# Battery (electricity) A battery stores [[Energy]] chemically and releases it as [[Electric_current|current]], carrying its reactants inside the package. That is the defining difference from a [[Fuel_cell]], which is fed from outside and runs as long as supply lasts — a distinction of plumbing rather than of [[Electrochemistry|chemistry]]. ## What the trade actually is Energy density decides how long a device runs; power density decides how hard it can be pushed; cycle life decides how long it lasts. No chemistry maximises all three, so selection is an [[Mathematical_optimization|optimisation]] against a duty cycle. Charging is [[Electrolysis]] in miniature — driving the reaction backwards — and the losses appear as heat, which makes thermal management a [[Heat_transfer]] problem and, at scale, a [[Fault_tree_analysis|safety]] one. On this spine the battery is the comparison case: the branch that stores electrons directly, against the [[Hydrogen]] branch that stores them as a gas. **Reads with:** *Electrolysers, Fuel Cells and Batteries: Analytical Modelling (2024)* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/electrolysers-fuel-cells-and-batteries-analytical-modelling) · [download](https://books.open.tudelft.nl/home/catalog/book/176). Seeded from the Thury Hydrodynamics Apex Spine book shelf. **On the spine:** [[Electrochemistry]] · [[Fuel_cell]] · [[Electrolysis]] · [[Energy]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Battery_(electricity)) : [Wikitube](https://en.wikitube.io/wiki/Battery_(electricity)) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Energy]]. --- *Apex Spine wave · 2026-09-10 · stub · microsim layer deferred.*