Depth of discharge (DoD) is one of the key aspects to consider when selecting batteries.
In this article, you’ll learn everything you need to know about depth of discharge, including:
What depth of discharge is
Why it’s important to know the depth of discharge
And much more!
Let’s dive in!
You may have seen “DoD” listed on a battery label before and wondered: What does “DoD” stand for in batteries?
The answer is that it stands for “depth of discharge.”
But what does that mean?
Put simply, it means how much of a battery’s actual power can be used out of its total power capacity.
Depth of Discharge vs. Battery Capacity: What’s the Difference?
To understand exactly what depth of discharge means, we need to talk about battery capacity.
But the thing is that you can’t usually use all that energy without damaging the battery.
So that’s where depth of discharge comes into play.
Depth of discharge is meant to tell battery users how much energy they can safely use from the battery without compromising its lifespan.
For example, let’s say you have a battery rated for 80% depth of discharge.
Now, what does 80% depth of discharge mean?
It means that you can only use 80% of your battery’s total rated capacity.
So if you have a 500 amp-hour capacity battery, you really only have 400 amp-hours to work with at 80% depth of discharge.
Depth of Discharge vs. State of Charge: What’s the Difference?
Battery state of charge (SoC) is exactly what it seems.
That is, it’s how much of a charge a battery has in any given condition.
So, a fully charged battery would have a 100% state of charge.
Often, it’s used in conjunction with depth of discharge.
Depth of discharge is simply the opposite of state of charge.
When that’s the case, state of charge is simply the inverse of depth of discharge.
Put another way, if a battery is at 100% state of charge, then its depth of discharge is 0%.
The opposite is also true.
If a battery is 100% discharged, its state of charge is 0%.
Depth of Discharge vs. Cycle Life: What’s the Difference?
A battery’s life cycle is the number of complete charge and discharge cycles it can go through before its performance begins to drop.
How Does Battery Discharge Affect Cycle Life?
Cycle life has an inverse relationship with depth of discharge.
In other words, the higher the battery’s depth of discharge and the more often it is discharged, the fewer cycle lives it will have.
For instance, a battery that’s continually discharged 80% will have fewer life cycles than if it were only discharged 20%.
This is why it’s generally not recommended to discharge a battery entirely.
Because doing so dramatically shortens its cycle life.
What Is the Cycle Life of a Battery?
Depending on the manufacturer, the type of battery, its depth of discharge, and the operating temperature, a battery’s cycle life can range from 500 to 8,000 cycles.
Assuming a 50% depth of discharge, the following are the common cycle lives for the most popular types of batteries:
A battery with a higher depth of discharge has the advantage because it means you can use more of the battery’s energy before it needs a recharge.
As you can see above, that’s a key advantage of using lithium-ion batteries.
These batteries can tolerate a higher depth of discharge - often between 80% and 100% - without losing cycle life.
A higher depth of discharge means being able to use your battery longer before needing to recharge it.
Thus, you can get more usage out of lithium-ion batteries than other types.
Of course, a higher depth of discharge does have its downside.
As we covered in the previous section, a higher depth of discharge means you’ll have fewer life cycles.
So, it’s a matter of balancing your ability to use more of the battery daily against how long you want your battery to last.
What Is the Average Recommended Depth of Discharge in Batteries?
The recommended battery DoD varies by the type of battery and manufacturer.
Let’s cover the average depth of discharge of some common batteries.
What Is the Depth of Discharge of a Lead-Acid Battery?
What Is the Recommended AGM Battery Depth of Discharge?
What Is the Depth of Discharge for a Gel Battery?
What Is the Depth of Discharge of a Lithium-Ion Battery?
Can You Fully Discharge a Lead-Acid Battery?
Never fully discharge a lead-acid deep cycle battery!
As we’ve said, the deeper you discharge the battery, the more its total cycle life reduces.
Most deep cycle batteries can handle only up to 50% depth of discharge, although some are built to handle up to 80% discharge.
Never fully discharge a lead-acid deep cycle battery!
If you frequently recharge your battery in a complete cycle, you can get just over 220 complete cycles if you drain it 80% each day.
But you could get up to 500 complete cycles if you only discharge the battery to 50%.
Is it Okay to Fully Discharge a Lithium-Ion Battery?
Answer: You should avoid doing so as much as possible.
Fully discharging a lithium-ion battery can potentially cause the individual cells to discharge at different states.
And when that happens, you can cause permanent damage to the battery.
The exception to this rule is when you rely on a power gauge fitted to the battery to give you the state of charge.
Because of something called “digital memory,” these gauges can get out of whack.
But by fully discharging the battery every 30 or so cycles, you can “reset” this digital memory and restore things back to normal.
What Happens If You Discharge More Than Your Battery’s Rated Depth of Discharge?
You’ll wear your battery out quicker
The battery will take longer to charge
You’ll void the warranty
Exceeding the rated depth of discharge causes the plates inside the battery to deteriorate more quickly.
And this has a direct effect on the ability of the battery to continuously charge and discharge its current.
Furthermore, you’ll be out of luck by continually over-discharging your battery.
That’s because your manufacturer will not honor the warranty.
Wondering how is depth of discharge determined?
Let’s go through just that.
The battery discharge calculation is as follows:
Discharge current (in amps) x Length of time discharged (in minutes) ÷ 60 minutes ÷ Nominal capacity (in amp-hours) = Depth of discharge
For example, let’s say you have a battery with a nominal capacity of 500 amp-hours.
Then, you discharge a load of 250 amps for 20 minutes.
The calculation is:
250 amps x 20 minutes = 5,000
5,000 ÷ 60 minutes = 83.333 (this figure is your state of charge)
83.333 ÷ 500 amp-hours = 0.16667
That equates to a depth of discharge of 16.667%.
There you have it: Everything you need to know about battery depth of discharge.
Now we’d like to turn it over to you.
What have you learned about depth of discharge?
Can you explain the difference between depth of discharge and state of charge?
What about how depth of discharge affects cycle life?
Please share with us in the comments section!