The present disclosure provides for advantageous electrolyte filling systems/methods utilizing predetermined pressure/flow cycles. The disclosed electrolyte filling system enables filling of at least one jelly roll assembly under vacuum. The disclosed electrolyte filling system is capable of simultaneously filling at least two multi-core lithium ion batteries. The disclosed multi-core lithium ion batteries include a plurality of jelly roll assemblies positioned within cavities, which are further positioned within a sealed enclosure. The disclosed jelly roll assemblies are electrically connected to at least one bus bar. The disclosed bus bars include at least one flexible connective structure to electrically connect to a sealed enclosure.