A charging method and system for rationing charge or energy supplied by a host to a portable device. The system includes a power switch, and a current sensing module connected to the power switch. The current sensing module detects instantaneous current drawn by the portable device. The system further includes a current register connected to the current sensing module for storing the instantaneous current value. A timing module generates timing information. The system also includes a charge register storing a cumulative charge drawn by the portable device. The cumulative charge is obtained by multiplying the instantaneous current value with the timing information. A threshold database stores a threshold value, and a rationing module connected to the host and the charge register continuously compares the cumulative charge value with the threshold charge value. When the cumulative charge value exceeds the threshold charge value, a control signal is generated.