In response to typical issues concerning lithium-ion battery systems for HEVs, a cell-control specialized IC and a scheme for the entire circuit mounting the IC (namely, no isolation and a ldquodaisy-chainrdquo communication architecture) were devised. The circuit was configured with the cell-control specialized ICs with 350 nm, BiDMOS, 50 V/3.3 V analog-digital mixed process. But the circuit can handle the 400 V HEV battery without any isolator between the ICs. As a result of these innovations, significant reductions in size and cost and improvement in reliability was achieved. That is to say, in comparison to a conventional product configured with a microcomputer, optocouplers, and discrete components, the developed IC and architecture achieved an 80% reduction of the board area, 90% reduction of component numbers and a reduced failure rate to 4%.