Depth of discharge explained

Depth of discharge, or DoD, is how much of a battery's capacity gets used. It's a share, shown as a percentage¹. Simple enough. Almost.

Two meanings, one word

The term has context-dependent meanings, and the exact meaning should be clear for any given context¹. Spec sheets tend to use it in one of two ways:

• The usable range. The largest share of the capacity that's meant to be taken out on a regular basis¹. A sheet that says "90% DoD" often means this.
• The current draw-down. The share of the capacity that has been removed right now, compared with the charged state¹.

The second one is the mirror image of state of charge, the "fuel gauge" reading. For a fully charged battery, DoD equals one minus the state of charge¹. When one goes up, the other goes down.

How it connects to usable capacity

If a sheet gives a DoD of a certain percentage, the usable capacity is that share of the total. A made-up example: a 10 kWh total and 80 percent DoD points to 8 kWh you can draw. The relationship comes from the definition of DoD as a share of capacity¹. The leftover reserve is the held-back part explained in the usable capacity note.

Be careful: sheets can name the DoD, the usable energy, or both. If you see two figures that don't match this simple maths, the maker's manual is where to look for how it defines each.

Why makers set a limit

The control electronics inside the pack may keep the battery from extreme levels of charge, which limits DoD to less than 100 percent¹. There's a link to lifespan as well. For almost all known rechargeable battery technologies, there's a correlation between depth of discharge and cycle life¹.

Wear matters at the large-system level too. Cycle ageing is generally higher at high charging rates and higher depth of discharge².

Older battery types show the idea clearly. Lead-acid leisure batteries usually shouldn't be discharged more than 50 percent of their rated capacity³. That's a limit on DoD, and it's the reason a lead-acid bank's usable energy is far below its label number. Lithium-based home batteries have their own limits, which are set in the maker's documents.

What to look for on a sheet

• Whether the DoD figure is a limit built into the product or a setting you can change in an app.
• Whether the warranty assumes a particular DoD. That matters on reading a battery warranty.
• Whether the capacity quoted next to it is total or usable.

Never change a battery's limits by modifying its hardware. Use only the settings the maker's app or manual offers.