Battery Balancing Is Like a Glass of Water

Question: When are large-scale distributed energy systems operated like a car?
Answer: Never!

Yet most battery management systems (commonly known in the industry as the BMS) that are used in stationary storage utilize software that was optimized for an electric vehicle (EV). That means it is focused on getting the most mileage possible out of a small to medium size battery for the average 30-50 miles it is driven each day to go to and from work, school, the grocery store, a coffee shop, etc. BMS software in an EV performs admirably for the use case and power output it is designed for, but batteries inside large-scale energy storage systems are utilized very differently.

A single containerized solution can have in the range of 125 kW to 250 kW of power with 0.5 MWh of capacity, and much larger systems can consist of dozens of containers (or more) in the same storage system to create installations multiple megawatts in size. Energy storage systems endure very punishing duty cycles where they are called upon to charge and discharge close to specified capacity multiple times in a day, depending on if the installation is focused on mitigating demand charges by supplying power during peak grid usage, smoothing loads from highly variable renewable energy sources, performing grid ancillary services like frequency regulation, or is part of a microgrid and is used as a diesel generator replacement.

Battery balancing is much like a glass of water (but not a box of chocolates…that’s a different analogy). If you have two glasses of water and want them to be perfectly even, you can simply move water from one glass to the other until the levels are uniform. Fairly simple coding there if you wanted to automate the process, though the robotics might be a challenge. If you invite some friends (say 10) over and you want to make sure everybody has the exact same amount of water, pouring from one glass to another (let’s assume there isn’t a faucet in the kitchen for this analogy) and measuring the amount to make sure they are exactly even becomes cumbersome. But with some straws or flexible tubing, some small water pumps, and some clever coding, you could still move water back and forth between the glasses.

Now you and your buddies decide to go to a concert at a football stadium and there are 50,000 glasses of water with people taking sips at different times. If we need to keep all of those glasses exactly even (like we’re in the middle of the latest Die Hard movie and the fate of New York or Los Angeles depends on it), then the problem becomes insurmountable. Moving water (or energy) from one glass (or battery cell) to another within a framework of tens (or hundreds) of thousands of individual units to keep them all at an even level is not just impractical, it is highly inefficient and extremely costly.

So what if we did it a different way?

The reason for battery balancing is to make sure no one individual cell gets over or undercharged, as that can rapidly degrade performance or lead to safety issues (just like some people having too much water in our analogy would lead to water being wasted while others go thirsty). But instead of keeping the whole system balanced by transferring power back and forth to try and achieve cell parity based on the total amount of stored energy, what if we treated each battery cell as an individual and instructed it to draw or release energy as needed to optimize itself to a pre-programmed level? This method is a much more practical way to balance multi-megawatt storage systems because every battery cell or module (i.e., the 50,000 glasses of water in the football stadium) does not need to be connected together by water pumps and hoses to transfer the water from glass to glass, and it is much easier and much cheaper to implement. This makes the whole system much less expensive, much more reliable, and much more efficient.

So if you are driving an EV, have confidence that your car’s battery management system is finely tuned to deliver optimum performance for your vehicle.

But if you are among the increasing number of utilities and C&I facilities that require energy storage, make sure your system utilizes a battery balancing system and software that is engineered for you and isn’t designed for the road.

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Virgil Beaston is the Senior Vice President & Chief Technology Officer (CTO) for Powin Energy and has been with the company since 2011. Beaston has over 10 years of experience in the battery and energy storage industry and is often asked to speak publicly as a subject matter expert (most recently at Intersolar North America, STUDIO, and ESNA in 2016). He views business and the rapidly growing storage sector from a unique perspective as he previously was a patent attorney at an intellectual property firm and then was the co-founder and CTO of Greensmith Energy Management Solutions. Beaston is an inventor, lawyer, and engineer with impressive academic credentials: Doctorate of Law, Masters in Electrical Engineering, and a Bachelor of Science in Applied Physics and Electrical Engineering.

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