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Batteries/Alkalline Secondary Page 080430
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Rayovac Varta  (Sept, 02) Rayovac and Varta Combine to Form Consumer Battery Business
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(Sept, 02) Q: Will the reusable Alkaline battery have a future?
(Ask Isidor)
A: The reusable alkaline was introduced in 1992 as an alternative to disposable batteries. The battery was promoted as a low-cost power source for wireless communications as well as medical and defense applications, but the breakthrough never occurred. Today, the Reusable Alkaline occupies a small market share and is used mainly for portable entertainment devices and flashlights. The lack of market appeal is regrettable when considering the environmental benefit of having to discard fewer batteries. It is said that the manufacturing cost of the regular and reusable Alkaline are very similar.
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 cycles 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.
The reusable Alkaline is designed for repeated recharge. Unfortunately, there is a loss of 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 Nickel-metal hydride. When the battery was discharged and recharged using the manufacturer’s charger, the reusable Alkaline settled at 60 percent, a capacity slightly below that of Nickel-cadmium. 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 high internal resistance, resulting in a 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 other rechargeable batteries. Whereas the Nickel-cadmium 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 nonreusable Alkaline that provides 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 reusable Alkaline achieved measurable cost savings in the low-intensity care unit in which the flashlights were used only occasionally. The high-intensity care unit, which used the flashlights constantly, did not attain the same result. Deeper discharge and more frequent recharge reduced the service life and offset any cost advantage over the standard Alkaline battery.
When considering the reusable Alkaline, one must realize that the energy density is lower than the on the standard Alkaline. The benefit of reusable Alkaline is expressed in the ability to recharge. Unlike a regular rechargeable battery, however, each subsequent recharge provides lower capacities. Overcoming this limitations and applying a charge before the energy is fully depleted will realize substantial cost savings for battery users.
Advantages
· Inexpensive and readily available — can be used as a direct replacement for non-rechargeable (primary) cells.
· More economical than non-rechargeables – 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.
Ed. Note: Before publishing Isidor’s story, we corresponded with him about the weakness of rechargeable Alkaline in that it fails after the first cycle if allowed to fully disharge. Because the battery is usually used in low cost applications, additional of electronics to safeguard it against excessive discharge, as done with Lithium-ion, are not present. The unknowing general consumer who places the rechargeable Alkaline in a radio or calculator, allowing total discharge, probably will not be able to recharge the cell. Although there could be a special situation with proper support hardware which would make rechargeable alkaline suitable, BD would caution the consumer in selecting rechargeable Alkaline. Common size, Nickel-cadmium and Nickel-metal hydride cells are usually more cost effective solutions. DKG
BD