Battery chargers and charging methods

The charging method is the process that dictates the method by which energy returned to the battery. If the energy is not returned to the battery in a way that it is compatible with the chemistry, the recharge can have detrimental effects on the battery. In the worst case, incorrect recharging can cause safety event (damage to equipment, personnel injury and environmental contamination). There are several generic charging methods, each of which is discussed below.

Constant Current Charging

Constant current charging is the simplest method of charging employing single low level current to the discharged battery. The current is set at a fixed rate that is usually selected at ten percent of the maximum rated capacity of the battery. Constant current charging is best suited for use on nickel cadmium (sealed) and nickel metal hydride batteries. The typical charge characteristics of a NiCd/NiMH cell can be seen in Figure 1. The type of charger is usually small and relatively inexpensive. The only disadvantage of this charging method is that, if the battery is overcharged, gassing and overheating of the battery may occur. This results in shorter performance and frequent battery replacement.

Figure 1 Charge characteristics of a NiCd/NiMH cell

Typical terminations methods (algorithms) for NiCd/NiMH cells as presented in Figure 2

Figure 2 Comparison of charge termination methods: TCO, ∆T/∆t, and -∆V

It was found that the cycle life and the capacity of sealed NiCd/NiMH batteries is strongly depends on charge termination method as described in Figure 3

Figure 3 Cycle life and capacity as a function of charge termination method for sealed NiCd/NiMH batteries.

Constant Potential Charging Constant potential charging allows the maximum current of the charger to flow into the battery until its voltage reaches a preset voltage limit. This system allows for higher charging currents, thus returning the battery to a full state of charge quicker. Once the voltage limit is reached, the current starts to taper to a minimum value see Figure 4

Figure 4 Charge stages of lead –acid cell. A multi-stage charger applies constant-current charge, toping charge and float charge.

As the current tapers to minimum, the maximum energy has been transferred into the battery. At third point the battery can be left on the charger until needed in what is referred to as a " Float Charge" which compensates for the normal self-discharge that occurs in any battery. This system works well especially with batteries that exhibit a voltage rise at the end of charge, such as the lead acid battery and NiCd pocket plate batteries. Constant potential charging is detrimental to NiCd sealed batteries, which exhibit a drop in voltage when the battery goes into overcharge and begins to heat up, causing the voltage to drop. Some other chemistry, especially Li-ion, are not able to absorb additional energy once fully charged and must be removed from the charger.

Constant Current/ Constant Potential Charging

Constant Current/ Constant Potential Charging is a combination of the two methods above. The system is designed to limit the maximum charger current until the battery voltage reaches the set limit. Then, the voltage control takes over, allowing the current to taper to a minimum value as the battery voltage nears full charge. The combination of Constant Current/ Constant Potential allows for fast charging without the problems of gassing and overheating

due to charging at high rates. This method is especially useful for sealed lead acid and pocket type NiCd batteries.

2.4.4 Smart Charging Smart charging (Figure 5) adjusts the voltage and current supplied to the battery based on the monitoring of critical battery parameters (temperature, cell balancing). Battery charger operation can be optimized by using a micro controller to carefully monitor and adjust the charging rate, the time, and, in some cases, the voltage. This optimization is used to increase charging efficiency, reduce charging time, or extend cell life. Use of smart charging is critical to charging rechargeable lithium chemistries to prevent the activation of the battery's internal safety features, which, if activated, render the battery useless. The charge rates and times are optimized to the specific battery chemistry and internal conditions during charging.

Figure 5 –Charge stages of Li-ion, Li-polymer batteries. Increasing the charge current on a Li-ion charger does not shorten the charge time by much. Although the voltage peak is reached quicker with high current, the topping charge will take longer.