A sodium-ion battery is a rechargeable battery that uses sodium ions as charge carriers instead of lithium ions. This practical guide explains how sodium-ion batteries work, their main components, advantages and limitations, and how they compare with lithium-ion batteries in energy density, low-temperature performance, cost, supply-chain considerations, and manufacturing maturity. It also explores potential applications in solar and energy storage systems, backup power, low-temperature environments, electric vehicles, and compact battery packs, while explaining why sodium-ion technology should be viewed as a complementary battery chemistry rather than a universal replacement for lithium-ion.