02. Battery Chemistries 9

GUIDE: Batteries in a portable world. 2. Battery Chemistries 9

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2.7 Reusable Alkaline Batteries

The idea of recharging alkaline batteries is not new. Although not endorsed by manufacturers, ordinary alkaline batteries have been recharged in households for many years. Recharging these batteries is only effective, however, if the cells have been discharged to less than 50 percent of their total capacity. The number of recharges depends solely on the depth of discharge and is limited to a few at best. With each recharge, less capacity can be reclaimed. There is a cautionary advisory, however: charging ordinary alkaline batteries may generate hydrogen gas, which can lead to explosion. It is therefore not prudent to charge ordinary alkaline unsupervised.

In comparison, the reusable alkaline is designed for repeated recharge. It too loses charge acceptance with each 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’ reusable alkaline cells showed a very high capacity reading on the first discharge. In fact, the energy density was similar to that of a NiMH battery. When the battery was discharged, then later recharged using the manufacturer’s charger, the reusable alkaline settled at 60 percent, a capacity slightly below that of a NiCd. Repeat cycling in the same manner resulted in a fractional capacity loss with each cycle. In our tests, 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 1V/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 mobile phones and video cameras.

The reusable alkaline is inexpensive but the cost per cycle is high when compared to the nickel-based rechargeables. Whereas the NiCd checks in at $0.04 per cycle based on 1500 cycles, the reusable alkaline costs $0.50 based on 10 full discharge cycles. For many applications, this seemingly high cost is still economical when compared to the non-reusable alkaline that has a one-time use. If the reusable alkaline battery is only partially discharged before recharge, an improved cycle life is possible. At 50 percent depth of discharge, 50 cycles can be expected.

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 using the flashlights only occasionally achieved measurable savings by employing 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.

In summary, the standard alkaline offers maximum energy density whereas the reusable alkaline provides the benefit of allowing some recharging. The compromise of the reusable alkaline is loss of charge acceptance after the first recharge.

Advantages and Limitations of Reusable Alkaline Batteries

Advantages

Inexpensive and readily available — can be used as a direct replacement of non-rechargeable (primary) cells.

More economical than non-rechargeable – allows several recharges.

Low self-discharge — can be stored as a standby battery for up to 10 years.

Environmentally friendly — no toxic metals used, fewer batteries are discarded, reduces landfill.

Maintenance free — no need for cycling; no memory.

Limitations

Limited current handling — suited for light-duty applications like portable home entertainment, flashlights.

Limited cycle life — for best results, recharge before the battery gets too low.

Figure 2-9:       Advantages and limitations of reusable alkaline batteries.

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