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Li-Ion is a commonly used battery nowadays..

"Lithium ion batteries can be formed into a wide variety of shapes and sizes, so as to efficiently fill available space in the devices they power.

The batteries are also lighter than equivalents in other chemistries — often much lighter. This is because lithium ions have an extremely high charge density — the highest of all known naturally occurring ions. Li ions are small and mobile, but more readily stored than hydrogen. Thus a battery based on lithium is smaller than one with hydrogen elements, such as nickel metal hydride, and with fewer volatile gases. The ions need fewer storage intermediaries, so more battery weight is usable as charge, instead of overhead.[citation needed]

Li-ion batteries do not suffer from the memory effect. They also have a low self-discharge rate of approximately 5% per month, compared with over 30% per month and 20% per month in nickel metal hydride batteries and nickel cadmium batteries, respectively.[citation needed]

According to one manufacturer, Li-Ion cells (and, accordingly, "dumb" Li-Ion batteries) do not have any self-discharge in the usual meaning of this word.[1] What looks like a self-discharge in these batteries is a permanent loss of capacity, described in more detail below. On the other hand, smart Li-Ion batteries do self-discharge, due to the small constant drain of the built-in voltage monitoring circuit. This drain is the most important source of self-discharge in these batteries."