Depth of Discharge

Energy (or energy capacity) of a battery

The energy capacity of a battery is typically defined as the discharge of energy, measured in kWh (or Wh, or MWh), from a state where the battery is fully charged, to a state where the battery reaches a certain minimum voltage called Cut-off Voltage.

Depth of discharge (DoD or DOD)

The depth of discharge of the battery measures how deeply depleted the battery is, compared to a state of full discharge when it would have discharged all of its energy capacity.

When a battery has discharged its full energy capacity, the Depth of Discharge, or DoD, is 100%. When it is anything less than that, the depth of discharge, measure in percent (%), is the ratio of the energy that would have been discharged by the battery to reach its present state of discharge from a state of full charge, divided by its energy capacity. When the battery is fully charged, its DoD is 0%.

It is sometimes possible for a battery DoD to exceed 100%, if the nominal energy capacity was estimated conservatively, or because of actual operating conditions different from the nominal operating conditions.

State of Charge (SOC)

The State Of Charge (100%) of a battery is the opposite concept – it is equal to 100% when the DoD is 0%, and 0% when the DoD is 100%.

Battery Life Span

Why is DoD important? The life span of many types of batteries (for instance lithium-ion batteries, indicated as Li-Ion, lead-acid batteries or nickel-iron batteries) depends heavily upon the number of charge/discharge cycles they undergo. The deeper the DoD per cycle, the shorter the life span.

Nickel_Iron_Battery_-_Depth_of_Discharge_life Wikipedia

Nickel-Iron battery life cycle – source Wikipedia

The lifespan of a battery is typically specified by two numbers:

  • the number of years the battery can function in a floating state (i.e. fully charged)
  • the number of charges/discharges (cycles) it can absorb while still providing nominal performance.

The problem with this specification is that the energy capacity of the battery depends upon the discharge current (in A. Amperes), and that the number of cycles in the lifespan depends upon how deep the discharge is.

So it is important to read the specifications of a battery system with great care, to understand properly what the manufacturers’ specifications actually mean, so that you can compare them to other manufacturers or technologies.

Many manufacturers of grid scale battery systems do not provide full public specifications for their products, nor price. This shows, to a degree, that the grid scale battery storage market is still quite immature, when manufacturers expect to gain competitive advantage by obscuring information.