By Joe McGarvey, Marketing Director |

Various factors impact the cost efficiency, longevity and overall performance of an energy storage solution. One of the most crucial — but often overlooked — energy storage metric is Depth of Discharge (DoD).

Understanding DoD, which is essentially a measurement of the percentage of usable energy in a battery or other energy storage medium, is key to optimizing the performance, potential lifespan and long-term costs of your energy storage solution. Equally important is recognizing that not every energy storage technology is created equal when it comes to DoD.

Depth of Discharge refers to the percentage of a battery’s total capacity that can be used before recharging. It is essentially the inverse of another important energy storage metric, State of Charge (SoC), which measures how much energy remains in the battery. For example, if a battery has a total capacity of 100 kilowatt-hours (kWh) and has discharged 60kWh, the DoD is 60%, while the remaining 40% is the SoC.

Deep vs Shallow

Batteries, such as lead acid and lithium-ion, and other energy storage sources, namely hybrid supercapacitors, are typically rated with a recommended DoD, which prescribes how much of the stored energy can be safely used without causing excessive wear or damage. Some energy storage mediums can be deeply discharged without significant degradation, while others require shallow discharge cycles to maintain long-term performance.

That degradation, which impacts lifespan and overall effectiveness, is one of the most important reasons to pay attention to DoD. All energy storage mediums are capable of a finite number of charge-discharge cycles, which essentially represents the medium’s lifespan. Deeper discharges tend to shorten the usable lifespan of storage mediums.

Lead-acid batteries, for example, have a recommended DoD of roughly 30-50%. Discharging beyond this threshold can significantly reduce their lifespans, which are typically 3-5 years or between 300 and 1,500 cycles.

Lithium-ion batteries are generally capable of an effective DoD rate between 70-90%, according to public sources, without suffering accelerated degradation. Lithium-ion batteries typically top out at around 2,000 to 7,000 charge-discharge cycles in their lifetimes.

Energize All Day

Hybrid supercapacitors, including ATX’s Areca family of energy storage solutions, support DoD rates of 90-100%. That means that MSOs and other businesses that use hybrid supercapacitors for standby power and other operations can essentially use the entirety of stored energy with minimal degradation over the medium’s lifespan of 20,000 or more cycles.

An apt analogy is variation in the human body’s capacity for exercise. Most of us have a finite tolerance for stressing the muscles, tendons, joints and other organs that make up our bodies. Exercise too hard for too long and our bodies start to break down and can no longer support the same levels of exertion. Hybrid supercapacitors, keeping with the exercise analogy, are more akin to robots than humans, able to function at maximum levels for nearly the entirety of their lifespans. 

Real-World Data

The ability of hybrid supercapacitors to deliver without degradation was on display in a 2023 study conducted by a major communication service provider comparing their life expectancy against lead-acid and lithium-ion batteries. In the study, each energy storage medium was used every other day for approximately two hours a day over eight weeks.

Data from the comparison showed that the lithium-ion battery backup runtime dropped to 68% after 13 lifecycles. The lead-acid battery backup runtime diminished to 30%. The hybrid supercapacitor module, though, remained within the expected runtime range during the full length of the trial, its rate dipping only slightly (98%), significantly outperforming the alternative energy storage mediums.

DoD also impacts overall energy efficiency and long-term cost effectiveness. An energy storage solution with a higher DoD rating increases the utilization of stored energy, potentially reducing the total number of batteries needed for a given application.

For example, a lead-acid battery with a 50% DoD or lower may need to be twice the size (and roughly three times the weight) of a hybrid supercapacitor with a 100% DoD rating to provide the same usable energy. Hybrid supercapacitors, due to their ability to use nearly all their stored energy, offer the best return on investment by maximizing usable energy and minimizing degradation over time.

Advanced Efficiency

By selecting an energy storage medium with a high DoD rating, businesses can be more efficient in matching energy storage capacity to actual need while also reducing long-term replacement costs. In addition, the high cycle counts of hybrid supercapacitors open the opportunity for additional savings by enabling MSOs and other businesses to participate in energy arbitrage applications, such as peak shaving.

With energy efficiency becoming increasingly important to telecommunications operators, both fixed and wireless, as well as datacenters, which are being stressed by the energy demands of artificial intelligence technology, maximizing the superior attributes of hybrid supercapacitor technology makes both strategic and economic sense.