Page 136 - Lindens Handbook of Batteries

BATTERY DESIGN 5.13

FIGURE 5.14 Temperature increase characteristics during charge of battery pack.

4. The design must provide for effective dissipation of heat to limit the temperature rise during
use and especially during charge. High temperatures should be avoided as they reduce charge
efficiency, increase self-discharge, could cause cell venting, and generally are detrimental to bat-
tery life. As previously mentioned, temperature gradients in the battery pack can also lead to cell
imbalance, which can also degrade performance and safety. The temperature increase is greater
for a battery pack than for an individual or separated cells as the pack tends to limit the dissipation
of heat. The problem is exacerbated when the pack is enclosed in a plastic case. This is illustrated
in Fig. 5.14, which compares the temperature rise of groups of cells with and without a battery
case. Note that the internal cell temperatures can be higher than the measured skin temperatures
of the cell, further increasing the detrimental effects.

5.4.4 Terminal and Contact Materials
Terminal material selection must be compatible with the environments of the battery contents as
well as the surroundings. Noncorrosive materials should be selected. Nickel-cadmium and nickel-
metal hydride batteries typically use solid nickel contacts to minimize corrosion at the terminal
contacts.
A number of factors must be considered when specifying contact materials. Several principles
apply to the substrate. The normal force provided by the contact must be great enough to hold the
battery in place (even when the device is dropped) and to prevent electrical degradation and any
resulting instability. Contacts must be able to resist permanent set. This refers to the ability of the