ABC OF RECHARGEABLE BATTERIES （2）

Published:2011/8/9 1:37:00 Author:Phyllis From:SeekIC

By Gert Helles

Charging batteries

A new rechargeable battery or battery pack (several batteries in one package) is not guaranteed to be fully charged; in fact, it is likely to be nearly discharged. The first thing to do, therefore, is to charge the battery/pack in accordance with the manufacturer’s chemistry-dependent guidelines.

Every charging operation applies voltage and current in a sequence that depends on the battery chemistry. Thus, a look at battery-cell chemistries reveals different requirements to be met by the charger and the charging algorithm. The terms most commonly found in battery charging are constant current (CC), used for NiCd and NiMH cells, and constant current/constant voltage (CC/CV), applied to lithiumion and lithium-polymer cells (Figures 1-6). Table 3 summarizes today’s most frequently used charging techniques and the associated parameters per cell type. The methods to determine the very important ’end of charge’ moment are listed separately in Table 4.

Charging nickel-cadmium cells

NiCd cells are charged by applying a constant current in the range 0.05 C to more than 1 C. Some low-cost chargers terminate the charge by means of absolute temperature. Though simple and inexpensive, that method of charge termination is not accurate. A better choice is to terminate charging when the condition of full charge is indicated by a drop in voltage. The -AV phenomenon is most useful for charging NiCd cells at 0.5 C or greater. The -AV end-of-charge detection should be combined with battery-temperature measurement as well, because aging cells and mismatched cells can reduce the voltage delta.

You can achieve a more precise full-charge detection by sensing the rate of temperature increase (dT/dt), and that method of charge detection is kinder to the battery than is a fixed-temperature cut-off. Charge termination based on a combination of AT/dt and -AV cut-off enables a longer life cycle by avoiding overcharge.

Fast charging improves charge efficiency. At 1 C, the efficiency is close to 1.1 (91%), and the charge time for an empty pack will be slightly more than one hour. When applying a 0.1 C charge, the efficiency drops to 1.4 (71%) with a charge time of about 14 hours.

Because the charge acceptance of a NiCd battery is close to 100%, almost all energy is absorbed during the initial 70% of charging, and the battery remains cool. Ultra-fast chargers use this phenomenon to charge a battery to the 70 percent level within minutes, applying currents equal to several times the C-rating without heat build-up. Above 70% the charging continues at a lower rate until the battery is fully charged. Eventually, you top off the battery by applying a trickle charge in the range 0.02 C to 0.1 C.

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