A Li-ion battery pack includes a Li-ion battery, a protection circuit, and multiple plugs connectable to a power hand tool. The positive pole of the Li-ion battery and the DC supply terminal of the protection circuit are electrically connected to two contact pieces of one respective plug. The two contact pieces are short-circuited by the positive contact terminal of the power hand tool. The negative contact terminal of the power hand tool is connected to a negative-pole contact piece of one plug, so that when the potential difference between the positive and negative poles of the Li-ion battery surpasses the low voltage level defined by the protection circuit, the protection circuit switches on the connection between the negative pole of the Li-ion battery and the negative-pole contact piece for enabling the positive and negative contact terminals of the power hand tool to obtain DC power supply from the Li-ion battery.
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