When charging multichemistry batteries with different cell capacities, the battery voltage can be higher or lower than the supply voltage at various stages of the charging. Thus, the supply voltage needs to be either boosted or attenuated to match the battery voltage. For example, a supply voltage of 3.3V must be attenuated, or bucked, when a single-cell NiMH (nickel-metal-hydride) battery with a typical voltage of 1.25V is being charged. When a single-cell, 4.1V lithium-ion battery is used, the input voltage needs to be boosted. Read more on EDN.