Depth of Discharge: Why Hybrid Supercapacitors Thrive Where Lithium-Ion Batteries Struggle
By: Julie Davis
When selecting an energy storage solution, it’s essential to consider factors such as longevity and performance. One common inquiry I receive revolves around the Depth of Discharge (DoD) of the Hybrid Supercapacitor and how it stacks up against VRLA and Lithium-ion batteries. Let’s explore this topic in detail!
Depth of Discharge (DoD) and Ambient Temperature: Understating Battery Usage
Depth of Discharge (DoD) measures the percentage of a battery’s capacity that has been utilized in one cycle. It’s determined by dividing the energy discharged from the battery by its total capacity. For instance, if a 100 Ah battery is discharged by 20 Ah, the DoD would be 20%. The extent to which an energy storage medium is discharged plays a significant role in its lifespan. The lower the depth of discharge in each cycle, the longer the battery will last. Frequent deep discharges, where a large amount of stored energy is released, can notably diminish both the available capacity and the number of cycles the battery can deliver over time.
Ambient temperature plays an equally important role in the cycle life of a battery. Using a battery in extreme heat is more destructive to a battery’s life than using a battery at the comfortable temperature of 70 Degrees F. When manufacturers offer cycle life rating, they typically provide specifications of cycle life based on a fixed temperature of 25C (77F). The cycle life of all batteries is impacted by both factors, DoD and ambient temperature. When it comes to discharge behavior, Lithium-ion batteries and Hybrid Supercapacitors exhibit distinct characteristics.
Cycle life (the number of times one can charge and discharge a battery) matters. If it’s not obvious, cycle life impacts how long batteries last before they must be replaced, how much material we have to extract from the earth to make the batteries and of course the overall cost of operating a system that requires batteries. In the world of batteries, cycle life is everything. Granted, there are some battery applications that are less dependent on cycle life, but even in those applications, a higher cycle life battery is going to have other qualities that make them a better choice.
Lithium-ion vs. Hybrid Supercapacitors: Discharge Behavior
Lithium-ion batteries have gained wide adoption for their high energy density, approximately 240 Watts per Kilogram (240W/Kg), and superior cycle life to older lead acid batteries. Lead acid batteries are being phased out of many applications because their extremely low cycle life and extremely low density. Lithium-ion batteries are known for their high energy density and durability compared to those older battery technologies. Lithium-ion batteries are found in things like smart phones, laptops, and automobiles. Another, lower density option of Lithium battery, known as Lithium Iron Phosphate, has a density of 140W/Kg. Both of these battery types face challenges related to their discharge behavior. Cycling near full capacity or deep discharge accelerates the aging process by thickening the Solid Electrolyte Interphase (SEI) layer on the anode, trapping lithium-ions, and reducing the battery’s capacity. Moreover, dendrite growth in Lithium-ion batteries poses risks of internal short circuits, overheating, which can lead to thermal runaway.
Let’s look at a typical cycle life chart for these two kinds of Lithium batteries, Lithium-ion, and Lithium Iron Phosphate (LiPO4). The chart commonly shared shows cycle life at various levels of DoD but with all ratings at the 25C temperature. As the ambient temperature increases above the 25C rating, the number of cycles decreases. The following chart estimates the number of cycles typical Lithium batteries can deliver at 25C before losing 30% of the battery’s capacity. Chart is courtesy of the Battery University, which can be found here.
We now have another option of battery with substantially improved performance of the Lithium battery options. Hybrid Supercapacitors are engineered to endure deep discharges without sacrificing cycle life. By merging the advantages of Lithium-ion batteries (high density) and Electric Double-Layer Capacitors (high cycle life), Hybrid Supercapacitors at a temperature of 25C can withstand over 20,000 cycles even at 100% Depth of Discharge (DoD). Also notice how well hybrid supercapacitors perform in a wider temperature range. Supercapacitors mitigate the dangers associated with dendrite growth in Lithium-ion batteries, as the absence of metal oxides in the cathode prevents excessive temperature rise and the likelihood of thermal runaway, making them a sound choice for high-performance energy storage applications. The following chart estimates the cycle life for a typical Hybrid Supercapacitor and includes the added factor of ambient temperature.
Battery degradation is the gradual loss of a battery’s ability to hold a charge or deliver power over time. As a battery degrades, its capacity diminishes, leading to reduced runtime and performance of the device it powers. There are many reasons why one would not want to discharge a battery system 100%, but being able to discharge 100%, knowing that we have not reduced the battery life to near zero is comforting.
Choosing the Right Fit
While Lithium-ion batteries offer high capacity, their sensitivity to depth of discharge and dendrite growth limits their suitability to applications requiring the highest energy density such as EVs. Hybrid Supercapacitors, on the other hand, excel in every other application, offering substantial energy density, more than 200 X more cycles, wider temperature range and enhanced safety thanks to their unique composition. All of these advantages mean using Hybrid Supercapacitors means longer time before batteries have to be replaced, less materials we have to extract from the earth, and of course the lowest overall cost of operating a system that requires batteries.