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Answers to your energy storage questions: Batteries 101

A few weeks ago I presented on an energy storage controls webinar which examined the relationships between principles, practice and project risk; and with 850+ registered attendees, we were bound to get a few questions! After sifting through many of the unanswered questions from the Q&A session, I realized that there were a lot of similarities. Many of you wanted to know about the different types of batteries, how long they last, the impacts of battery size, and so on. These questions influenced me to take some time to write a blog post to answer some of these general energy storage and battery questions. And if you still have questions after reading this blog, I’d be happy to set up a time with you to discuss your specific questions.

What types of batteries are used?

Li-ion batteries dominate present-day energy storage deployments. It feels strange saying this as only five years ago there was a lot of discussion about when and how they would become prevalent. Today, we speak of them as the incumbent. Price floors on battery cells have an incredible range between $150-$350/kWh, and roughly $500/kWh at a rackable modules scale with a battery management system (BMS).  This gives you an idea of how fast the market is growing. Consider discussions about mobile phones and “the world wide web” in 1996 vs. 2001; that’s an adequate analogy for projected market growth and activity over the next 5 years.

That being said, “Li-ion” is a very general term, and it helps to be more specific about the impact of chemistry on the application. It also helps to speak about other battery chemistries that are commercially available (you can buy them right now) that may serve in a storage application.

Li-ion is the incumbent, but it isn’t the only player. Perhaps the most important metric that determines the type of battery is the required duration for the application. Many RFOs in California are requiring a four hour duration. There are commercially available Vanadium Redox, ZnBr, and Zn air batteries that can achieve 4+ hour duration. Many of these batteries are commercial, i.e., you can call the manufacturer today and start a discussion on spec’ing your system and scheduling a delivery for your upcoming project.


Figure 1 – Common Lithium-ion Chemistries and Properties

Is it only batteries?

No, but it may be worthwhile to ask: Is there anything better? This question encourages you to think of the devil’s advocate view and helps to gain understanding around what’s driving the inquiry. We’ll never prescribe a solution without proper diagnosis, and that’s why we rephrase the question. The need determines the application and batteries tend to have the flexibility to meet many of the needs today. Therefore, anyone considering batteries for an energy storage solution should look at the project with a critical eye and prove to themselves whether it is the best solution. If the problem is examined with technical depth, with economic value at the forefront, the winning technology will be the best for the project, batteries or not.

Batteries can be deployed in aggregate or consolidated, are generally efficient and scalable, have no geographical limit, and can serve both behind-the-meter and in –front-of-the-meter solutions. Today, we see batteries used mostly by independent power producers and C&I applications (isolated and in aggregate). The aggregate of battery solutions is a utility solution, and hence the California RFOs from utilities are getting utility-scale storage from aggregated assets.

We also see thermal storage devices meeting some of the same needs as batteries, and they can also be distributed and aggregated to meet peak HVAC loads during the summer, for example. Other technologies like compressed air energy storage (CAES) and pumped hydro are still relevant but are frankly large and geographically limited and must operate in markets that can accommodate those requirements. If your needs for energy storage are broad and general, our ES Select Tool is a helpful energy storage platform that outlines relevant storage technologies today. At DNV GL, we also routinely perform general storage assessments for utilities, for example, to help them narrow down a broad list of storage technologies. ES Select provides a great overall view, but the needs of the customer dictate which storage is most suitable.

How do I size my battery for a project?

Battery size is entirely dependent on the application it is intended to serve and there are characteristics of the intended duty cycle that impact what the battery should do. Let’s look at some qualitative aspects of the duty cycle and how they might impact the battery size and chemistry:

  • Peak loads lasting multiple hours: A long duration battery may be required. Li-ion batteries serve up to 4 hour duration projects. For duration greater than 4 hours, flow batteries might be required but not without evaluating whether the power requirement can be met as well. A long duration application is a high energy application and Li-ion is technically capable of serving the need, though a multi-chemistry cost-benefit analysis of duration vs. cost is highly recommended.
  • Peak power: Unfortunately, physics won’t let us have both high power and high energy until we overcome some basic electrochemical principles. Some batteries are better for power than others.
  • Efficiency: Electrons lost in the fight are electrons that can’t be monetized. A high efficiency battery is a battery with better returns. The way the battery is being used, its optimum efficiency, and the sensitivity of efficiency to temperature, current, and duty cycle eccentricity factors are directly impactful to the project.
  • Response time: The readiness to dispatch is one of the factors that make batteries highly competitive in frequency regulation markets. It is important to grade its response time and match it to the application.

How long do they last?

This is one of the most frequently asked questions our energy storage team receives. At DNV GL we’ve developed a tool called Battery XT which predicts battery life based on its duty cycle. There are several factors that affect battery life. The duration of battery lifetime spent at the following factors dictates its total lifetime:

  • Temperature
  • State of charge (SOC)
  • Current or c-rate

When investigating a manufacturer warranty, it is enormously important to examine the manufacturer’s test data, match up the test data to the intended application, refine the match-up with the operational limits imposed by the ESS’s BMS, and make an educated engineering judgement on whether the system is up to the task.  Battery XT helps us take a step further by offering precise models that remove uncertainty around the application variables.  We can then limit uncertainty in the life prediction, and offer a quantitative (rather than qualitative) opinion about whether it will meet the warranty.


Figure 2 – DNV GL’s Battery XT Tool

How do controls impact battery life or warranty?

Controls affect how a battery is used. As mentioned above, the useful range of the battery has a direct impact on its life in the system. In order to squeeze as much cost efficiency as possible into battery systems, the battery management system may impose state of charge (SOC) limits that are right on the edge of accelerated degradation.  While these limits safeguard premature wear on the battery, if the controls system demands that the battery spend significant amounts of time near these limits, it will impact battery life.

The economics of an application have direct impact on the controls, since the controls are responding to prices. Spending time at high or low SOC limits, operating at high power, or running a system in warm conditions can accelerate its degradation. DNV GL also offers controls risk evaluations as part of technology review or due diligence projects to help both project developers and lenders understand how their selected technology supports the projected revenues.

Want to know more? Connect with me on LinkedIn or contact me to discuss your specific needs.

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