Higher Energy Density: The Rechargeable Nickel Metal Hydride Battery
The initial salvo in this wave of commercialization of new batteries was the rechargeable Nickel Metal Hydride (NiMH) battery. Using the same form factors batteries of this type offer much higher energy density (up to a maximum of 120Wh/kg) as identical Nickel Cadmium cells (up to a maximum of eight0Wh/kg). In addition, this type of battery consumes no cadmium, which is a highly toxic heavy metal. But the higher energy density of the NiMH battery comes with a shorter total battery life (often expressed in terms of total recharge counts). Until two to three years ago, these batteries were offered as cheaper alternatives to the more expensive lithium ion batteries first made commercially available in 1991, especially in some budget notebooks. But the rechargeable NiMH battery remains the most often-used form of electrical storage for other kinds of mobile devices to this day, both in terms of number of applications and overall number of batteries deployed.
Nearly Maintenance Free: Rechargeable Lithium Ion Batteries
These days, the lithium ion (Li-Ion) battery is absolutely THE battery for notebook use. Lithium is the lightest metallic element, and lithium-based battery cells offer very high energy densities (up to a maximum of 160Wh/kg) with a low rate of internal energy loss (about 10% of total combined cell capacity per month). By contrast, both NiCd and NiMH batteries have internal energy loss rates of 20 to 30%, and fare worse by comparison. With a maximum of up to one thousand recharge cycles, the total lifetime of Li-Ion batteries isn't quite as long as that for NiCd batteries (1,500 recharge cycles max) but at least twice as high as that for NiMH varieties (500 recharge cycles max). Better still, Li-Ion batteries exhibit none of the memory effects associated with the other two types just named. This means that Li-Ion batteries do not need to be fully discharged before recharging them, and makes them comparatively maintenance-free. Their higher per-cell output of 3.6 volts allows for battery packs to be designed with a serial connection of two or more cells for use in notebooks. And providing a small mobile device like a cellphone with electrical power usually requires only a single such cell. If you wanted to employ Nickel based batteries for the same uses, per-cell output levels around 1.25 volts each demands larger numbers of cells in a series to deliver the same power levels.