NiMH Charging Types

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Basics

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 the lithium-ion and lithium-polymer cells now typically used for portable devices.

NiCd cells are charged by applying a constant current in the range 0.05C to more than 1C. 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 -δV phenomenon is most useful for charging NiCd cells of 0.5C or greater. The -δV 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 a fixed-temperature cutoff. Charge termination based on a combination of δT/dt and -δV cut-off enables a longer life cycle by avoiding overcharge.

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

Because the charge acceptance of a NiCd battery is close to 100 percent, almost all energy is absorbed during the initial 70 percent 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 buildup. Above 70 percent 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 0.02C to 0.1C

Charging nickel-metal-hydride cells

Though similar to NiCd chargers, an NiMH charger employs the δT/dt method, by far the best method for this type of cell. The end-of-charge voltage depression for NiMH batteries is smaller, and for small charge rates (below 0.5C, depending on temperature) there may be no voltage depression at all.

New NiMH batteries can show false peaks early in the charge cycle, causing the charger to terminate prematurely. Moreover, an end-of-charge termination by -δV detection alone almost certainly ensures an overcharge, which in turn limits the number of charge/discharge cycles possible before the battery fails.

It seems there is no available -dV/dt algorithm that works well for charging NiMH batteries under all conditions: new or old, hot or cold, and fully or partly discharged. For that reason, don’t charge an NiMH battery with a NiCad charger unless it uses the dT/dt method for end-of-charge termination. And because NiMH cells do not absorb overcharge well, the trickle charge must be lower (about 0.05C) than that recommended for NiCd cells.

Slow-charging a NiMH battery is difficult, if not impossible, because the voltage and temperature profiles associated with a C-rate of 0.1C to 0.3C do not provide a sufficiently accurate and unambiguous indication of the full-charge state. The slow charger must therefore rely on a timer to indicate when the charge cycle should be terminated. Thus, to fully charge a NiMH battery you should apply a rapid charge of approximately 1C (or a rate specified by the battery manufacturer), while monitoring both voltage (δV=0) and temperature (dT/dt) to determine when the charge should be terminated.

Efforts should be made to charge at room temperature. Nickel-based batteries should only be fast-charged between 10°C to 30°C. Below 5°C and above, the charge acceptance of nickel-based batteries is drastically reduced. Table 2 summarises the charging techniques for the various battery types.

Last Amendment: 04/12/2006 22:38:20