samlexsolar SCC-30AB, Owner'S Manual, Page: 22 / 48

22
3
GENERAL INFORMATION: BATTERIES
The voltage efficiency is determined by the average discharge voltage (V
D
) and
average charging voltage (V
C
). V
C
is lower than V
D
particularly by internal resistance
of the battery.
η
V
=V
D
/ V
C
(3.5)
The overall efficiency (
η
S
) should be as high as possible, to be able to pass the biggest
proportion of the energy in the battery, which is generated by the PV generator
system, further to consumers.
- Self-discharge
The battery discharges itself even without load connected. This effect is caused by
secondary reactions at its electrodes and proceeds faster with higher temperature or
in older batteries. Thermodynamic instability of the active materials and electrolytes as
well as internal and external short-circuits lead to capacity losses, which are defined as
self-discharge. This loss should be small, particularly in respect of annual storage.
- Maintenance cost
The maintenance, e.g. water refilling in case of Lead-Acid batteries, should be kept
as low as possible.
- Easy installation and operation
Since batteries are often used also by non-experts, easy installation and operation
are, therefore, favorable.
- Power
In special cases, battery must be highly loadable for a short time, e.g. at the start of
diesel generators or in case of momentary power extension of PV systems.
There are many types of batteries potentially available for use in stand-alone PV
systems. Useful data of available batteries given in Table 3.1 shows approximated
values and are provided as a guideline.
Table 3-1: Comparison between selection criteria of available batteries
Type
Cycle life until 80
% DOD
Coulombic
Efficiency
η
I
[%]
Self-discharge
[%/month]
Temp. range
[°C]
Lead Acid 500...1500 > 80 3…4 -15°...+50°
Ni-Cd 1500...3500 71 6…20 - 40°...+45°
Ni-Fe 3000 55 40 0°…+40°