A rechargeable battery technology that dates back to the mid 1800s. Liquid-filled lead acid batteries are used in vehicles, but gel-filled units, which came out in the early 1970s, provide the most economical battery for UPS systems and electric carts (scooters, wheelchairs, etc.). Lead acid was also used in some early portable computers. Providing the least amount of charge per pound of the rechargeable batteries, the lead acid technology uses lead plates and an acid electrolyte. See batteries.