5 ELECTRIC VEHICLE BATTERY FAST CHARGING VRLA and NiMH batteries exhibit low internal resistance and are capable of very high discharge and charge rates. This is exhibited by modern battery modules, which are manufactured using thin electrodes with large active areas, allowing for charge and discharge rates of 10C to 20C. Excessive overcharging can on the contrary become catastrophic on these batteries.
THE FAST CHARGING PROCESS The fast charging technique for traction batteries account for the battery charge acceptance. The charger adjusts the charge rate continually to match the ability of the battery to accept the charge. Danger from excessive overcharging can be avoided, and the battery modules can arrive at the charge in 20 to 30 minutes. This fast charge also enhances the battery life and provides higher battery efficiency (charge recovery). Battery modules are sensitive to overcharge. The internal resistance of the battery is generally an obstacle to the fast charge and discharge process. Under room temperature conditions the maximum charge/discharge rate of the battery with low internal resistance may be as high as 15C to 30C at room temperature. The charge rate 1C or one-hour rate is the charge/discharge rate in amperes equal to the capacity of the battery in ampere-hours. Similarly, the 10C charge rate is 10 times higher and also referred to as the six-minute rate. A high-rate discharge of the battery does not cause a catastrophic failure. The battery voltage collapses, and the battery current to the load ceases. A high-rate charge on the other hand can result in serious battery damage. Thus making it important to know when to stop and how to stop. A fully discharged battery in a 0% SOC can accept current at the highest, initial charge rate. A fully charged battery at 100% SOC cannot be charged any further. Thus passing additional charge current through 95
