This invention offer a rechargeable-battery controlling device and an independent power system that is cheap and low power consumption. First current is created by amplifying and copying current flow through one diode or plural serially connected diodes, and first voltage is created by flowing said first current through resistance load and converting to voltage, and switching element is on-off-controlled by said first voltage, and the switching element works to prevent overcharge and overdischarge. This detects an alteration of voltage and the device protects from overcharge and overdischarge. By this, by small parts, the protection circuit for overcharge and overdischarge can be realized and power consumption can be suppressed.