Slow charge is usually defined as a charging current that can be applied to the batteryindefinitely without damaging the cell (this method is sometimes referred to as a tricklecharging).The maximum rate of trickle charging which is safe for a given cell type is dependent onboth the battery chemistry and cell construction. When the cell is fully charged, contin-ued charging causes gas to form within the cell. All of the gas formed must be able torecombine internally, or pressure will build up within the cell eventually leading to gasrelease 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 chemis-try, but also on the construction of the internal electrodes. This has been improved innewer cells, allowing higher rates of trickle charging.The big advantage of slow charging is that (by definition) it is the charge rate thatrequires no end-of-charge detection circuitry, since it can not damage the batteryregardless 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 RatesNI-CD: most Ni-Cd cells will easily tolerate a sustained charging current of c/10 (1/10 ofthe cell's A-hr rating) indefinitely with no damage to the cell. At this rate, a typicalrecharge time would be about 12 hours.Some high-rate Ni-Cd cells (which are optimized for very fast charging) can toleratecontinuous trickle charge currents as high as c/3. Applying c/3 would allow fully charg-ing the battery in about 4 hours.The ability to easily charge a Ni-Cd battery in less than 6 hours without any end-of-charge 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 asingle power resistor to limit the current.NI-MH: Ni-MH cells are not as tolerant of sustained charging: the maximum safe tricklecharge rate will be specified by the manufacturer, and will probably be somewherebetween 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.
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