Batteries in a Portable World 2^nd Ed.
       A Handbook on Rechargeable Batteries for Non-Engineers

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8. Choosing the Right Battery

What causes a battery to wear down — is it mechanical or chemical? The answer is ‘both’. A battery is a perishable product that starts deteriorating from the time it leaves the factory. Similar to a spring under tension, a battery seeks to revert to its lowest denominator. The rate of aging is subject to depth of discharge, environmental conditions, charge methods and maintenance procedures (or lack thereof). Each battery chemistry behaves differently in terms of aging and wear through normal use.

8.1 What’s the best battery for mobile phones?

When buying a replacement battery, the buyer often has the choice of different battery chemistries. Li-ion and Li-ion polymer batteries are used on newer phones, whereas the NiMH and NiCd are found in older models. If the buyer has a choice, the sales person may advise a customer to go for the highest capacity rating and to stay away from the NiCd because of the memory effect. The customer may settle for the slim-line NiMH because it offers relatively high capacity in a small package and is reasonably priced.

Seemingly a wise choice, an analysis in this chapter reveals that other chemistries may have served better. The NiMH offers good value for the price but falls short in expected cycle life. Although excellent when new, the performance trails off quickly after about 300 cycles due to decreased capacity and rising internal resistance. In comparison, the Li-ion can be used for about 500 cycles. The best cycle count is achieved with NiCd. Properly maintained, the NiCd delivers over 1000 cycles and the internal resistance remains low. However, the NiCd offers about 30 percent less capacity compared to the NiMH. In addition, the NiCd is being removed from the mobile phone market because of environmental concerns.

Switching to environmentally friendlier batteries is fitting, especially in the mobile phone market where the NiMH performs reasonably well and can be economical. The battery disposal issue is difficult to control, particularly in the hands of a diverse user group.

The NiMH and NiCd are considered high maintenance batteries, which require regular discharge cycles to prevent what is referred to as ‘memory’. Although the NiMH was originally advertised as memory-free, both NiCd and NiMH are affected by the phenomenon. The capacity loss is caused by crystalline formation that is generated by the positive nickel plate, a metal shared by both systems.

Nickel-based batteries, especially NiCd’s, should be fully discharged once per month. If such maintenance is omitted for four months or more, the capacity drops by as much as one third. A full restoration becomes more difficult the longer service is withheld.

It is not recommended to discharge a battery before each charge because this wears down the battery unnecessarily and shortens the life. Neither is it advisable to leave a battery in the charger for a long period of time. When not in use, the battery should be put on a shelf and charged before use. Always store the battery in a cool place.

Is the Li-ion a better choice? Yes, for many applications. The Li-ion is a low maintenance battery which offers high energy, is lightweight and does not require periodic full discharge. No trickle charge is applied once the battery reaches full charge. The Li-ion battery can stay in most chargers until used. The charging process of a Li-ion is, in many ways, simpler and cleaner than that of nickel-based systems, but requires tighter tolerances. Repeated insertion into the charger or cradle does not affect the battery by inducing overcharge.

On the negative side, the Li-ion gradually loses charge acceptance as part of aging, even if not used. For this reason, Li-ion batteries should not be stored for long periods of time but be rotated like perishable food. The buyer should be aware of the manufacturing date when purchasing a replacement battery.

The Li-ion is most economical for those who use a mobile phone daily. Up to 1000 charge/discharge cycles can be expected if used within the expected service life of about two to three years. Because of the aging effect, the Li-ion does not provide an economical solution for the occasional user. If the Li-ion is the only battery choice and the equipment is seldom used, the battery should be removed from the equipment and stored in a cool place, preferably only partially charged.

So far, little is known about the life expectancy of the Li-ion polymer. Because of the similarities with the Li-ion, the long-term performance of both systems is expected to be similar. Much effort is being made to prolong the service life of lithium-based systems. New chemical additives have been effective in retarding the aging process.

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