Battery chemistry and capacity determine runtime, weight and safety. Lithium-ion cells are common and offer reliable energy in a compact form, while some listings use lithium‑ion polymer which can be lighter and thinner but demands proper cooling and matching connectors. Capacity, expressed in watt-hours (Wh), directly translates to how long the battery can power the Latitude before recharging; higher Wh typically means longer use between charges. When choosing, pair the chemistry with a voltage of 11.1V and review the Wh to fit your daily workload. In this listing you’ll find both Li‑ion and Li‑ion polymer options designed for the Latitude E5400 family, so pick based on your priority—reliability or lighter weight.