Page 100 - Lindens Handbook of Batteries

FACTORS AFFECTING BATTERY PERFORMANCE 3.19

Low-rate (high-capacity) battery

High-rate battery
Capacity

Discharge current
FIGURE 3.22 Comparison of performance of batteries
designed for high- and low-rate service.

Another popular electrode design in the flat-plate construction, typically used in the lead-acid
SLI (starting-lighting-ignition) and most larger storage batteries (Fig. 3.21c). This construction
also provides a large surface area for the electrochemical reaction. As with the other designs, the
manufacturer can control the relationship between surface area and active material (for example, by
controlling the plate thickness) to obtain the desired performance characteristics.
A modification of this design is the bipolar plate illustrated in Fig. 3.21d. Here the anode and cath-
ode are fabricated as layers on opposite sides of an electronically conductive but ion-impermeable
material which serves as the intercell connector.
Most battery chemistries can be adapted to the different electrode designs, and some, in fact, are
manufactured in different configurations. Manufacturers choose chemistries and designs to optimize
the performance for the particular applications and markets in which they are interested.
In Fig. 3.22, the performance of a battery designed for high-rate performance is compared with
one using the same electrochemical system, but optimized for capacity. The high-rate batteries have
a lower capacity but deliver a more constant performance as the discharge rate increases.
Hybrid Designs. Hybrid designs, which combine a high energy power source with a high-rate
power source, are becoming popular. These hybrid systems fulfill applications more effectively (e.g.,
higher total specific energy or energy density) than using a single power source. The high energy
power source is the basic source of energy, but also charges a high-rate battery which handles any
peak power requirement that cannot be handled efficiently by the main power source. Hybrid designs
are being considered in many applications, ranging from combining a high-energy, low-rate metal/
air battery or fuel cell with a high-rate rechargeable battery, such as a lithium-ion system. Hybrid
electric vehicles use an efficient combustion engine with a rechargeable battery to handle starting,
acceleration, and other peak power demands, and use regenerative braking to recharge the battery.

Shape and Configuration. The shape or configuration of the cell will also influence the battery
capacity as it affects such factors as internal resistance and heat dissipation. For example, a tall,
narrow-shaped cylindrical cell in a bobbin design will generally have a lower internal resistance than
a wide, squat-shaped one of the same design and may outperform it, in proportion to its volume,
particularly at the higher discharge rates. For example, a thin AA-size bobbin type cell will have
proportionally better high-rate performance than a wider diameter D-size cell. Heat dissipation also
will be better from cells with a high surface-to-volume ratio or with internal components that can
conduct heat to the outside of the battery.
Volumetric Efficiency versus Energy Density. The size and shape of the cell or battery and the
ability to effectively use its internal volume influence the energy output of the cell. The volumetric
energy density (watthours per liter) decreases with decreasing battery volume as the percentage of