The lead-acid battery, the first rechargeable battery, was invented in 1859. More than 160 years later, they are still in wide use due to their low cost, ruggedness, broad temperature range of operation, and the ability to provide very high surge currents for applications such as starting engines. Although lithium-ion batteries are beginning to displace them in the market, all of the uninterruptible power supplies at Fourmilab use valve regulated lead-acid batteries which, since power outages are very rare, usually last from four to six years before requiring replacement.