Testing, scoring, and validating are all among the favorite terms of project developers and investors. After all, the practice reduces risk and increases trust for novel technology, whatever the industry. With that in mind, Norway-based technical consultancy DNV GL has introduced the third iteration of its Battery Scorecard.

This year, 22 companies entered batteries for the scorecard. Even if no company agreed to have their names disclosed, DNV GL says it will continue to work with its technology partners to foster transparency but it remains an ongoing process.

Looking at product trends over recent years, the analysts determined LFP (lithium-iron-phosphate) batteries are becoming increasingly popular again, including for stationary storage application. That chemistry – prismatic LFP – dominated from 2012-15 but was supplanted in 2016 by nickel-manganese-cobalt (NMC) and nickel-cobalt-aluminum (NCA) chemistries in pouch, prismatic and cylindrical form. Now, however, market dynamics primarily driven by CATL, BYD and other Chinese battery cell manufacturers are observing a trend back to LFP.

Bigger

Another clear trend is ever-larger cells for storage products with today's LFP cells having capacities of around 200 Ah. The reason is, larger cells enable savings on system bill-of-materials costs, as their intermediate arrangement into modules can be skipped and larger cells combined directly to packs.

Further innovation is expected in the choice of material for electrodes and in battery structure and system architecture, with grand innovations in battery chemistry not anticipated in the next few years. DNV GL expects lithium-ion chemistries to maintain their role as first-choice when it comes to storage. Rival mixes are not expected to usurp lithium-ion over the next three to seven years, said the consultant, with the battery chemistry benefiting from economies from scale as it is deployed in transport, consumer electronics, and storage applications.

With lithium-ion cells costing around $100/kWh at present, according to DNV GL, the analyst predicts whole-system prices will dip below that mark over the next decade.

Solar-plus-storage

The co-location of energy storage with solar or wind power assets is another trend observed by the analyst. As a result, project developers and owners demand the battery component of their systems has a service life of 20-25 years, to match solar arrays. Grid scale storage companies have responded to that demand, by including complete overhaul, augmentation, and operations and maintenance services in battery system prices.

The way batteries are used has also changed, said DNV GL. In the early days of energy storage, merchant ancillary services projects were the predominant front-of-meter asset type. Behind the meter, peak-load charge mitigation drove demand. Today, however, storage projects are increasingly used to time-shift solar power production from noon to later in the day. That prompts different technical requirements for batteries including their cycling stability and how they degrade under varying state-of-charge levels.

DNV GL tested the cycling stability of the scorecard's 22 products and determined the number of turnovers required to incur 1% of capacity loss. The analyst has shifted towards the term ‘turnover', in preference to ‘cycles', as the former implies degradation with each charging cycle. Hence, the number of turnovers is calculated as the sum of a battery's total cumulative discharge ampere hours divided by its nameplate capacity. Turnover is approximately the same value as cycles when the battery is new but the number of turnovers diminishes faster as the battery ages.

In this year's scorecard, the average turnover count was 381, with wide variation. For LFP products, the count was 135-448 turnovers; NMC batteries offered 180-849; NCA types yielded 143-330; and the sole titanate battery scored 1,067 turnovers per 1% loss of capacity.

Turnover count

The testing done at the BEST Test & Commercialization Center in Rochester, New York concluded, on average, a battery can offer 1,800 turnovers before falling to 90% of nameplate capacity. The scorecard highlighted such degradation must be understood as a function of temperature. If all cycling happens at 10 degrees Celsius, battery capacity drops to around 85%, on average, after just 1,000 turnovers. The DNV GL team observed such temperature sensitivity across all battery chemistries. The titanate device performed best in that respect, retaining 90% of nameplate capacity after 8,609 turnovers. Two undisclosed NMC products were the next best, offering 6,410 and 4,500 turnovers before losing 10% of their capacity.

In state-of-charge (SOC) terms, the scorecard found a 50-80% SOC window in which NMC batteries were more prone to intermediate degradation. LFP devices tended to experience the window at 30-40% SOC. DNV GL said it is essential to evaluate which degradation vector is dominant for a battery project: SOC, charge rate, or temperature. The latter two are usually the culprits in battery degradation. Operational ranges dependent on SOC can be a minor consideration, depending on cell characteristics, the scorecard researchers said.

C-rate

Charge rates, or the C-rate, are more relevant factors, with a lower C-rate generally better for batteries. In DNV GL's test set-up, LFP and titanate products generally sustained higher C-rates, though the testers pointed to the fact numerous NMC batteries also performed well. The primary defect at high C-rate was elevated temperature, which can be mitigated with the liquid cooling often found in today's storage systems.

The scorecard also noted developments in safety terms. Standards such as the UL9540A protocol demand improved testing and have initiated “at least a step toward transparency in safety.” According to DNV GL, however, there is confusion about new standards classification and the absence of pass-fail criteria. Containerized solutions are evolving such that compartmentalization means systems can be accessed fully from the exterior, avoiding the risks of operations and maintenance staff having to enter the container. The DNV GL scorecard added, many vendors are focusing on improving fire safety standards and preventing cascading thermal runaways.