Environmental Engineering Reference
In-Depth Information
(4.95)
(4.96)
The Wh efficiency is always lower than the Ah efficiency because the battery
voltage during charging is higher than during discharging. The Ah efficiency
of a lead-acid battery is between 80 and 90 per cent depending on the battery
type; the Wh efficiency is about 10 per cent lower.
Battery
self-discharge
, which causes additional losses, reduces the system
efficiency. The self-discharge rate increases with the temperature and is about
0.3 per cent per day or 10 per cent per month at temperatures of 25°C.
However, some battery types provide lower self-discharge rates.
The
usable capacity
of a battery depends on the discharge current, as
shown in Figure 4.42. The capacity decreases with higher discharge currents
and the end of discharge voltage is reached earlier.
To compare different rechargeable battery types, the capacity is often
reported in combination with the discharge duration.
C
100
means that this
capacity is usable when the discharge current is chosen so that the battery
Figure 4.42
Usable Capacity Related to C
100
= 100 A h of a Lead-Acid
Battery as a Function of the Discharge Current and Temperature
