Biomedical Engineering Reference
In-Depth Information
battery pack is integral to the device for reliability and sealing issues or where access to
the device is difficult, rechargeable batteries (secondary batteries) are used. Such devices
include prosthetic limbs and artificial hearts. In low-power applications where battery life
can be months and even years, then it is more practical to use primary batteries.
The oldest form of rechargeable battery still in use is the lead-acid battery. These are
wet cell
devices and mostly need to be kept upright and placed in well-ventilated areas
as they generate hydrogen if overcharged. One convenient alternative form is the gel cell,
which contains a semisolid electrolyte that prevents spillage.
Most portable rechargeable batteries are
dry cell
types, which are hermetically sealed.
These include nickel cadmium, nickel metal hydride, and lithium types. From a practical
perspective, the following rules of thumb can be used when selecting batteries for a specific
application:
•
Nickel cadmium (NiCd): These are good for devices that include motors and other
high-discharge requirements. They can accommodate heavy drain rates, but their
mAh rating is lower than more modern rechargeables and they also have a strong
memory effect.
•
Nickel metal hydride (NiMH): These batteries have a high mAh rating and can
sustain moderate to high current drain.
•
Lithium ion (Li-Ion): These have a very long shelf life and are excellent for mod-
erate to low-power applications.
•
Lithium polymer (Li-Po): These have similar chemistry to Li-Ion, but because
they are manufactured in flat sheets rather than cylinders they have a higher power
density.
A large number of primary battery types are available for different applications:
•
Zinc carbon: This is a low-cost battery good for light current drain applications.
•
Zinc chloride: This is similar to the zinc carbon but with a slightly higher power
density.
•
Alkaline: These are long-life batteries and are suitable for low and high current
drain applications. Their energy density is significantly higher than that of zinc
carbon types.
•
Silver oxide: This type is commonly used for hearing aids and watches where the
current drain is low.
•
Mercury: This type was formerly used in a wide range of devices but is seldom
used today because of toxicity issues.
•
Zinc air: These are generally used in hearing aids.
The main advantages of primary batteries are their high energy density, long storage life,
and ease of disposal (most primary batteries contain little toxic material). A regular alkaline
battery provides 50% more power than a Li-Ion, and a primary lithium battery has three
times the energy of a similar sized Li-Ion battery.
Because primary batteries have relatively high internal resistances, the maximum
current rating is limited. The energy density ratings shown in Figure 2-11 are determined
at the optimum discharge rate for that particular cell type. However, if the comparison is
made for a high-current mode application such as a prosthetic limb, then the usable energy
for each battery type is as shown in Figure 2-12.
