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Glossary · Energy industry & energy transition

Depth of Discharge

Depth of discharge DoD Battery storage state of charge cycle ageing service life

Depth of Discharge, or DoD for short, is the English term for the discharge depth of a battery (German: Entladetiefe). It indicates what share of the capacity was withdrawn during a discharge. An example: a battery storage system is discharged from 100 percent to 40 percent state of charge. The depth of discharge is then 60 percent. DoD is stated as a percentage. It describes neither the power of a storage system nor its absolute size, but only the used share of the storable amount of energy or charge.

What matters is what the 100 percent refers to. In data sheets for home and large-scale storage systems, this usually means the usable capacity released by the battery management system. In cell tests and standards, by contrast, DoD often refers to the nominal or rated capacity. Many battery systems have more physical cell capacity than they release during operation. The management system sets upper and lower limits so that the cells are neither overcharged nor discharged too deeply. When a manufacturer speaks of 100 percent DoD, it therefore usually means the fully used released operating window. It does not mean discharge down to the chemical limit of the cell. When comparing figures, you should therefore always check which reference value is used.

The obvious counterpart is the state of charge. The state of charge describes how full a battery is at a given point in time. The depth of discharge describes how far it was emptied during a discharge. In simplified terms: if the battery starts fully charged, a state of charge of 30 percent corresponds to a depth of discharge of 70 percent. In real operation the relationship is less rigid. Storage systems are rarely cycled from full to empty, but constantly move between different states of charge. A discharge from 80 to 30 percent and one from 60 to 10 percent have the same depth of discharge of 50 percent. However, they lie in different ranges of the state of charge, and this affects ageing.

Distinction from capacity, power and cycle count

DoD is often confused with capacity. Capacity describes how much a storage system can absorb or deliver. For cells it is often stated as an amount of charge in ampere-hours, for storage systems usually as energy content in kilowatt-hours or megawatt-hours. The depth of discharge, by contrast, describes what share of this is used in a single discharge. If a storage system with 10 kilowatt-hours of usable capacity is discharged by 5 kilowatt-hours, the depth of discharge is 50 percent.

This must be distinguished from power. It indicates how quickly energy is absorbed or delivered, for example in kilowatts or megawatts. Two storage systems can reach the same depth of discharge and still work at very different speeds. A home storage system is often discharged over several hours. A battery storage system for balancing power, on the other hand, must be able to react within seconds. The depth of discharge therefore says nothing about the speed of discharge. For ageing, however, the current relative to capacity, the so-called C-rate, also matters. A deep discharge at a high C-rate stresses the cells differently than the same depth of discharge at a low C-rate.

The cycle count does not replace DoD either. A cycle is often understood as a complete discharge and recharge. In operation, however, partial cycles predominate. These can be combined arithmetically into equivalent full cycles. Two discharges of 50 percent DoD each amount, in energy terms, to one equivalent full cycle. For ageing, they can nevertheless be more favourable than a single discharge of 100 percent DoD. For irregular state-of-charge profiles, ageing models often decompose the time series using a counting method from material fatigue, rainflow counting. Each partial cycle is assigned its own depth and its own mean state of charge. How strong the effect of all this is depends on the cell chemistry, the temperature, the state-of-charge window used, the charge and discharge rates, and the operating strategy.

Why depth of discharge matters for storage operation

The depth of discharge influences the lifetime of a battery. In many cell types, regular deep discharges accelerate cycle ageing. This also applies if a battery remains at extreme states of charge for a long time. Ageing does not primarily mean a sudden failure. Typically, the usable capacity decreases and the internal resistance increases. As a result, efficiency deteriorates and the available power may decrease. This condition is often expressed as state of health. The NREL lifetime models for lithium-ion batteries explicitly describe degradation as a function of temperature, state of charge, depth of discharge, and charge and discharge rates. How strongly DoD acts differs depending on the cell chemistry. For lithium iron phosphate cells (LFP), the effect is often less pronounced than for cells with nickel-rich cathodes. Blanket rules of thumb can therefore not be transferred from one chemistry to another.

The battery management system therefore monitors cell voltages, temperatures, currents and states of charge. It determines which range of the battery may be used in operation. This operating window protects the cells and is at the same time an economic decision. A narrower window reduces the usable capacity but can extend the lifetime. A wider window provides more energy in the short term but can accelerate ageing. For home storage systems, electric vehicles and large-scale storage systems, this trade-off turns out differently because usage profile, warranty conditions and cost structure differ.

Depending on the application, very different depths of discharge arise. A storage system that shifts electricity from photovoltaics from midday to the evening often runs a regular, fairly deep daily cycle. A storage system that provides balancing power for frequency maintenance, on the other hand, performs many small charge and discharge movements. A storage system that limits load peaks may be used rarely, but is then discharged deeply. For an economic assessment, capacity and power are therefore not sufficient. A large storage system is of little benefit if its use has to be heavily restricted because of ageing. A smaller storage system, on the other hand, can be worthwhile if it allows many shallow cycles with low degradation. Indicators such as the levelized cost of storage make this relationship visible. They distribute the costs over the energy that a storage system actually delivers over its lifetime.

Depth of discharge in data sheets, warranties and EU law

Manufacturer specifications can only be compared if the test conditions are known. Warranties often state a number of cycles, a guaranteed residual capacity after a certain time, or a maximum energy throughput. A warranty of 6,000 cycles at 80 percent DoD must be assessed differently from the same number of cycles at 50 percent DoD. In the first case, the storage system arithmetically delivers significantly more energy. Without information on depth of discharge, temperature and C-rate, a cycle count therefore says little about the amount of energy actually deliverable.

In the EU, the Batteries Regulation (EU) 2023/1542 governs such information. Since 18 August 2024, rechargeable industrial batteries with a capacity of more than 2 kWh, batteries for light means of transport and batteries of electric vehicles must be accompanied by a document. It contains values on electrochemical performance and durability according to Annex IV Part A, including rated capacity, capacity fade and expected lifetime in cycles and calendar years. Annex IV Part B requires an explanation of how these values are determined. This expressly includes the depth of discharge in the test of cycle life. For stationary industrial batteries above 2 kWh, the regulation additionally provides for minimum values. The Commission sets these in a delegated act. They are to apply at the earliest from 18 August 2027 or 18 months after the entry into force of this act, whichever is later. Batteries with exclusively external storage are exempt.

Typical misunderstandings

  • 100 percent DoD does not mean that the battery is chemically completely empty. Usually the window released by the manufacturer is meant. Below it lie protective reserves that are not available to the operator. Especially with lithium-ion cells, deep discharge below the permissible voltage can permanently damage the cells and increase safety risks.

  • A high depth of discharge is not automatically wrong. It can make sense if an application requires rare but energy-intensive discharges or if the revenue exceeds the additional ageing. It becomes problematic when operating models do not reflect the ageing costs. Then a storage system appears more capable in the short term, while part of the costs is shifted into the future.

  • Shallow cycles are not free either. They are gentler on many cell types than deep cycles, but still contribute to ageing. In addition there are calendar ageing, temperature stress and long phases at a high state of charge. A battery therefore also ages when it is little used. DoD explains the stress from cycles, but not the entire degradation.

Connection with the electricity market and grid operation

In electricity trading, energy is shifted in time: charge cheaply, discharge at a higher price. The larger and more frequent the price differences, the more intensively a storage system can be used. The depth of discharge per cycle helps determine the energy throughput and the ageing. An operator therefore must not only check whether there is a price difference. They must also check whether it is large enough to cover conversion losses, grid charges, taxes, levies, operating costs and ageing. Optimisation models for storage dispatch therefore often treat ageing as costs that depend on DoD.

For ancillary services, the picture can look different. In some applications, what matters above all is that power is available quickly, while only little energy is moved. Many small changes in the state of charge with a low depth of discharge then occur. To bridge longer phases with little wind and solar power, on the other hand, a lot of energy would be needed. Technically, batteries can also take on such tasks. With long discharge durations, however, they reach limits in terms of costs, required capacity and ageing.

In grid operation, DoD plays a role when storage systems shift loads or relieve local grid bottlenecks. A storage system can, for example, absorb a high feed-in peak from photovoltaics and release it again later. How deeply it may be discharged determines whether it can absorb enough energy again the next day. Storage operation therefore requires forward-looking dispatch: a full storage system cannot absorb further energy, an empty storage system cannot cover a load. The current state of charge and the possible depth of discharge for the next application are therefore directly linked.

Depth of discharge thus describes how a battery storage system is actually used: as the withdrawn share of the released or nominal capacity. It must be clearly distinguished from power, installed capacity, state of charge and cycle count. It is important for the power system because the depth of discharge has consequences for ageing, warranty and costs. Megawatt-hours alone do not adequately describe a battery. Only the interplay of state of charge, depth of discharge, power, operating window, temperature and application shows which tasks it can reliably take on in the power system.

Sources

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Edited by eCMS on 11.11.2026 18:54

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.