The European Union Aviation Safety Agency (EASA) is the EU body responsible for civil aviation safety regulation and certification. EASA develops Certification Specifications (CS) and Acceptable Means of Compliance (AMC) for aircraft, engines and equipment across Europe.
For electric and hybrid-electric propulsion systems and their battery energy storage, EASA has issued Special Condition E-19 (SC E-19), which provides the regulatory framework covering battery system design, thermal runaway containment, failure mode analysis and crew warning requirements for high-power traction batteries.
EASA Standards on batterydesign.net
References: European Union Aviation Safety Agency (official site)
See also: Federal Aviation Administration (FAA)