Linear adds high precision to stacked Li-ion battery monitor
Linear Technology has introduced a high voltage battery monitor for measuring stacked Li-ion cells to a new level of precision.
Designed for electric vehicles, but also used in stacked-battery systems used in renewable energy systems and back-up power, the latest LTC6804 battery monitor can measure up to 12 series connected battery cells at voltages up to 4.2V with 16 bit resolution and better than 0.04% accuracy.
Previous generations of the chip (LTC6802/3) incorporated a bandgap voltage reference, for this new device Linear has designed in a buried Zener voltage reference similar to references used in precision instrumentation.
This gives the monitor the higher level of precision and tolerance to temperature and aging.
“This level of accuracy is important because the discharge characteristics of a Li-ion cell are very flat and it is necessary to measure very small voltage variations,” said Greg Zimmer, product marketing engineer at Linear Technology.
Typically multiple devices are daisy-chained together to monitor large battery stacks with a total voltage of 400V and higher.
Interconnect between the devices has used the SPI interface. This is fine when cells are stacked in a single battery pack. But Linear has found there is a need for distributing the cells in separate battery packs.
For interconnecting monitor devices over the longer distances the company has designed a proprietary 2-wire isoSPI interface which provides up to 1Mbit/s data links over several metres of twisted pair. So it is possible to monitor battery packs distributed around a vehicle or building.
Two communication options are available: with the LTC6804-1, multiple devices are connected in a daisy chain with one host processor connection for all devices; with the LTC6804-2, multiple devices are connected in parallel to the host processor with each device individually addressed.
The monitor has six operating modes which also includes the low pass response of a built-in 3rd order noise filter. In the fastest mode, all cells can be measured within 290µs.
“Our aim was to get the conversion time below 1ms and we have more than achieved this with the new chip,” said Zimmer.
Residual power consumption has also been reduced, and in sleep mode, the LTC6804 draws less than 4µA from the batteries.
General purpose I/O pins are available to monitor analogue signals, such as current and temperature, and can be captured simultaneously with the cell voltage measurements.
Additional features include passive balancing for each cell with a programmable balancing timer for up to two hours, even when the LTC6804 is in sleep mode.
The chip has I2C interfaces for connecting to temperature sensors, ADCs, DACs and EEPROM. Local EEPROM can be used to store serialization and calibration data, enabling modular systems.
It has been designed to meet the environmental, reliability and safety demands of automotive and industrial applications. The LTC6804 is fully specified for operation from -40°C to 125°C. It has been engineered for ISO 26262 (ASIL) compliant systems and a full set of self-tests ensure that there are no latent fault conditions.
Along with the LTC6804, Linear Technology is introducing the LTC6820 isoSPI transceiver. The LTC6820 enables bidirectional transmission of the Serial Peripheral Interface bus (SPI) across an isolated barrier up to 100m.