Civil Engineering Reference
In-Depth Information
FIGURE 10-8
Temperature effect on internal resistance in 25 Ah nickel-cadmium cell.
FIGURE 10-9
Charge efficiency versus state-of-charge at various charge rates.
charge rate of C/40, on the other hand, the charge efficiency at 60 percent
SOC is only 50 percent.
10.4.6
Self-Discharge and Trickle Charge
Even with open-circuit terminals, the battery slowly self-discharges. In order
to maintain the full state of charge, it is continuously trickle-charged to
counter the self-discharge rate. This rate is usually less than one percent per
day for most electrochemistries in normal working conditions.
After the battery is fully charged, the charge efficiency drops to zero. Any
additional charge will be converted into heat. If overcharged at higher rate
than the self-discharge rate for an extended period, the battery would over-
heat posing a safety hazard of potential explosion. For this reason, battery
