What is Depth of Discharge (DoD)? The Ultimate Battery Guide

Are you searching for batteries for your home solar kits, systems, or other backup power sources? If so, you must have heard the term “depth of discharge.” It is closely related to the performance of rechargeable batteries, as it lets you know when you should recharge the battery.

When it comes to planning for a backup power system, it is essential to consider the depth of discharge, especially for applications such as portable electronics, electric vehicles, and energy storage systems.

If you are wondering what is depth of discharge, don't miss reading through this guide, as it presents the definition of this concept, how it impacts the battery life, how you can calculate it, and more.

What is the depth of discharge (DOD)?

The percentage of charge removed from the total capacity of a battery is commonly called the depth of discharge. Simply put, if you want to know how empty or full a battery is, depth of discharge will lend you a helping hand. For example, if half of your battery is discharged, its DoD will be 50%.

In other words, the depth of discharge shows the amount of energy left in a specific battery and lets you know how long you can use it before putting it on the charge. Generally, it is not a good practice to discharge the battery fully, as it may damage the whole system.

Considering this factor, manufacturers usually specify the maximum value for DoD. It helps you determine the maximum power you can get from the battery without damaging it. For example, if 80% is the depth of discharge of a 5kWh battery, you should not use more than 4kWh without recharging it.

Depth of discharge vs. state of charge (SoC)

When it comes to measuring the available energy level of a battery, both the depth of discharge and the state of charge play a crucial role.

DoD determines how much energy a battery has already discharged or used. It measures how empty the battery is after use. In contrast, the state of charge is the current energy level available in the battery. It shows how empty or full a battery is at a given moment.

Let's understand the concept with a real-life example. Think of a glass of water. The depth of discharge shows how empty the glass is or how much water is taken out of the glass. On the other hand, the state of charge shows how full the glass is or the current level of water available inside the glass.

If you want to understand the energy status of the battery, both DoD and SoC are equally important to consider. If you consider both of these parameters, you will be able to manage the battery usage effectively. Plus, they will tell you when to recharge the battery.

If we talk about calculating the depth of discharge from the state of charge, the DoD will be 100% minus SoC. For example, if a battery's state of charge is 40%, its DoD will be 60% (100-40). Let's now learn what impact DoD has on the battery's life cycle.

How does the depth of discharge affect battery life?

DoD has a great impact on the battery's life. A higher value of the depth of discharge means a large percentage of electricity is used before recharging. Generally, this practice leads to a shorter lifespan of a battery. The batteries that are frequently discharged deeply degrade faster than usual. Plus, they lose their capacity with time.

Let's go through some potential points about the depth of discharge and the battery's life. When you deeply discharge a battery, it puts added stress on its internal components. As a result, the plates may be damaged and the capacity may be reduced.

Similarly, DoD is directly related to the number of a battery's charge-discharge cycles before it loses its capacity over time. Remember, different batteries come with different values of DoD, and we will discuss it in detail later in this article.

When a battery delivers a high amount of energy at a given time, its rate of discharge is higher. Some batteries are designed to do this, while others show this behavior when they lose capacity. Continuing to discharge the batteries deeply may eventually lead to their short life.

Depth of discharge vs. Battery capacity

The battery's DoD and capacity are two different aspects of measuring its performance. When we talk about capacity, it means the total amount of energy a specific battery can store. It is similar to a tank's size, which shows how much fuel you can put into it. In contrast, the depth of discharge lets you know about the amount of energy that has already been used, as stated earlier.

DoD is a variable value that determines what percentage of capacity has been used. On the other hand, capacity is a fixed value that represents the total energy a battery can have. Let's understand this concept with an example.

Suppose the total capacity of a battery is 100 units, and 50 units are already consumed. It means the DoD will be 50%. In this case, DoD is half of the battery's total capacity. Understanding both the depth of discharge and capacity makes us familiar with the used and remaining energy level of a battery. In addition, it helps us know how long we can use a battery before connecting it with the charger.

If you want to achieve optimal battery performance, you should balance DoD and avoid discharging the battery deeply (still using some portion of its total capacity).

Lithium-ion vs. Lead-acid batteries

Lithium-ion batteries are a more recent technology compared to lead acid batteries. Though both come with unique features and a set of applications, Li-ion batteries are considered better than lead acid batteries in many aspects. Let's look at a few of them.

Li-ion batteries have a higher energy density than lead-acid batteries, which means they can store energy or electricity. When it comes to overall efficiency, lithium-ion batteries lead their lead-acid alternatives. Similarly, the lifespan of Li-ion batteries is longer than that of lead-acid ones.

Along with these parameters, lithium-ion batteries offer a better depth of discharge compared to lead-acid batteries. It means you can obtain more energy relative to the total capacity of a Li-ion battery than a lead-acid battery.

The DoD of lead-acid batteries is not more than 50%, which means you can't discharge more than half of their total capacity. On the other hand, at least 85% of the total capacity can be used for Li-ion batteries.

Suppose the total capacity of a lead-acid and a lithium-ion battery is 5kWh. Since the DoD of a lead-acid battery is 50%, it will only be able to deliver 2.5kWh of electricity, indicating half of the total capacity. If you want to buy reliable and premium-quality lithium batteries, explore a decent collection at Renogy.

How to calculate the depth of discharge?

It is super simple to calculate the battery depth of discharge. The steps you should follow while calculating DoD are as follows.

Step 1: Determine the battery's initial capacity. This is usually presented in watt-hours (Wh) or amp-hours(Ah).

Step 2: Now, estimate or measure the amount of energy that has been used from the battery's total capacity.

Step 3: It is time to divide the amount of discharged energy by the battery's total capacity.

Step 4: Multiply the answer with 100 to get the percentage value.

You can easily calculate the depth of discharge of your battery with the formula given below.

Depth of discharge = (used energy/initial capacity) x 100

Suppose a battery has a total capacity of 200Ah and 100Ah of energy has already been used out of the total. In that case, the depth of discharge will be 50% ((100Ah/200Ah)x100). As mentioned earlier, DoD is a variable value that can differ for each discharge cycle. Therefore, don't forget to calculate the DoD of your battery every time you use it.

Does a higher depth of discharge always mean better performance?

No, not at all. The better performance of a battery is not necessarily connected to a higher depth of discharge. For most batteries, it is advised to avoid high depth of discharge.

When we say a battery has a higher DoD, it means we can use more energy before recharging it. Though it may provide more electricity in the short term, it may negatively impact the battery's lifespan. At the same time, overall performance is equally compromised.

Frequent deep discharges are one of the batteries' biggest enemies, as they accelerate battery degradation. As a result, the battery's usable life is shortened, and its capacity is reduced.

If you want to make your battery last longer, remember to keep its depth of discharge within the limits recommended by the manufacturer, usually between 80% and 95% for Li-ion batteries. Following these limits not only extends the battery's life but also improves its health. In addition, it ensures that the battery is able to function properly for a long time.

The impact of temperature on battery DoD

All kinds of batteries usually perform well at room temperature. Extreme temperatures (low or high) have always been a trouble for most batteries. However, this is the area where lead-acid batteries outperform both lithium-ion and lithium iron phosphate batteries.

Batteries discharge faster in higher temperatures as the chemical reactions are accelerated. As a result, the lifespan is reduced. On the other hand, extremely low temperatures slow down the chemical reactions within the batteries, reducing their total capacity. Both extremely high and low temperatures increase the battery's depth of discharge.

If you want your battery to deliver electricity efficiently, keep it in the ideal temperature range. In addition, always follow proper charging practices and use active cooling systems to keep batteries away from extremely hot conditions.

Frequently asked questions

1. What does 80% depth of discharge mean?

When a battery has an 80% depth of discharge, it means we can use 80% energy out of its total capacity before putting it on charge.

2. What is the best depth of discharge for a battery?

It largely depends on the battery type, as it varies from one type to another. The best depth of discharge for lead-acid, lithium-ion, AGM, and Gel batteries is 50%, 80-100%, 80%, and 75, respectively.

Conclusion

The amount of charge or power discharged from the battery is known as the depth of discharge. It shows how empty a battery is at a given time and lets you know its current energy level. Though DoD is closely related to the battery's performance, a higher value is not always needed for better performance.

Always review the manufacturer's guidelines to ensure the DoD of your battery is within the recommended limits. It will help the battery stay healthy and last longer. Avoid deep discharges and keep the batteries charged.