Energy Storage - Wind and Solar Power Systems - page 165

FIGURE 10-7

Temperature effect on charge/discharge ratio.

Therefore, the round trip energy efficiency is as follows:

12
145 11

. ⋅ C

η energy = = 075 . or 75 percent

. ⋅ . ⋅ C

10.4.4 Internal Resistance

The above efficiency calculations indicate that 25 percent energy is lost per
charge/discharge cycle, which is converted into heat. This characteristic of
the battery can be seen as having an internal resistance R

. The value of R

i i

is a function of the battery capacity, operating temperature and the state of
charge. The higher the cell capacity, the larger the electrodes and the lower
the internal resistance. The R varies with the state of charge as per

Equation 10-1. It also varies with temperature as shown in Figure 10-8 , which
is for a high quality 25 Ah NiCd cell.

i

10.4.5 Charge Efficiency

The charge efficiency is defined as the ratio of the Ah being deposited
internally between the plates over that delivered to the external terminals
during the charging process. It is different from the energy efficiency. The
charge efficiency is almost 100 percent when the cell is empty of charge, the
condition in which it converts all Ah received into useful electrochemical
energy. As the state of charge approaches one, the charge efficiency tapers
down to zero. The knee point where the charge efficiency starts tapering off
depends on the charge rate ( Figure 10-9 ). For example, at C/2 charge rate,
the charge efficiency is 100 percent up to about 75 percent SOC. At a fast