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Glossary · Energy industry & energy transition

Depth of Discharge (DoD)

depth of discharge Depth of discharge DoD Battery state of charge cycle stability Battery storage Battery management

Depth of discharge, or DoD, indicates what share of a reference capacity is drawn from a battery during a discharge. It is usually stated as a percentage. An example: a battery storage system has 10 kilowatt-hours of usable capacity. If 8 kilowatt-hours are drawn from it, the depth of discharge relative to this capacity is 80 percent. The figure is only unambiguous if it is clear what it refers to: the nominal or rated capacity, the usable capacity approved by the manufacturer, or the current capacity reduced by ageing.

Two common definitions

The closest related term is the state of charge (SoC). It describes the fill level of a battery at a given point in time. For depth of discharge, there are two readings in engineering and the literature that should be kept apart. In the first, depth of discharge is the counterpart of the state of charge, measured from a full battery: at a state of charge of 30 percent, the depth of discharge is 70 percent. In the second reading, which is common in ageing studies, depth of discharge is the swing of a cycle, i.e. the difference between the highest and the lowest state of charge. If a battery is discharged from 90 to 30 percent, the depth of discharge under this definition is 60 percentage points, whereas under the first reading it is 70 percent at the end. The state of charge therefore describes a state. Depth of discharge in the sense of a cycle swing describes a change.

Whether calculations are made in ampere-hours or kilowatt-hours also makes a difference. In cell technology, capacity is usually stated as an amount of charge in ampere-hours, whereas for storage systems it is usually stated as energy in kilowatt-hours. Because cell voltage varies with the state of charge, the two percentage values are not always exactly the same. Depth of discharge also says nothing about power. A storage system with high power and small storage capacity can be deeply discharged very quickly without having delivered much energy. For system design, power, usable capacity, discharge duration and depth of discharge must therefore be considered separately.

Gross, net and the role of the battery management system

Many systems do not make the entire electrochemically available cell capacity accessible. The battery management system releases only a usable window and thus keeps away very high and very low states of charge, which can increase ageing and safety risks. A displayed “100 percent depth of discharge” or “0 percent state of charge” then usually only means that this usable window has been exhausted. The cells are not completely empty. A protective reserve remains below the window. Data sheets that do not clearly distinguish between gross and net capacity can therefore only be compared to a limited extent.

The battery management system monitors voltages, currents and temperatures at cell level. Cells do not age completely uniformly. If a weaker cell reaches its lower voltage limit earlier, the system must end the discharge even though the storage system as a whole does not yet appear empty. Cell balancing partially compensates for such differences. The displayed depth of discharge is therefore always also the result of measurement, state estimation and protection logic. The EU Battery Regulation requires, for stationary battery storage systems and further battery categories, that the battery management system record, among other things, deep discharge events and the number of equivalent full cycles.

Influence on ageing

Batteries age at rest (calendar ageing) and through use (cycle ageing). How much a cycle stresses a lithium-ion cell depends, among other things, on temperature, average state of charge, charge and discharge current (C-rate) and depth of discharge. Measurement series and lifetime models from the US National Renewable Energy Laboratory (NREL) explicitly represent ageing as a function of these variables. For many cell chemistries, the general tendency is that frequent shallow cycles within a medium state-of-charge range tend to be gentler on the cell than repeated cycles over almost the entire range. How strong this effect is differs depending on the chemistry, for example between LFP and NMC, and depending on the cell manufacturer. Generic cycle counts per depth of discharge are therefore reliable only as a manufacturer specification for a particular cell under particular test conditions.

A widespread simplification needs to be corrected here. Two discharges of 50 percent depth of discharge each add up to one equivalent full cycle in terms of energy throughput. For ageing, however, they are not automatically equivalent to one cycle over 100 percent. In many empirical ageing models, the stress per cycle increases disproportionately with depth of discharge. Two half cycles then cause less damage than one full cycle. Counting in full-cycle equivalents therefore measures energy turnover. It does not replace an ageing model.

With ageing, the depth of discharge also shifts by itself. If the available capacity decreases (see State of Health), the battery must be discharged more deeply for the same amount of energy. If, on the other hand, operation management adheres to fixed state-of-charge limits, each cycle delivers less energy as the battery gets older.

Distinction from efficiency and cycle count

Depth of discharge and efficiency are different quantities. Efficiency, usually stated for storage systems as round-trip efficiency, describes what share of the stored energy is released again. Depth of discharge, by contrast, describes how far the storage system is discharged. A storage system can have either a high or a low efficiency at 80 percent depth of discharge. The cycle count, in turn, indicates how often charging and discharging take place. Depth of discharge describes the extent of these cycles.

Warranties, data sheets and law

Manufacturers usually state service life as a combination of calendar years, cycle count or energy throughput, and a minimum residual capacity, often at a particular depth of discharge. Anyone who compares only cycle counts can therefore be mistaken: many cycles at low depth of discharge can mean less total energy than fewer cycles at high depth of discharge. For investment decisions, what matters most is the total energy delivered over the service life and the cost per kilowatt-hour delivered.

In the EU, Battery Regulation (EU) 2023/1542 governs this information. Since 18 August 2024, rechargeable industrial batteries with a capacity of more than 2 kWh, batteries for light means of transport and electric vehicle batteries must be accompanied by documentation with performance and durability parameters in accordance with Annex IV. These parameters include the depth of discharge used in the cycle life test, alongside the C-rate and the expected service life in cycles. This makes cycle figures for different products more easily comparable. Binding minimum values are to follow by delegated act. For industrial batteries that have only an external storage, these minimum values are exempted.

Significance in the power system

In the power system, depth of discharge helps determine how much energy a storage system can shift per use. This involves a trade-off between the energy per cycle and the ageing budget. Depending on the application, the balance turns out differently:

  • Frequency containment and other balancing power: Here, fast changes in power matter most. The state-of-charge swings per event are often small. At the same time, the storage system must hold reserve in both directions and is therefore usually operated around a medium state of charge.

  • Arbitrage and shifting over several hours: Here, the amount of energy that can be shifted matters. A greater depth of discharge brings more revenue per cycle but accelerates ageing. It makes sense as long as the price differences exceed the additional ageing costs.

  • Home storage: A greater usable depth of discharge can increase the self-consumption of solar power in the evening. However, warranty conditions and the backup power reserve limit how deeply the storage system is discharged.

The practical question is therefore rarely whether deep discharges should be avoided in principle. Rather, it is which depth of discharge is permissible and economical for a given cell chemistry, temperature, warranty, operating strategy and revenue situation. Depth of discharge thus links the cell level with questions of flexibility, costs and operation management.

Sources

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Edited by eCMS on 11.11.2026 19:47

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