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XIII International PhD Workshop OWD 2011, 22–25 October 2011

BatMan

Battery Health and Safety Management

Mattias Tjus, Bochum University 29.08.2011

Keywords: Electrical Vehicles, Battery Management, BMS, Battery State of Health.

Abstract

The paper discusses the first testing and plans for a Battery Management System for Electric Vehicles belonging to a project at Hochschule Bochum (eng. “Bochum University of Applied Sciences”) as well as some Battery and Battery Management basics.

1. Introduction

The BatMan-Project deals with the management and protection of lithium ion batteries for use in electric vehicles. In addition to ensuring battery safety, this also involves giving users of electric vehicles as much useful information as possible about the status and well-being of the batteries.

Market acceptance of electric vehicle development is steadily increasing and that which seems to be the weak link of an electric vehicle, is the energy storage. Today, lithium ion batteries show great potential in enabling a practical and price competitive electric vehicle solution as an alternative to internal combustion engine vehicles. These batteries however, are still quite expensive and due to the almost non-existent real world examples demonstrating adequate durability of the batteries, comparisons to mobile phones and laptops are made, for which battery life expectancies are of the order of 3-5 years. To relieve anxiety and to support and possibly to improve on performance of the batteries, a management system must be in place to aid manufacturers and users. The BatMan-Project contains two parts to address this; one is to develop a Battery Management System from industry available integrated circuits and components with the ability to manage a battery pack and to report its state, such as controlling and reporting voltage, current, temperature, state-of-charge and state-of-health. The second part is to evaluate the possibility of using an impedance spectroscopy method to

monitor and determine the aging, the state-of-health, of a battery pack. This paper discusses the first part of the project.

2. Electric Vehicles

2.1 What is an Electric Vehicle?

A vehicle, [from Latin vehiculum, meaning carriage] describes any (land) transporter for persons or goods. An Electric Vehicle, whence infers a transporter propelled by an electric motor, may it be an electric motor assisted bicycle (commonly E-bike), a motorcycle, car or buss, or an other type of human constructed transporter. Electricity can be seen as a means to carry energy, conversely, it is not a practical means to store energy. Electrical energy can be stored with, e.g. capacitors where the energy is stored in an electric potential.

In practice, there are several means to make a functional electric vehicle, most commonly, either the energy is stored as chemical potential energy in batteries (Fig. 1), transmitted to the vehicle contin-uously during powered motion, (Fig. 2) or the energy is generated withing the vehicle (Fig. 3).

Fig.1. Bicycle with electric drive train from BionX. A diesel train, despite its name, is in some ways

similar, the electric power is generated by an on-board diesel engine instead of coming through power lines from larger Power Plants. This type of hybrid electric vehicle, see Fig. 2, is called Series Hybrid since the engine, generator and electric motor are all in a series connection.

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Fig.2. Simplified presentation of a train.

A diesel train, despite its name, is in some ways similar, the electric power is generated by an on-board diesel engine instead of coming through power lines from larger Power Plants. This type of hybrid electric vehicle, see Fig. 3, is called Series Hybrid since the engine, generator and electric motor are all in a series connection.

Fig.3. Simplified presentation of a diesel train.

Alternatively, a parallel hybrid system, where the wheels can be driven either by the engine or the electric motor, or mixtures of the two versions can be used to complement strengths and weaknesses of the different motor types.

3. Batteries

Three components are vital to an EV; the electric motor and generator, the battery and the power electronics to control these. Motors and controllers & power electronics in use in industry and consumer markets are, if not perfectly adapted, very well matured, and often offer good performance at low efficiency losses [2]. Batteries with high enough energy and power densities to be used in transportation, other than heavy and slow warehouse loaders and fork lifts, are new and still with needs for further development. Looking at data for taxi drivers as an example of vehicle usage, Fig. 4 [5] and energy usage as described by Tesla Motors [3] of 188Wh/km, most taxi drivers could manage their day with approximately 60kWh of energy. With an energy density of around 200Wh/kg, as a laptop battery has [6], this equates a battery weight of 300 kg. As for price, with an estimated €400/kWh [argonne], the battery alone would cost €24 000.

Fig.4. Average daily distance traveled by taxi drivers.

3.1 Lithium Ion Batteries

Lithium is the lightest solid element and it has a very high electric potential [1] which makes it a desired candidate for a lightweight and powerful battery.

Today, most, if not all, laptops and mobile phones and also many other portable electronics devices use lithium based rechargable batteries. There are however a few drawbacks with lithium being very reactive since it makes it unstable. To go around this type of problem, lithium ion battery producers use lithium salts instead of pure lithium. Common examples are LiCoO2, LiFePO4, LiNiO2 and mixtures of lithium-Nickel-Manganese-Cobalt (NMC).

3.2 Cathode

When a battery is referred to as a lithium ion battery, it means that lithium ions are the charge carriers within the battery. The lithium ions, or atoms, come mainly from the cathode which contains the lithium salts. The reasons for the many different versions of lithium batteries is that the different compounds give different abilities for the complete cell. To achieve a high cell potential, a high positive electrical potential is wanted at the cathode.

Lithium Cobalt batteries for example give high energy density but at the cost of having reduced life expectancy.

Lithium Manganese give lower energy density but makes for more abuse tolerant batteries which is preferred in power tools.

Lithium Iron Phosphate have even lower energy density but are very tolerant and have high life expectancy.

“NMC” are currently very common and simply put mixes the abilities of the above mentioned battery chemistries.

3.3 Electrolyte & Separator

In order for the lithium ion to move from the cathode to the anode and back an electrolyte of a lithium salt, often LiPF6, in a solvent is used. Withing the electrolyte, a separator is needed to prevent single electrons from being transfered internally between the anode and cathode. The separator is mostly made from a porous plastic foil of polypropylene or polyethylene.

3.4 Anode

The anode of a lithium battery needs to be able to store lithium ions or atoms and have as low potential as posible. The most used material with

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these abilities is Graphite. Graphite, as seen in Fig. 5, can “store” atoms between its two dimensional C6 layers.

Fig.5. Lithium atom between C6 layers.

Due to the extra atoms withing the graphite, a mechanical swelling occurs causing stress on the anode structure. A way to reduce mechanical wear, lithium titanate anodes can be used instead of graphite, this on the other hand has a higher potential giving a lower total electrical gap between the cathode and anode as well as being much more expensive. A third anode material is lithium silicate, which is used in primary batteries (non rechargeable).

3.5 Protection

Lithium ion batteries are dangerous because lithium is highly reactive and since the electrolytes are, in most cases, highly flammable. For these reasons, most consumer sold lithium ion batteries contain several internal and external protection mechanisms to reduce possible dangers.

A few examples are: fuses, PTCs, built-in weak spots and added flame retardants in the electrolyte. 3.6 Charge & Discharge

In order to give a description of charging and discharging reactions within a battery, a LithiumCobaltDioxide (LiCoO2) cathode and a graphite (C6) anode example battery will be discussed in following section.

While charging, an external potential, negative side at the lithium salt-electrode and positive at the graphite electrode, is applied over the battery. The potential must be high enough to separate an electron from its lithium atom and causing freed lithium ions to fall through the electric potential through the electrolyte and the separator, to the anode, while the electron is led through the charging device.

Negative Electrode, Cathode:

LiCoO2 → xLi+ + xe- + Li1-xCoO2 At the graphite anode, the lithium regain an

electron as they get trapped between the 2D layers in the graphite. Positive Electrode, Anode: C6 + xLi+ + xe- → LixC6

The discharge process is the reverse of the charging process: Positive Electrode:

LixC6 → C6 + xLi+ + xe- Negative Electrode:

xLi+ + xe- + Li1-xCoO2 → LiCoO2 Due to the interstitial lithium atoms in the

graphite in the charged state, the physical size of the anode may be up to 10% [1] larger than in the discharged state.

In the first experiments within the BatMan-project, cylindrical laptop-style battery cells of the type UR18650F from Sanyo (now Panasonic) were used and discharge curves from these can be seen in section 5.3.

3.7 Aging

Laptops and cell phones tend to get decreased operating time between each charge as they get older, this is due to aging of their batteries. While each battery producer strive to achieve as well defined and controlled material structures as they can within the batteries, these structures do change with time, the speed of the change depends on potential unwanted reactions in the batteries, the thermal motion of the battery constituents and possible mistreatment. Unwanted reactions and thermal decomposition are strongly temperature dependent – while wanted cell reactions (charge and discharge) are helped by increasing temperature (ions move faster), higher temperature usually means faster aging. A rule of thumb is that lithium ion batteries prefer room temperature.

The electrolyte is the part of the battery which is the most sensitive to temperature differences; higher than desired temperatures causes faster decomposition of the solvents and lower than desired temperatures decreases the allowed ion transfer and may cause local ion shortages causing either the electronics to do a low voltage cutoff or it may also cause damage to the battery [1].

As mentioned in Charge/Discharge, material stresses while charging and discharging occur. These can lead to structural damage to the batteries which often can cause increased internal resistance.

4. Battery Management

A battery, or Battery Pack, is one or more electrochemical cells used in series and/or in parallel to reach a wanted battery energy or power capacity.

When more cells are used jointly, even the individual cell protections are often inadequate; e.g. if two cells are used together, with one having slightly lower capacity than the other, this cell can be pushed lower than its normal minimum voltage during discharge and higher than its maximum during charge causing it to age more rapidly than it would have otherwise. For an electric vehicle, with a very high amount of

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individual battery cells, this problem could cause a very short life expectancy of the complete battery.

4.1 Safety

A BMS is usually designed to allow a battery to function within preset values of Voltage, Current and Temperature. For example, a model BMS might only allow maximum discharge current within a small voltage interval, but still allow lower current within a larger interval, see Fig. 6 for a simple graphical representation. If a parameter should fall outside given allowed space, the BMS should be designed to partially or fully limit this parameter.

Fig.6. The area in gray represents the allowed battery states.

4.2 Regulation

A BMS is used to regulate many crucial but not directly fatal behaviours of batteries, one of the most important is Cell Balancing. To ease the load on weaker cells in a larger battery and avoid having to cut power completely, many BMSs include a balancing method. There are multiple ways to practically do this, for example: - A BMS can bleed extra capacity off of stronger cells by leading current from them over resistors. - An energy efficient way can be to bypass weak cells and only use the stronger ones until even capacity is reached. - Capacity can be pumped from stronger cells to weaker using coils and capacitors.

4.3 Communications Interface

In many applications, a non-communicative BMS is enough, such as a power tool, where the user notices low charge by decreased performance, but in many other situations, a user, such as a human or a higher ranking system, often need information from the BMS. One example of this is a laptop computer, which receives information about the state-of-charge of its battery from the built-in BMS in the battery pack and can automatically (user

defined) change to power consumption and performance depending on available capacity.

4.4 Texas Instruments

Integrated circuits currently being reviewed in the BatMan-project include ICs from Texas Instruments, TI. TI have a large portfolio of BMS-ICs for many areas, such as Power Tools, E-Bikes and Laptop Computers. The tests covered below all use an IC with the name BQ78PL116 together with BQ76PL102. This IC enables a very potent BMS and is design for power tools, portable medical equipment, back-up power systems and E-bikes (and more). Each BQ78PL116 can control four battery cells alone, and with six simpler BQ76PL102-ICs it can control up to 16 cells with a capacity of up to 300Ah. The main IC (BQ78PL116) includes many safety features, high precision measurements, SMBus communication, charge pumping balancing and many more features at a very low power consumption. [4]

4.5 Small VS Large

When considering the layout of a BMS, it is important to consider the rated power of the application. For a laptop, available BMSs work impressively well, but they also only need to handle power of the order of 50W. A power tool, e.g. a drill, with modern lithium ion batteries needs to handle of the order of 1kW of power. Higher current and high voltage transients caused by the electrical motor and its controller, infers the need for sturdier BMSs, thankfully, by using more tolerant batteries, the functions in the BMSs can at the same time be simpler.

5. BatMan BMS

The goal of the BatMan-project is to develop a BMS with the functionality of a laptop-BMS or better, with accurate State-of-Charge and State-of-Health estimates as well as ensuring the safety and giving maximum possible expected life out of the batteries at the same time as it should be able to handle very high power loads – an electric car might need power in the order of 100kW during acceleration and of the order of 10kW during constant speed.

To take advantage of the accuracy achieved with the BQ78PL116-IC, as well as to be able to use the high power needed in an electric vehicle, it is intended to use multiple 16-cell BMSs together, as per Fig. 7. A configuration as per Fig. 7, with two 16-cell BMSs, would be able to deliver up to 15kW of power safely and a system using five in series (80

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series cells) would equal a total of approximately 300V at 100A, or 30kW.

Since the BQ78PL116-IC is not originally intended to be used as suggested above, research and safety measures are needed to ensure safe usage, which will be done in future work.

Fig.7. Model of a 32 series cells battery.

5.1 TI BQ78PL116 + EVM

As discussed above, the IC intended for use is the BQ78PL116 from TI. The IC uses a QFN-48-package, quadratic with 7mm side, see Fig. 8.

Fig.8. Model of a BQ78PL116 integrated circuit.

Texas Instruments provides an EValutation Module for their buyers, the BQ78PL116EVM, in order to evaluate the functions of the BMS-IC.

Fig.9. BQ78PL116EVM – a complete BMS with multiple extra connections for mesurements.

5.2 Batteries

To test and evaluate the IC and the EVM, laptop-style batteries have been used, Sanyo UR18650F [6], with 3.7V nominal voltage, 4.2V maximum and 2.1Ah capacity, connected (welded) 5 in parallel and 12 five-packs in series for a total of 466Wh. All testing reported below are done with this setup.

Fig.10. Laboratory setup.

5.3 EVM Testing

After tuning in the EVM, a discharge without using any balancing shows that the cells are not 100% matched in performance. When the batteries are discharged from 95% SOC, where they are balanced to within 5mV voltage difference, to 0% SOC, the difference between the weakest and the strongest cells are at around 300mV, see Fig. 11,. This difference remains even when the cells are allowed to rest.

Fig.11. Discharge curves, balancing inactive. With the balancing active, it allows for a deeper

discharge, but as seen in Fig. 12, with high discharge rates, it may not be able to keep the cells fully balanced.

Fig.12. Discharge curve with active cell balancing. During a rest phase after the discharge, the cell

balancing eventually minimizes the voltage differences, see Fig. 13.

Fig. 11-13 show plots made in a program, BQ Wizard 3, together with the Evalutation Module and the discharge was made at 0.5C (with the battery

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capacity of 10.5Ah, this equates to approximately 5A) with a maximum balancing current of 100mA.

Fig.13. During rest, the batteries are eventually fully balanced.

6. Conclusions

The BMS-IC from Texas Instruments does show great promise in it multitude of funcions, as it has a high level of accuracy and high flexibility. Further research will be done to gather more knowledge of its functionality at more (and less) demanding tasks aswell as durability over time in various situations.

7. Acknowledgements

Funding for the project comes from the German Bundesministerium für Bildung und Forschung, BMBF (Förderkennzeichen 17026X10).

8. Bibliography And Authors

8.1 Author

Mattias Tjus, M.Sc.E mattias.tjus@hs-bochum.de Lennershofstraße 140 44801 Bochum Germany

8.2 References

[1] A. Jossen, W. Weydanz: Moderne Akkumulatoren richtig einsetzen, Reichardts Verlag, München, 2006

[2] F. Pautzke: Radnabenantriebe, Shaker Verlag, Aachen, 2010

[3] Tesla Motors, http://www.teslamotors.com, As of January 2011. [4] Texas Instruments, Data Sheet:

http://focus.ti.com/docs/prod/folders/print/bq78pl116.html

As of August 2011 [5] Schaller Consulting: Taxi Trip and Fare Data:

A Compendium, October 1991 [6] Sanyo, Data Sheet, UR18650F, As of

December 2009