not fully charged and remains discharged. Sulfated batteries should be charged slowly to prevent damage. SLI batteries (starting, lighting, and ignition) are not designed for deep discharge, and their life is reduced when subjected to this. Starting batteries have plates designed for increased surface area and thus high instant current capability, whereas marine (hybrid) and deep cycle types will have thicker plates and more room at the bottom of the plates for spent plate material to gather before shorting the cell. Car batteries using lead-antimony plates require regular topping-up with pure water to replace water lost due to electrolysis and evaporation. By changing the alloying element to calcium, more recent designs have reduced the rate of water loss. Modern car batteries have reduced maintenance requirements, and may not provide caps for addition of water to the cells. Such batteries include extra electrolyte above the plates to allow for losses during the battery life. Some battery manufacturers include a built-in hydrometer to show the state of charge of the battery. The primary wear-out mechanism is the shedding of active material from the battery plates, which accumulates at the bottom of the cells and which may eventually short-circuit the plates. This can be substantially reduced by enclosing one set of plates in plastic separator bags, made from a permeable material. This allows the electrolyte and ions to pass through but keeps the sludge build up from bridging the plates. The sludge largely consists of lead sulfate, which is produced at both electrodes. Environmental impact Battery recycling of automotive batteries reduces the need for resources required for the manufacture of new batteries, diverts toxic lead from landfills, and prevents the risk of improper disposal. Once a lead–acid battery ceases to hold a charge, it is deemed a used lead-acid battery (ULAB), which is classified as hazardous waste under the Basel Convention. The 12-volt car battery is the most recycled product in the world, according to the United States Environmental Protection Agency. In the U.S. alone, about 100 million auto batteries a year are replaced, and 99 percent of them are turned in for recycling. However, the recycling may be done incorrectly in unregulated environments. As part of global waste trade, ULABs are shipped from industrialized countries to developing countries for disassembly and recuperation of the contents. About 97 percent of the lead can be recovered. Pure Earth estimates that over 12 million Third-World