that the operator can freely dispose of. It is part of the technical protection concept.

A manufacturer can therefore advertise 100 percent DoD even though the cells are never physically discharged completely. What is usually meant is that the released net capacity may be used in full. For operation, this specification is meaningful. For comparing different storage systems, it is useful only if it is clear what it refers to. Two systems with the same nominal capacity can deliver different amounts of energy in everyday use if their protective reserves or their permissible discharge depth differ. In addition, the energy that can actually be withdrawn depends on temperature, discharge power and state of ageing.

Discharge depth and ageing

Discharge depth is one of several factors influencing ageing. Lifetime models for lithium-ion batteries from the US research laboratory NREL describe ageing as a function of temperature, state of charge, discharge depth, and charge and discharge rates. Test series on NMC cells show ageing patterns that depend on temperature, charge rate, DoD and SoC. In addition to cycle ageing, that is, wear caused by charging and discharging processes, there is calendar ageing. It continues even at standstill and is usually stronger at a high state of charge and high temperatures. How strongly the DoD acts depends on the cell chemistry. Iron phosphate, NMC and lead cells behave quite differently in this respect.

For service life, therefore, not only the number of cycles counts, but also their depth. Two discharges of 50 percent DoD each arithmetically amount to one equivalent full cycle. For the cells, however, shallow cycles are often less stressful than deep ones, even if the same amount of energy is turned over. A model example from research shows how large this effect can be: there, halving the discharge depth from 100 to 50 percent extended the service life to roughly two to four times, depending on the failure criterion. Such values apply only to the respective cell and the respective model. They cannot be generalised. Experts therefore often break down irregular load profiles into individual partial cycles of different depth using counting methods such as rainflow.

This leads to a common misinterpretation. A high permissible discharge depth does not automatically make a storage system more economical. It increases the available amount of energy, but can accelerate wear if operation regularly pushes this limit. Conversely, a lower permissible discharge depth is not a sign of poor technology. Often it is a deliberate limit to safeguard service life and warranty. Warranties therefore frequently combine a calendar duration, a maximum energy throughput, operating limits and an assured residual capacity.

Legal framework in the EU

In the EU, the Batteries Regulation (EU) 2023/1542 governs the information on service life. Since 18 August 2024, rechargeable industrial batteries with a capacity of more than 2 kWh, LMT batteries and batteries for electric vehicles must be accompanied by documentation with values on electrochemical performance and durability in accordance with Annex IV Part A. This includes the service life in cycles. To explain these measurements, Annex IV Part B expressly requires information on the depth of discharge in the cycle life test, on the charge and discharge rate, and on the power-to-energy ratio. Binding minimum values for rechargeable industrial batteries above 2 kWh, excluding those with exclusively external storage, are not envisaged until 18 August 2027 or 18 months after the entry into force of the associated delegated act, whichever is later.

Distinction from related parameters

  • Efficiency: The DoD describes the depth of discharge, not the losses. The round-trip efficiency indicates what share of the stored energy can be withdrawn again. Both parameters belong in the same operating calculation, but they do not replace one another.

  • C-rate: The C-rate describes how quickly a battery is charged or discharged relative to its capacity. At 1C, the capacity under consideration would arithmetically be fully discharged after one hour. A high C-rate can additionally stress the cells even if the discharge depth remains moderate.

  • Cycle life: The cycle life indicates how many cycles a battery achieves under defined conditions before its capacity falls below a certain share of the rated capacity. Without specifying DoD, temperature, charge and discharge rate and the end-of-life criterion, this figure is hardly meaningful. Very high cycle numbers often apply only to shallow cycles.

Significance for storage operation and the power system

Power in kilowatts determines how much electricity a storage system can absorb or deliver at a given moment. Capacity in kilowatt-hours determines approximately how long it can sustain that. The DoD determines which part of this capacity is actually used in operation. For home storage systems connected to photovoltaic installations, it helps decide how much solar power can be shifted from the day into the evening. For large-scale batteries, it affects arbitrage, balancing energy and other forms of flexibility. For balancing energy, operators often deliberately keep an SoC window free so that they can provide both positive and negative power.

Anyone operating a storage system in the market must therefore include, in addition to the price difference between charging and discharging, the wear costs per kilowatt-hour turned over. A deep discharge pays off if the revenue is high enough. With small price differences, the same discharge can be uneconomical because it consumes part of the limited service life.

In system analyses, too, a storage system must not appear merely as installed gross capacity. A battery with high power and small usable capacity can support grid frequency in the short term, but cannot bridge a multi-day Dunkelflaute (prolonged period of low wind and solar output). Models that do not represent usable capacity, power limits, efficiency and ageing easily overestimate the contribution of storage systems to security of supply.

Misunderstandings in economic viability calculations and tenders

In investment calculations, the DoD is sometimes treated as a simple adjustment lever: the higher the discharge depth, the more usable kilowatt-hours, the better the storage system. What is overlooked is that each additional kilowatt-hour in a deeper window can cause different ageing costs. The economic value of a storage system results from revenues, electricity procurement costs, losses, operating and capital costs, and wear. If support programmes, tenders or technical requirements state only a storage capacity without specifying gross or net, bids can hardly be compared. A warranty on residual capacity is also worth less than it appears if the permitted energy throughput or the DoD window is heavily restricted. For operators, what ultimately counts is the usable energy over the entire service life.

The DoD is not a market role and not a grid function, but a technical operating parameter with economic consequences. Used precisely, it links capacity, service life, operating strategy and revenue model. Used imprecisely, it makes storage systems appear larger, more flexible or longer-lived than they are in actual use.

Sources

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Edited by eCMS on 11.11.2026 18:55

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