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The battery in our OLPC XO review sample was a 4-cell 3,100mAh LiFePO4 (lithium iron phosphate) unit, although 5-cell nickel metal hydride batteries (NiMH) are also used. Among the advantages of lithium iron phosphate are the absence of heavy metals and the ability to support more charge/discharge cycles than conventional Li-ion cells (OLPC quotes 2,000 in this case). Battery life figures vary, but in our simple rundown tests we got around 3.5 hours with the screen backlight on (colour/transmissive mode) and 4.5 hours with the backlight off (greyscale/reflective mode).
The Intel Classmate's battery is not designed to be easily removed — you need to undo four screws to get the protective cover off and two more to release the battery itself. Having done this, you discover a bulky and relatively weighty 6-cell 4,000mAh Li-ion unit. Intel claims around four hours' usage for the Classmate on battery power: this seems optimistic in our experience, although we have yet to formally test this.
The Eee has the most compact battery pack, a 4-cell 4,400mAh Li-ion unit for which ASUS claims 2.8 hours' life, which seems reasonable in our (so far anecdotal) experience.