Why/When should rechargeable batteries be used instead of disposable batteries?
Disposable batteries such as alkalines, heavy-dutys, etc. are relatively cheap and commonly available. Why and when should disposable batteries be substituted for the more expensive and less commonly available rechargeable batteries?
Take alkalines as an example for disposable batteries. An AA sized alkaline battery has a typical capacity of 2800mAh. Think of this as the amount of energy stored in a battery. The bigger the number, the more energy it can supply and hence, all things being equal, it should provide an equipment with longer operating hours. Current rechargeable batteries such as an AA sized Nickel Metal Hydride (Ni-Mh), on the other hand, has a typical capacity of 1600mAh (1800mAh AA sized Ni-Mhs are already available in the market today). It would appear that alkalines should provide longer operating hours compared to even a rechargeable battery with the highest rated capacity. The truth is, this is not always the case. Apart from capacity ratings, there are other factors that determine the useful lifespan of a battery.
An AA sized disposable alkaline battery is typically rated at 1.5V. During usage, voltage will drop linearly from 1.5V down to less than 1.0V. A Ni-Mh rechargeable battery, on the other hand, starts from 1.4V when fully charged. During usage, it will drop to 1.2V and will stay around this voltage for most of the time. Once it is almost drained, voltage will drop off sharply from 1.2V down to 1.0V. For high drain devices such as digital cameras, flash units, etc., the equipment will stop operating once voltage drops to about 1.0V. Although the alkaline battery has a higher capacity, the additional energy stored cannot be utilised under such circumstances. The additional energy stored is basically wasted. Comparatively, the Ni-Mh rechargeable battery is capable of maintaining a voltage of 1.2V for most of its usable lifespan. This is the reason why rechargeable batteries provide much longer operating hours for high drain equipment as compared to disposable alkalines. Due to their operating characteristics, rechargeable batteries can also be used instead of disposable alkalines for most applications.
