It’s no secret that the energy storage market is growing fast. Storage incentives in markets like Connecticut and Maine are making battery systems more advantageous. Concerns about grid reliability and resilience in markets like California and Louisiana are making batteries more sought-after. With the increased customer demand, more battery options are flooding the market than ever before. 

Despite battery prices dropping significantly in recent years, batteries are still a sizable investment for customers. It’s important to be well versed in the various characteristics that define battery performance to help customers be well-informed and at ease with their new investment, regardless of which battery type they choose.

While there are certainly some characteristics that are more critical than others, gaining a general understanding of these attributes will help you select the best battery for each application as well as communicate the value to end users.

Let’s briefly go through each of them.

Nominal Voltage 

Nominal (or named) voltage is the average voltage a cell outputs when charged. This is how batteries are marketed even though the actual voltage will vary. For example, a 12V lead acid battery is typically named as such, even though when it is fully charged the voltage will be actually closer to 12.7V. The measured voltage will decrease as it discharges.

Battery Capacity

Battery capacity is the maximum amount of energy that can be extracted from the battery under specified operating conditions. It is typically measured in amp-hours (Ah). Battery systems are sized by looking at a customer’s load (in watt-hours, Wh) then converting to Ah at the system’s DC voltage. Batteries are typically sized very conservatively to accommodate critical loads for a fixed number of days. 

Internal Series Resistance

Batteries will always have some resistance and it is best for there to be low internal resistance within a battery. The lower the resistance within a battery, the less constraint the battery faces when delivering energy at higher usage spikes. 

Self Discharge Rate

Self discharge is when stored energy within a battery escapes, slowly reducing the battery’s charge over time. A battery’s self discharge rate is affected by the environment and usage. Different battery chemistries have different factors that slow self discharge. Typically, many batteries do best in cool, dry conditions. 

Energy Density

Energy density is the amount of energy a battery contains compared to its weight. The higher the energy density, the longer the battery can function in relation to its size. Batteries with high energy density are favored when there is limited space.

Cut-Off Voltage

The cut-off voltage is the voltage when a battery is considered fully discharged. It is determined by the manufacturer, so that users can utilize the maximum capacity of a battery while not damaging the battery through excessive discharging. 

Efficiency

Battery efficiency is the ratio of the amount of energy you can get from a battery relative to the amount of energy put into it. When a battery has a high charge efficiency, the discharge loss is small. 

Life Cycle at Depth of Discharge

A battery’s depth of discharge is the amount of a battery’s storage capacity that is utilized before it is charged again. The amount of cycles a battery can handle depends on how deep it is discharged each cycle. Some battery types are able to be discharged much lower than others and still last many years. 

Temperature Range

Batteries have the ability to operate at a wide range of temperatures, but typically have more specific temperature requirements for charging. Different battery chemistries function within different temperature ranges, so it is important to match the application with an appropriate battery. 

Battery Lifetime

Battery lifetime is typically defined by the number of charge and discharge cycles under certain conditions until the battery can no longer maintain its original capacity. While all batteries deteriorate over time, factors related to improper usage or charging as well as its environment can cause some batteries to degrade more quickly than expected. 

Maintenance Requirements

Some batteries require regular maintenance schedules to maintain the health and function of the batteries. One of the most common types of batteries requiring maintenance are lead acid batteries, which require regular preventative maintenance activities like checking and maintaining water levels within the battery bank. It is important for end users to understand maintenance requirements and have a plan for the continued maintenance. 

Interested in learning more about storage? Check out Wes Kennedy’s Comprehensive Solar + Storage Course.