NI-CD/NI-MH CHARGING INFORMATION
In the realm of battery charging, charging methods are usually separated into two general categories: Fast charge is typically a system that can recharge a battery in about one or two hours, while slow charge usually refers to an overnight recharge (or longer). Slow Charge Slow charge is usually defined as a charging current that can be applied to the battery indefinitely without damaging the cell (this method is sometimes referred to as a trickle charging). The maximum rate of trickle charging which is safe for a given cell type is dependent on both the battery chemistry and cell construction. When the cell is fully charged, continued charging causes gas to form within the cell. All of the gas formed must be able to recombine internally, or pressure will build up within the cell eventually leading to gas release through opening of the internal vent (which reduces the life of the cell). This means that the maximum safe trickle charge rate is dependent on battery chemistry, but also on the construction of the internal electrodes. This has been improved in newer cells, allowing higher rates of trickle charging. The big advantage of slow charging is that (by definition) it is the charge rate that requires no end-of-charge detection circuitry, since it can not damage the battery regardless of how long it is used. This means the charger is simple (and very cheap). The big disadvantage of slow charge is that it takes a long time to recharge the battery, which is a negative marketing feature for a consumer product.
Slow Charge Rates
NI-CD: most Ni-Cd cells will easily tolerate a sustained charging current of c/10 (1/10 of the cell's A-hr rating) indefinitely with no damage to the cell. At this rate, a typical recharge time would be about 12 hours. Chester Simpson N National Semiconductor Some high-rate Ni-Cd cells (which are optimized for very fast charging) can tolerate continuous trickle charge currents as high as c/3. Applying c/3 would allow fully charging the battery in about 4 hours. The ability to easily charge a Ni-Cd battery in less than 6 hours without any end-ofcharge detection method is the primary reason they dominate cheap consumer products (such as toys, flashlights, soldering irons). A trickle charge circuit can be made using a cheap wall cube as the DC source, and a single power resistor to limit the current. NI-MH: Ni-MH cells are not as tolerant of sustained charging: the maximum safe trickle charge rate will be specified by the manufacturer, and will probably be somewhere between c/40 and c/10. If continuous charging is to be used with Ni-MH (without end-of-charge termination), care must be taken not to exceed the maximum specified trickle charge rate. Fast Charge Fast charge for Ni-Cd and Ni-MH is usually defined as a 1 hour recharge time, which corresponds to a charge rate of about 1.2c. The vast majority of applications where Ni-Cd and Ni-MH are used do not exceed this rate of charge. It is important to note that fast charging can only be done safely if the cell temperature is within 10-40°C, and 25°C is typically considered optimal for charging. Fast charging at lower temperatures (10-20°C) must be done very carefully, as the pressure within a cold cell will rise more quickly during charging, which can cause the cell to release gas through the cell's internal pressure vent (which shortens the life of the battery). The chemical reactions occurring within the Ni-Cd and Ni-MH battery during charge are quite different: The Ni-Cd charge reaction is endothermic (meaning it makes the cell get cooler), while the Ni-MH charge reaction is exothermic (it makes the cell heat up). The importance of this difference is that it is possible to safely force very high rates of charging current into a Ni-Cd cell, as long as it is not overcharged. The factor which limits the maximum safe charging current for Ni-Cd is the internal impedance of the cell, as this causes power to be dissipated by P = I2R. The internal impedance is usually quite low for Ni-Cd, hence high charge rates are possible. There are some high-rate Ni-Cd cells which are optimized for very fast charging, and can tolerate charge rates of up to 5c (allowing a fast-charge time of about 15 minutes). The products that presently use these ultra-fast charge schemes are cordless tools, where a 1 hour recharge time is too long to be practical. The exothermic nature of the Ni-MH charge reaction limits the maximum charging current that can safely be used, as the cell temperature rise must be limited. At present, there are no makers of Ni-MH batteries that recommend charging rates faster than 1.2c (and the chances of that changing are not very good).
