While other industries have codes, standards, and guidelines that dictate the components necessary in a system and that Battery Systems design based on full-scale, real-world testing, for energy storage projects the responsibility has historically landed on the system owner or integrator’s shoulders. Too many have given in to the siren song of lowest cost, and the industry has seen a series of fires and failures as a result. There hasn’t been an easy way to know which products are truly ready for prime time and which will fail under the extreme conditions these projects demand.
Thermal cycling breaks connections over time
The expansion and contraction of battery cells during each charge and discharge cycle is a common phenomenon. While the movement is minimal in a single cell, if you magnify it across a utility-scale Battery Energy Storage System which contains thousands of cells in a racked configuration, the cumulative mechanical stress applied on fasteners is significant and continuous.
Traditional fasteners were not made to face such a situation. When exposed to repeated thermal cycling, the clamping force provided by a standard fastener degrades. Hence, a bolt which was torqued to its specification at the time of installation can slowly become loose over the months with no apparent evidence of failure until the connection point begins to arc or overheat. Engineers are, thus, increasingly opting for fasteners which have torque retention features engineered inside in addition to the spring-loaded-like Belleville washer hardware or any similar counterparts which will constantly apply pressure even when the surrounding materials begin to shift.
There is no wiggle room when it comes to making the connection right. A loose connection on a busbar which is conducting hundreds of amperes will generate heat. Heat in a densely packed battery compartment is nothing but the starting points of thermal runaway.
Dissimilar metals create a hidden corrosion problem
Aluminum battery cases, copper busbars, and steel mechanical hardware are all necessary components of a battery energy storage system – all readily available, lightweight, and cost-effective materials. Individually, each metal brings unique properties to the whole. Together, though, they create galvanic corrosion risk. Galvanic corrosion is an electrochemical reaction that occurs when two dissimilar metals come into electrical contact in the presence of an electrolyte. The less “noble” metal – aluminum in this case, which is at the anode of the electrolyte – corrodes while the more “base” metal sacrifices it.
This is an established process that has been studied for centuries and is why you might see zinc blocks bolted to the hulls of naval ships. The problem with ships, in this analogy, is that batteries keep getting more expensive and the zinc block stays the same price. If there’s a way to minimize the size of that zinc block or even get rid of it altogether, you do it. In the battery business, you do that with plating. Zinc-nickel or tin plating, to be precise. Project developers working with specialized platforms like BESSBASE can cross-reference system specifications and verify that the fasteners and electrical hardware coming into direct contact with battery cells are coated with the right zinc-nickel or tin plating before a single rack gets installed.
Fire containment standards are reshaping structural hardware
UL 9540A testing has altered the performance expectations for what structural racking and enclosure hardware needs to do. The standard quantifies how thermal runaway propagates from one cell module to the adjacent ones, and the results directly influence the necessary design and construction of the rack.
Hardware that mechanically collapses and allows a thermal event to escape a cabinet also allows fire to spread laterally and vertically through a cabinet. Racking systems must now be engineered to not only hold load under normal conditions but maintain integrity under the localized extreme heat that a single runaway cell can generate. Likewise, with metals to disperse that heat, non-metallic brackets and insulators used within these assemblies are also quantified, with the UL 94 V-0 flammability rating as a new baseline requirement for plastic components in proximity to cells.
Outdoor installations raise the bar on environmental sealing
Over the years, the industry has learned many of these lessons the hard way. It’s not unusual to find a site where latches or hinges have already begun rusting on structures put in service in the past decade. Retrofitting components isn’t cheap, but capex to replace aging hardware is less of a concern than the lost operational expenditure from a premature failure. In some cases, BESS OEMs and integrators are taking proactive steps in the design phase to specify components intended for 10+ year operating lifetimes, even as they maintain every-six-month torque checks on each mechanical connection of their existing customer fleets.
High-voltage architecture demands electrical isolation at the hardware level
The transition from 1000V to 1500V DC systems meant not only new inverters and cabling but also a change in the role of structural hardware as an electrical component. At 1500V, the risk from arc flash events is high enough to turn hardware clearances, grounding paths, and non-conductive coatings of structural components into design requirements, rather than design considerations.
Integrated grounding schemes, where the hardware itself is part of the electrical safety architecture, are considered best practice on well-designed systems, and therefore hardware selection cannot be partitioned from electrical engineering. A structural bolt in the wrong location, without the right coating or isolation, is a potential fault path.
Furthermore, as global cumulative energy storage capacity is projected to exceed 500 GW by 2030, the scale of that sort of supply chain cannot support the level of custom-fabricated hardware that goes into small-build energy storage systems today. It takes standardized, pre-fabricated, utility-grade balance-of-system components, meeting all applicable safety standards, to make that sort of buildout possible, and work on utility-grade standard-setting bodies to raise the bar with each new generation of larger systems.
