The idea of recharging alkaline batteries is not new. Although not endorsed by the manufacturers, ordinary alkaline batteries have been recharged for over 15 years for such applications as flashlights on commercial aircraft. There is a danger in recharging ordinary alkaline, however, in that they pose a risk of generating hydrogen gas which can lead to explosion.
One of the major differences between a reusable and a standard alkaline is in the energy density. The standard alkaline offers maximum energy density whereas the reusable alkaline provides the benefit of allowing some recharging but compromises on the energy density after the first recharge. The longevity of the reusable alkaline is a direct function of the depth of discharge; the deeper the discharge, the fewer cycles the battery can endure.
Tests performed by Cadex on "AA" size cells showed a very good discharge capacity on the first discharge. In fact, the energy density was much higher than that of an equivalent NiCd battery. After a discharge and recharge with the manufacturer’s charger, however, the rechargeable alkaline settled at 60%, a capacity slightly below that of a NiCd. Repeated cycling in the same manner resulted in a fractional capacity loss with each cycle applied. For our test, AA cells were used and the discharge current was adjusted to 200mA (0.2 C-Rate or one fifth of the rated capacity); the end-of-discharge threshold was set to one volt per cell.
An additional limitation of the reusable alkaline system is its low load current capability of 400mA (lower than 400mA provides better results). Although adequate for portable AM/FM radios, CD players, tape players and flashlights, 400mA is insufficient to power most cellular phones and transceivers.
The purchase price of the reusable alkaline is very low but the cost per cycle is high when compared to the NiCd. Whereas the NiCd checks in at $0.04 per cycle based on 1500 cycles, the reusable alkaline costs $0.50 based on 10 cycles. For many applications, this seemingly high cost still represents a large saving when compared to non-reusable alkalines.
To compare the operating cost between the standard and reusable alkaline, a study was done on flashlight batteries for hospital use. The low-intensity care unit that used the flashlights only occasionally achieved measurable savings by using the reusable alkaline. The high-intensity unit that used the flashlights constantly, on the other hand, did not attain the same result. Deeper discharge and more frequent recharge reduced their service life and offset any cost advantage over the standard alkaline battery.