As grid energy storage systems become more complex, it grows more difficult to design them for safe
operation. This paper first reviews the properties of lithium-ion batteries that can produce hazards in
grid scale systems. Then the conventional safety engineering technique Probabilistic Risk Assessment
(PRA) is reviewed to identify its limitations in complex systems. To address this gap, new research is
presented on the application of Systems-Theoretic Process Analysis (STPA) to a lithium-ion battery based
grid energy storage system. STPA is anticipated to fill the gaps recognized in PRA for designing complex
systems and hence be more effective or less costly to use during safety engineering. It was observed that
STPA is able to capture causal scenarios for accidents not identified using PRA. Additionally, STPA enabled
a more rational assessment of uncertainty (all that is not known) thereby promoting a healthy skepticism
of design assumptions. We conclude that STPA may indeed be more cost effective than PRA for safety
engineering in lithium-ion battery systems. However, further research is needed to determine if this
approach actually reduces safety engineering costs in development, or improves industry safety
standards.