he alkaline-manganese battery is an example of the type of battery that until some years ago was available only in non-rechargeable (primary) form, but has since become available in rechargeable (secondary) form. It must be said, however, that this is primarily so in the USA and Canada; in Europe they are still very scarce, but demand (and supply) is growing. Manufacturers that supply rechargeable alkaline manganese cells and batteries include Union Carbide (Eveready), Ray-O-Vac (both USA) and Pure Energy Corporation (Canada). Today alkaline-manganese batteries are available also in many online stores.
These batteries use a unique electrochemical system, are maintenance free, hermetically sealed, and will operate in any position. Their discharge characteristics (voltage decrease, internal resistance and self-discharge) are very similar to those of a primary alkaline-manganese battery. Their capacity is comparable to that of a nickel- metal-hydride (NiMH) battery, that is, somewhat higher than that of a NiCd battery, but rather lower than that of a primary alkaline-manganese battery. Like NiMH batteries (but in contrast to NiCd batteries), rechargeable alkaline-manganese batteries do not contain heavy metals. They are somewhat dearer than NiCd batteries, but cheaper than NiMH batteries.
Charging (pulsed charging at a constant voltage of 1.8 V) is rather different from that of nickel-based batteries. The charging time for AA/R6/HP7 size batteries is 16 – 18 hours: fast charging is not (yet) possible. On the other hand, the battery can be charged at any time, irrespective of the state of residual charge: discharging it beforehand is not necessary.
The charge retention properties of secondary alkaline manganese batteries are as good as those of primary batteries.
The cell uses electrodes of powdered zinc and manganese dioxide with an electrolyte of potassium hydroxide. These are put together as shown and the cell is then hermetically sealed. The voltage per cell is 1.5 V. Batteries of higher voltage are made by connecting the requisite number of (similar) cells in series and sealing them in a metal case.
When the cell is being discharged, the manganese dioxide gives off hydrogen, which reduces its mass, while the zinc reacts with the hydrogen to form zinc oxide. When the cell is being charged, the zinc oxide is reduced to zinc again which is made possible, among others, by the separators (made of non-woven fabric) that are much stronger than those in a primary cell.
When energy is withdrawn from the cell, the terminal voltage drops slowly. The total voltage drop for a given energy withdrawal increases as the number of discharge/charge cycles rises. Furthermore, the available energy per cell diminishes with each discharge/charge cycle although the e.m.f. remains constant. If the power demands exceed the rated battery capacity, the cells cycle life will decrease more quickly. Nevertheless, when the cell is discharged at the maximum rate for a period of time and then recharged as recommended by the manufacturers, the discharge/charge cycle can be repeated many times before the cell e.m.f. will drop below 0.9 V.