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Li ion Battery Monitoring Solutions: Accuracy and Safety

Mon, 12/21/2009 - 4:11am
Andrew Glascott-Jones & Thierry Masson, e2v

Li ion is an obvious choice to power future electric vehicles (EVs) and hybrid electric vehicles (HEVs).

It has a number of advantages: a high-cell voltage of 3.6 Volts means that fewer cells, with their associated connections and electronics, are needed for high voltage batteries. It has very high energy density (about 4 times better than Lead acid) and very high-power density.

However there are a number of challenges: the internal impedance is higher, it has a flat discharge characteristic which means that the state of charge is difficult to determine and the high-energy density has safety implications. This energy, if released in an uncontrolled way through a short circuit or physical damage, can have catastrophic consequences. These challenges mean that most Li ion battery stacks operate today with the use of a monitor system, and the high-voltage battery stacks common in EVs/HEVs require a large amount of complexity in their monitoring systems.

Requirements
Battery stacks can have over 200 cells which provide a challenge for the integration of the monitor system. One of the main functions of battery monitor design is cell protection. The purpose of cell protection is to provide the necessary monitoring and control to protect the cells from out of tolerance ambient or operating conditions and to protect the user from the consequences of battery failures

Requirements for Monitoring Systems are as follows: 
• Ability to accurately measure multiple cells; the typical Battery Monitoring System (BMS) IC will measure from 6 to 12 cells. 
• Temperature Measurements, since over temperature degrades performance and can lead to thermal runaway. 
• Precise measurement of charging/discharging current.

Other features of a Li ion BMS would be 
• Cell equalization 
• Protection from over/under charge 
• Cell disconnection detection

The large number of cells in a typical EV/HEV stack means that dedicated monitor ICs become attractive due to cost and size issues.

Fig.1-e2v

 

Li ion technology issues
Li ion cells can be extremely dangerous if mistreated. They may explode if overheated or if charged to an excessively high voltage. Furthermore, they may be irreversibly damaged if discharged below a certain voltage.

Cell balancing: if overheated or overcharged, the cell degrades and in an extreme form this can result in fire or explosion. So hardware and software protection is needed to prevent this. However, series cells can result in cell imbalance which can also cause degradation of battery lifetime. There are different tolerances between cell in a battery stack and weaker cells exhibit high voltage at full charge. In discharge, the weak cells have lower voltage. Balancing is performed by passive bleeding or active current transfer between cells.

High accuracy required in the monitor system because the slope of the State of Charge (SOC) vs. cell voltage is not very steep. In fact, for some materials it is almost non-existent which places added importance on the precision of the cell current measurement circuitry. To be able to measure the SOC with greater accuracy means that battery life can be extended. For a 5V cell 0.1% error means 5mV accuracy over the full temperature range. The accurate measure of SOC also has direct financial implications since the charge remaining in a battery will have a value should the battery be sold on or downgraded for other less rigorous applications, just like the petrol in the tank of a car.

In addition, because of high-load transients, the cell voltages should be sampled with a small amount of jitter, less than 10µs being ideal. Also, this should be synchronised with the measure of the current.

Safety aspects
The number of safety incidents involving Li ion technology is rising. There have been recent recalls of Li-Ion batteries used in certain mobile computers. The Sony Li-ion recall was also widely publicized in 2006. 

Safety critical monitoring of cell over-voltage, cell under-voltage, excessive charge/discharge currents, under/over temperature situations are just a few of the measures that must be taken.

High-power cells can be particularly dangerous. They contain large amounts of energy which, if released in an uncontrolled way through a short circuit or physical damage, can have catastrophic consequences. In the case of short circuits, currents of hundreds of amps can build up in microseconds and protection circuits must be very fast acting to prevent this. The BMS IC should also perform open circuit monitoring and a number of self-check verifications to ensure that its operation is not compromised.

Other safety features also required for commercial Li-ion batteries include: 
• shut-down separator (for overtemperature), 
• tear-away tab (for internal pressure) 
• vent (pressure relief) 
• Shock detector for shutdown in the event of a crash

Implementation
The high-battery stack voltage means that isolation is required between BMS components while still allowing communication down the stack.

This stack communication can use capacitor or current steering techniques to achieve isolation: a capacitive solution has the advantage that true isolation is obtained.

Another consideration in the design of the IC architecture is the varying accuracy if a potential divider approach is used for the measurement. Another approach might use a separate ADC for each cell, but this would add cost to the IC. One novel architecture which solves this problem is the use of an isolating multiplexor with sample and hold.

Fig.2-e2v

The Ideal Solution
The ideal solution would incorporate: 

• A resolution of 14-bit with low jitter simultaneous sampling. 
  
• An accuracy of 2mV 

• A temperature measurement channel per cell 

• A separate failsafe over/under voltage monitor for each cell 

• Many other safety functions (cell open/short voltage, test signals, alarm signals). 

• Integrated discharge transistors 

• A fast, secure communications channel with a response time of 1ms to read the stack 

Fig.3-e2v

 A BMS IC with these characteristics would enable the optimum lifetime for a battery while maintaining high levels of safety.

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