Degree of discharge

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The degree of discharge , also known as the depth of discharge or DOD for short in technical literature (also DoD ; from English depth of discharge ), indicates what proportion of the nominal capacity of an electrical energy store ( batteries , accumulators , capacitors ) is used during operation.

It describes the ratio of the maximum amount of electrical charge that can be drawn during operation (usually in ampere-hours , Ah) to the nominal capacity.

The capacity used in the operation of a battery is often limited compared to the nominal capacity in order to achieve a longer cyclical or calendar service life of the battery. The indication of the discharge degree alone does not define in which state of charge range (SoC English state of charge , this capacity is used). This differs depending on the technology.

The usual degree of discharge for lithium-ion batteries is often 60–90%, depending on the application. Since accelerated aging occurs with both low and high states of charge, the middle state of charge range is usually used: A degree of discharge (DoD) of 70% can e.g. For example, this means that the SoC area is used between 10% and 80%. In addition, the discharge rate from the can battery management system depending on ambient conditions or aging state (SoH English state of health ) are also adjusted the battery over time.

In the case of lead-acid batteries , on the other hand, the upper state of charge range is used whenever possible, since only the lower range is detrimental to the service life. Here, a degree of discharge of 40% usually means that the range from 60% SoC to 100% SoC is used.

Since batteries z. If, for example, due to safety reserves or manufacturing fluctuations, they may have a higher capacity than their nominal capacity stated on the nameplate, a degree of discharge over 100% is possible (e.g. 55 Ah for a 50 Ah battery, which corresponds to a degree of discharge of 110% ).

Alternative use of the term

Less often (and in contradiction to the above definition) the degree of discharge is also used to describe the current state of charge of an energy storage device: The specification "60% DoD" can mean that the state of charge (SoC) is now just 40%. In industry, however, it has become established to only use the SoC variable to describe the state of charge (ie how “full” the battery is at the moment).

literature

  • Thomas B. Reddy: Linden's Handbook of Batteries . 4th edition. McGraw-Hill, New York 2010, ISBN 0-07-162421-X , p. 71 (English).