disposable batteries must compete with cheap wind-up, wind-powered and rechargeable devices that have proliferated. Comparison between primary and secondary cells Secondary cells (rechargeable batteries) are in general more economical to use than primary cells. Their initially higher cost and the purchase cost of a charging system can be spread out over many use cycles (between 100 and 1000 cycles); for example, in hand-held power tools, it would be very costly to replace a high-capacity primary battery pack every few hours of use. Primary cells are not designed for recharging between manufacturing and use, thus have battery chemistry that has to have a much lower self-discharge rate than older types of secondary cells; but they have lost that advantage with the development of rechargeable secondary cells with very low self-discharge rates like low self-discharge NiMH cells that hold enough charge for long enough to be sold as pre-charged. Common types of secondary cells (namely NiMH and Li-ion) due to their much lower internal resistance do not suffer the large loss of capacity that alkaline, zinc–carbon and zinc chloride ("heavy duty" or "super heavy duty") do with high current draw. Reserve batteries achieve very long storage time (on the order of 10 years or more) without loss of capacity, by physically separating the components of the battery and only assembling them at the time of use. Such constructions are expensive but are found in applications like munitions, which may be stored for years before use. Polarization A major factor reducing the lifetime of primary cells is that they become polarized during use. This means that hydrogen accumulates at the cathode and reduces the effectiveness of the cell. To reduce the effects of polarization in commercial cells and to extend their lives, chemical depolarization is used; that is, an oxidizing agent is added to the cell, to oxidize the hydrogen to water. Manganese dioxide is used in the Leclanché cell and zinc–carbon cell, and nitric acid is used in the Bunsen cell and Grove cell. Attempts have been made to make simple cells self-depolarizing by roughening the surface of the copper plate to facilitate the detachment of hydrogen bubbles with little success. Electrochemical depolarization exchanges the hydrogen for a metal, such as copper (e.g. Daniell cell), or silver (e.g. silver-oxide cell), so called. Terminology Anode and cathode The battery terminal (electrode) that develops a positive voltage polarity (the carbon electrode in a dry