The several advantages of cordless power for portable power tools and certain kitchen and domestic appliances have led to the development of a wide range of sizes of power- or battery-packs, that is, a contained group of power cells. These power cells may include nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium, or lead-acid cells, etc.
FIG. 2 shows illustrating a typical voltage and temperature curves of a battery during charging. As shown in the voltage curve V, the voltage of the battery typically does not increase linearly. Instead, the battery voltage increases until an area A is reached, where the battery voltage increases slowly. The battery voltage then rapidly increases until a voltage peak B is reached. Full charge of the battery occurs just before voltage peak B.
If the charging process is not terminated, the battery would then be overcharged, possibly damaging the battery. As shown in the voltage curve V, the battery voltage also decreases when the battery is overcharged.
During the charging process, the battery temperature also varies. FIG. 2 shows a typical temperature curve T for a battery being charged, when the battery temperature before charging is around room temperature. As indicated in temperature curve T, the battery temperature begins to rise around or after the area A is reached. The temperature then continues to increase until the charging process is terminated. If the charging process is not stopped when the battery is fully charged, the battery could be overcharged and thus damaged by the rising temperature. Accordingly, battery temperature or battery voltage are usually monitored as indicators of the full charge condition.
Among the voltage monitoring methods, the Saar double inflection termination method described in U.S. Pat. Nos. 4,388,582 and 4,392,101, is preferred to detect a battery reaching full charge. However, while the double inflection method avoids overcharging of a battery that was completely discharged, the method is difficult to implement with already fully or mostly charged batteries, as well non-expressive batteries, such as NiMH batteries, which have depressed voltage curves.
Other voltage monitoring methods more typically employed are (1) the minus-delta-voltage method, (2) the peak detect method, and (3) the voltage slope detect method. In the minus-delta-voltage method, a sample of the battery peak voltage is stored and compared to the most recent voltage. Termination occurs when the most recent voltage falls below a set point, usually within between 0.5% and 1.0% of the stored peak, or about 10 to 20 millivolts per cell.
The peak detect method is more modem version of the minus-delta-voltage method. Basically, the same method is used, except the set point can be set closer to the peak by using more accurate instrumentation. Both of these methods, however, tend to overcharge the battery, reducing battery life.
The slope detect method is another voltage monitoring method. According to this method, the voltage peak B is detected by calculating the slope of the voltage curve V, or voltage change rate (dV/dt). Termination occurs when the voltage change rate is 0 or negative. This method also tends to overcharge the battery, reducing battery life.
However, the slope detect method has another drawback. Under this method, the area A may be confused with voltage peak B because of the small slope. This could cause the termination of the charging process, resulting in undercharged batteries.
Current temperature monitoring methods are also problematic. Temperature monitoring methods typically employed are (1) absolute temperature termination and (2) temperature change rate termination. Absolute temperature termination relies on the temperature rise that occurs when the battery is fully charged. Under this method, the charging process will be stopped when the battery temperature reaches a certain temperature. However, because the largest temperature rise usually occurs after the battery is frilly charged and during the overcharge period, battery life and performance are adversely affected.
The temperature change rate termination method requires monitoring the changing slope of the battery temperature, or temperature change rate (dT/dt), during the charging process. Termination occurs when the temperature change rate reaches and/or exceeds a predetermined rate. In other words, termination occurs when a trip point is reached and/or exceeded. However, selecting the appropriate trip point is problematic, especially under conditions of varying ambient temperature conditions. Accordingly, the method may cause undercharged or overcharged batteries.
It is preferable to provide a charging and monitoring method that will not result in undercharged or overcharged batteries.