More on Measuring the State of Health and State of Charge of a Battery

In my last blog, I gave an overview of how to measure the state of health and state of charge of an ultracapacitor. In an ultracapacitor, end of life does not arrive abruptly and without warning, as it may with batteries. An ultracapacitor will conceivably continue to operate until there is not enough energy left in the device when fully charged to do the job. At that point, an orderly change out of the energy storage device can take place, but it is unlikely to occur for many years and many cycles, as long as it is operated within the specifications. Batteries, by contrast, have a failure characteristic of just falling off a cliff when their ability to perform ends quickly due to failing electrochemical function.

To understand the state of health and state of charge of a battery, one needs more sophisticated measurements and complicated predictive algorithms than those needed to understand the health and charge in an ultracapacitor. Having a generally flat voltage profile in the battery means the energy contained in the device has to be determined differently than just measuring voltage as with an ultracap. Predicting the end of life of the battery system is also a complex process that carries a lot of potential for error. This leads system designers to design battery systems with highly conservative specifications and operating profiles for fear of miscalculating and experiencing premature end of life of the device. These design practices will always result in a battery larger (and more costly) than could otherwise be designed if state of health were readily, predictably, and reliably determined. Despite this conservatism in design, batteries are often known to reach end of life earlier than desired or expected.

With their extraordinary power capability and readily determined health and state of charge, ultracapacitors can be used in a hybrid energy storage system along with batteries to mitigate the effects of uncertainty in batteries' state of health determination on the overall system reliability. This is because, in an appropriately configured system, the ultracapacitors can considerably prolong the lifetime of the battery by carrying the significant high-power loads that the battery might otherwise experience. In turn, this preserves the battery's health. Further, when a gentler battery operating cycle is enabled through a combination with ultracapacitors, the state of health will remain high for longer, reducing the risk for the battery designer and the system operator. This is just one of the areas of significant value that ultracapacitors bring to the overall system when operating in conjunction with batteries.

So if the application calls for a high-value, predictable, and readily characterizable energy storage system, one can rely on ultracapacitors to provide long lifetime and the highest reliability — something not found in any but the highly oversized and conservatively designed battery-only systems.

Follow us at @Maxwell_Tech on Twitter for more conversations about the complementary features of ultracapacitors and batteries.

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