Building Reliable High Power Battery Packs: Here Are Some Practical ConsiderationsBy Tom Bartley • Mar 20th, 2009 • Category: Battery Technology for Hybrid and Electric Cars
For the most part, today’s car batteries are driven by low cost mass production economies of scale. They are still the same fast discharge, slow charge energy storage device required to start an engine. Technology improvements have evolved in the areas of chemistries, shelf and cycle life, performance environment, maintenance, packaging, and recycling. They all contribute toward offering more choices to the discriminating battery consumer.
Lead-acid is still the most popular choice for battery chemistry because the elements are cheap and production techniques are well known. Recycling efforts have reduced much of the damage previously done by discarding old lead-acid batteries.
No matter what the chemistry, however, battery selection always starts with power and energy; the good old P and E. The battery has to have sufficient power over a long enough time to meet the peak load demand, e.g., turn the starter motor for a minimum of 3 10-second starting attempts. Because power equals current times voltage
resistance heat losses (which is also expressed as a form of power) equal the current squared times the resistance
it is desirable to keep the voltage high and the current low for better efficiencies. Also, I2R not only describes the quantitative losses but also is real heat that must be dissipated to prevent high temperature damage to nearby materials. The resistance that causes heat loss can come from many components within the electrical system including parts of the battery itself or the wiring or connections.
Minimizing the resistance is always a challenge. And because there is a limit to minimizing resistance in the battery or heavy-duty cables, the discriminating vehicle designer must look toward using higher voltage batteries to get the power needed to drive the hybrid or electric car. That is why over 50 years ago car manufacturers switched from 6-volt starting batteries to the 12-volt car battery that is now common. Many heavy-duty buses and trucks use two 12-volt batteries in series to provide electrical power for 24-volt accessories. Several years ago there was an attempt to move to 42-volt car and truck batteries, but it didn’t catch on because of the proliferation of inexpensive 12-volt and 24-volt accessories.
In practice, for vehicle applications it is highly desirable to keep power currents less than 200 amps with only momentary spikes going higher. For example, if the power circuit has the small resistance of 1 ohm, at 200 amps the waste heat generated (I2R) is 40 kW, about the same heat put out by 40 hairdryers.
Making Battery Packs – The Key to Higher Voltage
The only way to get higher voltage battery packs is to “stack” or connect battery cells in series. Cells are essentially individual chemical batteries, and can be technically defined as “a vessel containing various chemicals which produce electricity as a result of the reactions taking place between these chemicals.”
So far so good, we’ll just stack the cells in series to get higher voltage and use larger cells or parallel stacks to get higher current and the problem is solved, right? Not so fast.
The first real world problem is that each of the stacked cells is not identical to the other cells in the stack because of slight differences in the fabrication processes and materials, i.e. manufacturing tolerances. The net result of differences in cell capacity, cell resistance, and leakage current is to cause the voltage to vary across each battery cell in a stacked pack.
When charging or discharging a series connected battery pack, each cell has exactly the same current as the whole pack. Because of the difference in resistance from cell to cell, repeated charge/discharge cycles of the pack cause the voltage variations between the cells to increase. Also, for the same charging current a lower capacity cell will wind up with a higher voltage than a cell with higher capacity; and for the same discharging current the lower capacity cell will have a voltage that drops faster than the cell with a higher capacity. Even if the pack sits on the shelf, over time the cells will have different voltages because of the variation in leakage currents (parasitic self discharge).
All this means that, as the state of charge (SOC) variations among the cells increase over time, some cells will eventually get excessively charged and some cells will get excessively discharged. In either case the cell is destroyed, typically resulting in a high cell resistance. Higher resistance means more heat generated during high currents and can result in a high enough temperature to melt materials and cause battery fires. If we’re lucky enough, the damaged cell will just short out, and the overall pack voltage and corresponding power capability will just be reduced. In either case, “unbalanced” voltages can result in destruction of the pack rather quickly as the best cells and the worst cells continue to be picked off from either overcharging or over-discharging. In a series connected pack it only takes one cell to fail in the open condition or “burn out” for the whole pack to fail.
So now you have some understanding on how high voltage battery packs can be built to power hybrid and electric cars. There are always challenges involved, and the best designs will have to take advantage of new materials, designs, and compromises between many important physical and operational considerations the system must withstand in both typical usage and worst-case driving scenarios.
How does all this affect my hybrid and electric car? Look for my next post.
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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