Page 76 - North American Clean Energy January/February 2019 Issue

energy storage
Warranties and Performance
Guarantees in the Stationary
Energy Storage Market
How are they validated?
by Davion Hill, Ph.D.
e growth of the energy storage industry comes with greater exposure to commercial risk. Battery manufacturers are addressing this risk with warranties built on self-developed testing and models. But this creates inconsistency on warranty o erings in terms of cycles or throughput and operational limitations. Buyers and sellers, seeking a way to objectively match technology to its intended market, need a standardized testing approach that supports guarantees with objective data.
Most battery manufacturers have developed standard (unadjusted) warranties for stationary storage, based on their experience in the automotive industry. e legacy automotive battery system was expected to meet a 10-year, 100,000-mile warranty. In the late 2000’s, an electric car was expected to have a range of 30-60 miles, and charge once per day. is use case amounted to 300-365 cycles per year, a 10-year calendar life, and an average total cycle life of ~3,000-3,650 cycles, until reaching 80 percent remaining capacity. Today, battery manufacturers are still using this data to market their product.
However, this convention is no longer relevant to stationary storage, or electric vehicles. Modern electric vehicles have 200+ miles of range with larger battery packs. Battery costs are almost 1/10th of what they were 10 years ago, and a vehicle may only be charged once per week. In a 10-year period, this amounts to 500-600 cycles, with the battery pack spending most of a week in intermediate states of charge (SOC), especially if the vehicle is opportunistically charged; the cycle life is less important than the amount of time the battery spends at di erent SOCs. Because of these uncertainties, automotive manufacturers have invested millions in their own battery testing facilities to perform their own diligence on battery lifetimes. Recent, independent testing has shown that battery degradation is dependent on the SOC rest states of a battery, and that the most sensitive SOC conditions vary greatly across batteries.
Another precedent from the lead acid and uninterruptible power supply (UPS) battery industry is the concept of “Depth of discharge” (DOD), which implies the battery must always start from a full SOC before discharging. e DOD is measured as the percentage of discharge, i.e., if the battery discharges to 75 percent SOC, the DOD is 25 percent. In many stationary markets, Li-ion batteries can operate at many SOC conditions without reaching a fully charged state each cycle. erefore, the term DOD has very little meaning for Li-ion batteries, which make up 95 percent or more of the market in stationary storage.
When energy storage project developers issue requests for proposals (RFP) from stationary storage vendors, these legacy conventions are still evident in the responses. e standard o ering from a battery manufacturer is a 10-year warranty, guaranteeing 80 percent remaining capacity at the end of life, under an assumed DOD of 100 percent. e senior engineering, procurement, and construction contractor (EPC) will usually base their capacity guarantee on this warranty. Yet, these criteria are inappropriate for most stationary storage markets.
Once the vendor is selected, the battery manufacturer will likely revise the warranty o ering with linear capacity degradation models (based on data that the manufacturer will not disclose). However, publicly available testing results demonstrate that degradation is not linear; it’s greatly dependent on the SOC, temperature, and current (Amps) to which the battery cells are exposed. erefore, the original warranty o erings in RFP responses have no lasting impact, and should not be considered empirical in the evaluation of preliminary bids. e later revisions to the warranty lack transparency, and introduce technical risk to the contractual guarantees in the project.
e cycling needs of a stationary storage battery are rarely as simple as one cycle per day with a 100 percent SOC setpoint. In frequency regulation markets, the battery transits the entirety of the SOC range multiple times daily, in partial cycles, with an intermediate SOC setpoint. In a demand charge avoidance use case, the battery may cycle infrequently, and idle in a partially charged state. In a weather-dependent solar self-supply scenario,
a battery is charging with solar power during the day, while remaining in a partially depleted state overnight. Given these complexities, it is better to discard the convention of “cycle life”, and instead think in terms of “throughput”, or the condition-dependent
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