In the invention, single battery core is grouped. Each group includes cascaded single battery in quantity no more than four pieces. Using protection IC suitable for 1-4 pieces of cascaded lithium ion batteries obtains overcharge signal and overdischarge signal of each group of battery core. Through not common grounding electrical level conversion, overcharge signal level in each group is adjusted unitedly, and synthesized as one overcharge signal. Similarly, one overdischarge signal is synthesized. Output control switch of cascaded batteries is controlled by the said synthesized overcharge signal and overdischarge signal so as to realize protection for batteries. Using protection IC suitable for 1-4 pieces of cascaded lithium ion batteries, the invention realizes protection for more than five pieces of batteries. Features are simple structure and easy of implementation.