Specifications

will be charged reverse from how they should be charged. Essentially, deep

discharging solves the issue of premature voltage dropoff, but it also creates a

bigger problem of prematurely ending the battery pack’s life. So with either case,

a battery with limited usefulness happens after only a few uses.

The size for such a battery is acceptable, however, as is the cost for the battery.

The optimal nickel cadmium battery would be a common 6V hobby battery pack,

commonly used in many electronics. A dimensioned image of this battery pack

can be seen in Figure 2.11.2-1. While the specific milliamp hour (mAh) rating of

the battery may change as the power requirements for the device change, the

overall battery pack will still be very similar.

Figure 2.11.2-1: A 7.2V nickel cadmium battery pack. Reproduced with

permission from www.batteryspace.com .

2.11.3 Nickel Metal-Hydride Battery

Nickel metal hydride batteries are another widely used battery technology. While

they offer fewer recharge cycles over its useful life compared to a nickel

cadmium battery, it offers the advantage of not being susceptible to the

previously mentioned “memory” effect. Like nickel cadmium, nickel metal hydride

batteries are also a mature technology with well understood charging schemes.

In addition, the environmental penalty of a metal hydride type battery is less than

that of a cadmium-based battery, as it can be recycled easier and with less toxic

cost to the environment.

The primary disadvantages with this technology are the higher self-discharge

rate compared to other technologies, and also the aforementioned reduced

number of recharge cycles. While this is a concern, it is not as much of a problem

as the issue with the memory effect when recharging nickel cadmium batteries.

The self-discharge rate can be as high as 30% per month. However, since the

battery will be stored in the device and often charged under normal

circumstances, this should not be a concern, as the high discharge often refers to

conditions in open air with an open circuit.