Two types of rechargeable solar batteries — nickel-cadmium (NiCd) and nickel-metal-hydride (NiMH)— power many commonly used handheld devices. The attributes of these batteries make each one appropriate for specific applications. Common sizes include AAA, AA, C, D and 9 volts for NiCd batteries and AA and AAA for NiMH batteries. A solar battery charger usually requires placement in window that faces the Earth's equator, where it will receive the most sunlight. During evening hours or on cloudy days, the solar charger can be placed under a lamp to absorb the infrared heat, although this will charge the batteries much more slowly than if the charger is in sunlight.
Rechargeable solar batteries have an established temperature range for charging and for discharging. Most battery manufacturers recommend that users avoid charging the batteries in extreme weather conditions. They also recommend charging the batteries in moderate temperatures.
Nickel-cadmium batteries have been one of the main rechargeable solar batteries on the market since the 1980s. These batteries power devices such as solar lights, emergency medical equipment, two-way radios and power tools that require high capacity and a long product life. Nickel-cadmium batteries work well in low temperatures. They also have a higher tolerance than NiHM batteries for heat. When stored, NiCd batteries have a self-discharge rate of about 20 percent a month.
NiMH batteries have been in use since 1990. These batteries' include a hydrogen-absorbing alloy, which makes them environmentally friendly and recyclable. It has a capacity that is 30 to 40 percent higher than that of a conventional NiCd battery. NiMH batteries often power devices such as cameras, laptop computers, cellular phones and hybrid automobiles.
These batteries have the same energy capacity as lithium-ion batteries, but if they are stored, they have a higher discharge rate. NiMH batteries provide a consistent power supply over a longer period of time than NiCd batteries. The life cycle of NiMH rechargeable solar batteries is shorter, however, which means that they can be recharged fewer times before they must be replaced. NiCd batteries might last for about 1,000 cycles or more, but NiMH batteries might be able to be recharged only about 500 times or less.