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Linear Technology develops battery monitor IC for electric vehicles

Posted: 03 Feb 2015     Print Version  Bookmark and Share

Keywords:Electric vehicles  hybrid vehicles  LTC6804  battery monitor IC 

Advances in battery technology have enabled some of the most interesting innovations in the automotive market, giving rise to new generations of electric vehicles (EVs) and hybrid/electric vehicles (HEVs). New applications such as energy storage systems (ESS) are also emerging, which could revolutionise how energy is created, distributed and stored. Designers of these systems face significant challenges of cost, design flexibility, battery pack reliability and lifetime, and safety.

The battery management system (BMS) plays a defining role in how well a battery stack meets each of these design challenges. At the heart of the BMS is a battery monitoring integrated circuit (IC). This IC measures individual cell voltages which are used to determine state of charge and battery stack health.

The most critical characteristics of a battery monitor IC are accuracy, data robustness and fault detectability to ensure safety. The accuracy of the monitor IC directly affects system cost, battery pack reliability and lifetime. Each cell has a limited capacity which must be carefully managed. Overcharging can cause safety and reliability issues, while over-discharging can affect the lifetime of the cell. Using a less accurate monitor IC requires that the system designer use larger "guard bands" to protect against overvoltage and undervoltage, therefore limiting the amount of total available capacity for the vehicle. A higher accuracy monitor IC can make use of more or each cell's total capacity, reducing the total cost of the battery stack system.

LTC6804

The LTC6804 from Linear Technology can measure up to 12 series-connected battery cells at voltages up to 4.2V with 16bit resolution and better than 0.04 per cent accuracy.

For the best accuracy over time and harsh operating conditions, Linear Technology uses a sub-surface Zener voltage reference in the LTC6804 Battery Monitor IC. This results in a guaranteed total cell voltage measurement error of less than 1.2mV. To maintain the best accuracy measurements in the presence of electrical noise and transients from inverters, actuators, switches, relays, etc., the LTC6804 uses delta-sigma ADCs with built-in third order noise filtering.

Ideally, a battery stack would be divided into smaller modules which are distributed throughout the vehicle for better design flexibility and weight distribution. The challenge is that the battery modules need to communicate sensitive measurement data in an electrically noisy and physically harsh environment. CANbus was originally designed to provide robust communications in harsh automotive environments, but the data throughput demands of raw BMS data and component costs have kept CANbus from being adopted in EVs and HEVs. For this reason, Linear Technology has created the isoSPI interface to provide low cost robust communication for up to 100m cable lengths.


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