Battery chargers are used to recharge batteries which, after discharge, can be restored to a fully-charged state. Rechargeable batteries or secondary cells differ in terms of design and chemistry. Iron electrode batteries feature good energy density and low self-discharge, but have a low efficiency and provide poor low-temperature performance. They are more expensive than lead acid batteries, a type of rechargeable battery that accounts for almost 60% of all batteries sold worldwide. Battery chargers for lithium batteries are designed to support backup power sources for electronic equipment. When the battery is charged, lithium ions are driven from the cathode into the anode; when the charge is removed, the lithium ions flows back to the cathode. Other types of rechargeable batteries include metal or air batteries, nickel-cadmium or Ni-Cd batteries, and batteries that use chemistries such as nickel-hydrogen (Ni-H₂ ), nickel-metal hydride (NiMH), nickel-zinc (Ni-Zn), and silver oxide (AgO). Rechargeable zinc, alkaline, and manganese dioxide batteries are good low-cost alternatives, but their useful capacity is only about two-thirds that of primary cells. Battery chargers for these products are commonly available.