Specifications

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While there are many varieties and configurations of nickel metal hydride
batteries, there are no significant variances as far as the discussed advantages
and disadvantages. The dimensioned battery under consideration for this project
is shown in Figure 2.11.3-1. There are new developments in the consumer
market for higher efficiency nickel metal hydride cells, such as the Sanyo
Eneloop line of batteries. These batteries have significantly reduced self-
discharge rates, higher shelf life, and increased recharge cycles. However, due
to their availability generally only in AA size, they are not considered here for
reasons similar to the physical issues mentioned in the alkaline section.
Figure 2.11.3-1: A 7.2V nickel metal hydride battery pack. Reproduced with
permission from www.batteryspace.com
2.11.4 Lithium Ion Battery
Lithium ion batteries are a somewhat recent technology for the consumer market.
Adaption was driven primarily by the need for a type of battery with no memory
effect and with a high number of recharge cycles in use in mobile and portable
phones. As such, it is also being considered for the portable unit of this project.
Although lithium ion batteries are constantly being improved, their overall
disadvantages and advantages are still valid when comparing this battery type to
others being considered.
There is no significant size penalty for lithium ion. Indeed, it is the most compact
of all the solutions considered. As mentioned previously, it is also able to
withstand partial charges and recharges better than other types of batteries.
However, it can also degrade due to age (rather than use) as well. Most lithium
ion batteries do not typically make it past two or three years. However, this may
be acceptable considering the target use and consumer of this product.