Design Considerations For Good Battery Pack Design & IntegrationBy Tom Bartley • May 26th, 2009 • Category: Battery Technology for Hybrid and Electric Cars
Battery pack technology is all about managing the temperature of the cells and low resistance of the interconnecting conductors, connectors, and switches. Battery pack technology is not the same as battery chemistry technology. The latter is about building functional individual battery units, while the former is about making those units work together effectively and safely to provide much higher capacities.
There is still a lot of room for creative pack designs in electric and hybrid-electric vehicles with different form, fit and function topologies and connection schemes. The most common requirement is the need for temperature management for high power performance and operational life of a pack. High current charging and discharging through parasitic resistances in the cells, connections, switches, and wire cables generate heat that must be dissipated without damaging the surrounding materials.
Here are some key factors governing good battery pack design.
Internal Heating Factor
Knowing where the heat originates is the first step in designing a cooling system. Just like ultracapacitors, cylindrical cells are rolled up like a jelly roll with an outside cover and dissipate the most heat through the electrode connections in the center at either end of the cylinder. Again just like ultracapacitors, prismatic cells are in the form of flat sheets and dissipate the most heat through the sides of the cell. Cells in the middle of the pack and at the end of the circulating air or coolant stream will be the hottest and deserve the most attention. The most stress on a cell occurs at high temperature and high voltage such as immediately after braking regeneration charging while going down a hill.
Quick, low cost, low resistance connections between cells are a challenge. One bad or corroded high resistance connection can create enough heat to destroy an entire pack very quickly during high current charging or discharging operations. Even if the cell chemistry can accept high current “quick” charging and discharging, resistive heating losses in the connections and wires can easily drop the stored energy efficiency to 50% from the 80%-90% efficiency of more moderate power operations.
MES-DEA Zebra Z5 battery pack Courtesy MES-DEA.
The challenge is to keep all the cells at a uniform cool temperature to prevent uneven cell aging and premature pack failure. It would be clever if the cooling system would start early in anticipation of high current downhill charging and startup accelerations. Today’s position locations systems make this a real possibility.
Low Ambient Temperature Factor
Keeping a pack warm is generally not a problem except during cold weather startups. Some batteries lose a significant amount of capacity at extremely low temperatures. Using part of the stored energy to power a heating blanket could work nicely prior to startup.
The “Zebra” Nickel Sodium Chloride (NiNaCl) battery requires an internal temperature of 300 °C (572 °F) to keep the NaCl electrolyte melted. Using the stored energy of the battery pack and excellent insulation, the temperature can be maintained over a number of days from the stored energy alone. The downside is that the battery must be preheated before use. Prematurely charging a cold Zebra will destroy an expensive pack.
Lithium Ion Factor
Lithium ion (Li ion) titanate is a unique battery chemistry that is endothermic (cools by absorbing heat from the environment) during moderate to low operational power currents. However, at high currents and with corroding connections over time a Li ion titanate pack has the same heat dissipation problems.
Tesla Roadster battery system – Courtesy TeslaMotors.com
The Tesla electric sports car battery pack uses an older Li ion battery chemistry that is more susceptible to thermal runaway. Even when passively sitting fully charged in a garage the Tesla pack uses the equivalent of two refrigerators power to continuously keep the pack cool.
Electromagnetic Radiation Factor
Often ignored, one final consideration in pack design is the high magnetic field surrounding the high current carrying conductors. The high current DC power of the battery leads to an inverter controller for control of the AC induction motor used in most electric vehicles. The high current AC coming out of the inverter creates enough electromagnetic radiation to drown out any nearby AM radio, so good shielding practices have to be followed in the wire and cable installation. Alternatively, this problem can be ignored if functioning AM radio reception is not a priority.
The Bottom Line
Very few battery pack manufacturers have successfully integrated all the pack thermal and electrical requirements into a mechanical structure that has to withstand the shock and vibration environment of transportation applications. With at least five new plug-in battery cars coming on the market within the next two years, resulting in thousands of electric cars driving on our roads, let’s hope the manufacturers are successful with their battery pack integration.
Tom Bartley is an industry veteran with 30 years of experience in general business, marketing, project and product management, and engineering research and development. Mr. Bartley provided executive management support including technical and business oversight to heavy-duty hybrid-electric prototype projects as they evolved into production. He developed cost models for energy storage and fuel savings, and power models for ultracapacitor packs. Mr. Bartley is well known throughout the industry of heavy-duty hybrid-electric buses and trucks, having delivered many papers and presentations since 2003. Mr. Bartley maintains a blog at TomBartleyIdeas.com. Follow twitter.com/TLBartley.
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