Finding a technically attractive and cost–efficient way to store energy from intermittent sources, such as solar and wind power, is a major challenge, but one with many possible solutions. Obviously, there is no single “best” solution here, as it depends on the needed electrical capacity, charge, discharge, and use cycles, physical siting, costs, and many other factors. The list includes but is not limited to stored water, gravity and weights, flywheels, molten salts, compressed gases, and batteries, of course.

There is even a battery option for these electrical storage systems (ESS) with an unusual twist: the use of “retired” battery packs (that’s a euphemism for “used”), which are generally (but not exclusively) taken from cars and trucks of various types.

These used batteries can be from vehicles that have reached the end of their road life, those salvaged from vehicles in accidents, or from used cars that are being refurnished by the manufacturer, dealer, or even an independent shop. The widely used standard is to declare the battery “done” for its initial applications when its capacity drops to 80% of the original value.

(Personal note: I generally ignore projections that look more than a few years out, or I give them at least a ±30% error band despite any stated precision. However, my personal error band for data related to cars and trucks is much tighter, as the present numbers are known with great accuracy and many of the projections are derived from the “momentum” of these numbers, which is fairly well understood.)

A recent article in The Wall Street Journal identified some of the many commercial installations already using these batteries or that will soon be turned on. Some are small–scale setups for homes and small buildings, while others are supporting much larger offices, factories, shopping malls, and neighborhoods.

At first glance, using these batteries in a so–called “second life” mode for ESS makes a lot of sense for many reasons. These batteries are widely available, don’t require major construction and siting efforts to user, are transportable and can be containerized, are quiet, have no moving parts, and are modular and scalable in capacity.

Equally important, there is a lot of expertise and standard modules available for managing battery packs and using these DC energy–storage units as sources for AC grid–like supplies; much of this is an extension of the experience with EVs and other larger–scale battery projects.

However, there are concerns that can’t be ignored with an energy–storing configuration. First, use of lithium–based batteries and their high energy density by volume (one of their major virtues) also means that these large configurations need complex, multilevel monitoring of charge, discharge, temperature, and many other parameters, along with fail–safe shutdown arrangements and even special fire–squelching systems.

A second issue is the additional useful life of these batteries, which are already 20% degraded when they are installed. The cited article says that second–life batteries are deemed to be useful until they drop to 60% of their initial capacity, which is typically after 10 to 15 years of ESS use. If so, is that long enough to justify all the effort and expense of installation if the batteries need to be replaced every decade?

Finally, there are battery–management issues. Because the constituent batteries and packs—even of the same nominal type—have likely had different charge/discharge cycles, thermal operation, and in–use and even storage abuse of various types, each second–life battery will have a different operating profile and need very careful individual management and possible replacement cycles. To use a cliché, managing such a large disparate collection of batteries could be the electrical analog of “herding cats.”

Still, the idea of repurposing these batteries in a second–life scenario is obviously attractive, at least for some situations (their third–life stage is recycling, which is a complicated story for another time). It certainly seems more sensible with respect to various cost, reliability, and footprint than using huge cranes to raise and lower large weights, or trucking water down an incline (see “Related content”).

As always, it’s filling in the technical details and the many specifics of the situation that makes or breaks the final decision. In addition, market dynamics can be hard to figure: One credible blog from Circular Energy Storage Research and Consulting explains why, under some circumstances, the price of used batteries can be higher than it is for new batteries—go figure that one.

What’s your view on the broader viability of basing an ESS system on used rechargeable batteries in a second–life arrangement? Do you think that the possible negatives make it sensible only for smaller installations, where there is less to manage and fewer variables, or perhaps for larger ones, where the engineering and management effort is spread over a larger array? How do you think it compares with other ESS solutions?

By: DocMemory
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