NOVELTY - The system has a voltage detector i.e. voltage-reading element, linking batteries to detect voltage of the batteries. A micro control unit (MCU) (20) on a battery protection board receives signals from the voltage detector for characteristic differences between the batteries balanced by an equalizer and the batteries with similar charging and discharging efficiency. A digital interface i.e. universal asynchronous receiver/transmitter (UART) (10), is connected between the MCU and an upper-level control system as an interface of signal transmission.    USE - Battery system for use in a vehicle-borne system i.e. electric car-borne system.    ADVANTAGE - The system developed in a battery monitoring system delivers direct serial connection of the batteries, which have same specifications so as to monitor digital signals required by the system. The system allows a serial and parallel connection of the batteries and allows the batteries to be freely changed during maintenance, thus reducing costs in battery design and maintenance. The system simplifies complexity of a digital interface circuit for series-parallel batteries, and effectively shrinks an area of a circuit on the digital interface. The system is designed such that voltage level of a UART signal at an off stage is one, so that an inverted signal with a phase shifted from an actual signal by 180 degree is utilized in the up-link interface, thus effectively reducing power consumption at the off stage.    DETAILED DESCRIPTION - The digital interface comprises an up-link interface and a down-link interface that deliver voltage level protection signals including a low-voltage protection (LVP) signal, over-voltage protection (OVP) signal and a wake-up signal to wake up the MCU. The up-link interface/down-link interface is made up of a voltage isolator and a logic gate circuit. The voltage isolator is selected from a group consisting of a photo-coupler, photo-MOSFET and a photo-replay. The logic gate circuit is constructed with standard logic gates, which are composed of bio-polar junction transistors (BJT), FET and a MOSFET.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram illustrating signals communicated between a MCU and a digital interface in a battery system.    UART (10)    Low voltage protection signal (15)    Over voltage protection signal (16)    UART transmit signal (17)    MCU (20)