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Li Ion Battery Management in Automotive Applications

Li Ion Battery Management in Automotive Applications

Full Description

Safety, Performance and Reliability are the three attributes required of the battery pack in an automotive application. Battery management in automotive goes far beyond just managing the battery cells and keeping the pack healthy, it involves ensuring the safe operation of the pack, interfacing to the load and keeping the battery working under ideal conditions right across its State of Charge (SoC) range.

Safety again

This is a reoccuring theme and with good reason. The 300 -400volts and up to 1000amps that a modern EV battery can supply momentarily are sufficient to instantly vaporise metal and cause serious injury including burns and death, if mishandled!

A number of features need to be in place to prevent this happening:

  • • Main contactors on both legs of the pack to isolate it when not in use and during emergencies
  • • Separate power supply for powering the BMS and contactors, the pack itself cannot be used to power the BMS and switching otherwise it would be impossible to isolate the pack as well as be able to reconnect the pack when desired. This means a separate battery needs to be onboard. The complete system should form a circuit such that when power is interupted to the BMS, all of the contactors open by default (HVIL), preventing accidental electrocution during servicing or tampering with the pack.
  • • Ground fault detection. This is a check of the isolation between the -ve leg of the pack and chassis ground (earth) and is expressed in ohms per volt. If the measured resistance drops below 500ohms/volt say, this would suggest poor isolation of the battery pack from the rest of the vehicle structure, due to anything from water ingress, to a cable touching part of the vehicle chassis
  • • Excessive current flow monitoring. This is to prevent short circuits or levels of current that would cause the batteries and or the cabling to overheat damaging the pack or even starting a fire. Fast reacting fuses help here, additionally high frequency current measurement allows the BMS to react and communicate with the load to reduce current or open the contactors in emergency situations. Opening the contactors while current is flowing will cause arcing that may damage the contactor. This can be avoided by incorporating a semi conductor switch in series with the main contactor so that the current can be "strangled" before opening the contactor

Contactor welding. If closing a contactor completes a high voltage circuit where high current can flow, such as when connecting to a capacitive load (motor inverter), just as the contactor closes, arcing will occur across the contactor poles, and the subsequent bringing together of these poles when the contactor closes may fuse them together, preventing them from opening again. This means that the pack cannot be isolated and is unsafe. To avoid this, most battery packs are equipped with pre charge circuits where a parallel leg with a resistor in series is closed first allowing the current to flow in a controlled manner before the main contactor is closed. Additionally, the BMS can check for welded contactors by making a voltage measurement on the pack (battery side) and on the load/bus side (the otherside of the open contactors) while the contactors are still open. If the measured voltages are exactly the same although the main contactors are open, then most likely a contactor is welded.

Performance

Getting the best performance from a battery is a specialist science! By best performance, we mean the maximum energy and or power, without damaging the battery or shortening its lifespan. The safe power density of a battery cell in charge or discharge modes changes as a function of SoC - state of charge, temperature and SoH - state of health. To maximise the performance of the battery cells, the BMS needs to contain sufficient information about the battery cell characteristics as a function of the previously named parameters (battery model) so that the safe allowable current draw can be calculated and transmitted to the engine controller so that the batteries are not driven outside of their comfort zone. This not only prolongs cell life, but also allows more of the theoretical energy held in the cell to be safely used.

Balancing the pack

In most cases, the battery pack is charged and discharged with the cells connected in series. This measns that any slight variation in the cell characteristics will lead to the cells charging unevenly, with some charging faster than others, due to their reduced capacity. Each charge and discharge cycle will worsen this problem, and as the battery pack performance is determined by that of the weakest cell, then a capacity reduction takes place as the cells within the pack become more unevenly charged or discharged. The process of aligning the cells back to a uniform state of charge across the pack is called balancing. Balancing can be done passively, or actively.

Passive balancing involves disipating or bleeding energy from the higher cells to bring them down until all cells are equal. The level of charge in the cell is determined by its voltage. Passive balancing can be performed during discharge, charge and at the end of charging.

Battery cells have an internal impedance that varies as a function of SoC, temperature and age. If a cell voltage is measured while current is flowing in or out of the cell, there will be an observed voltage drop (V=IR) proportional to the product of the cell's instantaneous impedance and the current flowing. This makes it challenging to determine the actual open circuit (no load) cell voltage and in turn the SoC. This makes balancing during discharge or fast charging imprecise.

Currently the most reliable method is to balance at the top of charge. This involves balancing the pack when the cells are within the last 10% of SoC. The benefit here is that in this range all cell types/chemistries show a clear relationship between cell voltage and SoC, making it easier to balance the pack. Additionally, as this is during the charging process, the charging current can be reduced to a point where the cell impedance has a negligible effect on the measured voltage.

The disadvantages of this method is the increase in charging time as now balancing needs to take place as well and the fact that HEV (Hybrid Electric Vehicle) normally keep their batteries within tight SoC limits below 80% and in the linear range of the SoC vs Voltage curve. This makes it more challenging to deduce accurate SoC per cell, and more importantly, to then balance the pack. There are a range of novel ways to overcome this problem, some better than others and some are the subject of patents.

Active balancing involves equalising the pack by moving charge between cells. Whereas this seems logical, it presents a number of technical challenges for the traction batteries used in automotive. Firstly, there are typically 100 cells in series in an automotive battery pack, the ability to move energy between any two cells would require a huge cable spagehetti. As most high voltage packs are built up of modules, limited active balancing within a module is a better compromise, but still requires that modules then be balanced across the pack. Currently, there a no high voltage automotive grade battery management systems capable of doing this in volume production.

Reliability

Warrantee issues rank high among the concerns of automotive OEMs when it comes to the development and sale of electric vehicles. There are many issues that contribute to reliability including the mechanical design of the pack, the thermal management of the cells and the electrical configuration of the pack. Additionally, you have the management of the battery cells by the BMS.

Mechanical

Vibration and shock reduction are important as they can lead to loose connections, which in turn lead to hot spots due to their increased resistance. Cables and buss bars, if improperly mounted and routed can lead to shorting across the pack. The BMS needs to have strategies for recognising system failures by monitoring excessive heat build up or checking for ground isolation faults continously. The BMS itself needs to be robustly designed so that it does not suffer failures due to components breaking away from the board. The use of SMCs (surface mount components) will help prevent this.

Thermal

Proper pack design with the provision of thermal managment is essential in most climates. Batteries may get too hot in the summer and are often too cold in the winter to be safely charged or have decent range. The battery management system needs to monitor and manage temperature within the pack by switching on cooling devices (a fan, or opening a vent) or heating devices. If a pack is overheating due to excessive current draw, the BMS needs to signal the load to reduce current draw and allow the cells to cool.

Electrical

Improper switching arrangements, such as the lack of a pre-charge circuit, having a BMS that draws too much current in idle mode, discharging the pack or even worse, takes energy from cells unequally, unbalancing the pack in the process. Although not strictly electrical, EMC emittance and imunity are very important in automotive

There are a host of over detail features that also belong in this discussion, but have not been covered here, but our hope is that this article will give you some thoughts when choosing a BMS for an automotive application.

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