Don’t Forget About System Efficiency

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April 4, 2013

As engineers, we can be obsessed with solving problems. If a circuit doesn’t work right, we debug it to find the root cause. If a component fails, we look for the root cause to prevent future failures. If you are given a specific improvement to make in a product, you focus on that specification or feature to improve.

I was recently tasked with improving the range in an EV. The vehicle featured a 14.4 KWh, 72 Volt (nominal) Lithium Iron Phosphate (LiFePO4) traction battery (24 cells, each 200Ah and nominally 3.0V). Vehicle weight was approximately 2100 lbs. Using a standardized test cycle, this vehicle routinely achieved an average of just over 60 miles range (driven on the same course, at similar speeds, throttle position/load, and environmental conditions). The target was to improve the range by at least 30%, to approximately 80 miles.

There are many ways to affect range in an EV. The primary target for most designers is the battery pack. Increase the capacity (KWh) of the pack, and you increase range (generally at a 1:1 ratio). Weight in an EV is range, and generally  proportional as well. You can also work to eliminate system-level inefficiencies, which may range from connector and cable resistance, mechanical inefficiency, control electronics (such as FET or IGBT) resistance, etc.

If you have ever taken on a weight reduction program you will know that reducing weight by 30% (or a major portion of that) is a task of Titanic proportions. In this case, the time and budget was not available to undertake this. Likewise, space for a larger battery pack was very limited. With battery packs, capacity (KWh) can be increase by physically larger cells (increased Ah capacity) or by adding additional cells to increase voltage. The former is a very transparent change, other than physical space needed. The latter is one that requires a review of system capability (maximum voltage of components, number of cells the battery management system can handle, etc.).

In order to maximize the space available for an increased battery pack it was decided to increase the battery pack capacity by adding cells (larger Ah cells were not available to fit within existing space constraints). 8 additional cells could be located in the existing battery compartment, bringing the total number of cells to 32, system voltage to 96V (nominal), and total pack to 19.2 KWh. As it would happen, the system was designed with sufficient overhead to tolerate the higher voltage with minimal parts changes.

The expectation was that the increase from 14.4 to 19.2 KWh would yield a 33.3% increase in capacity. This should provide a direct increase in range after compensating for the weight increase. Each cell weighs 12.8lbs, resulting in a 308lb 72V pack and a 410lb 96V pack. The ~100lb increase in pack weight is an approximately 5% increase in vehicle weight, which should translate to a ~ 28% overall increase in range. Actual testing resulted in a very different story, however.

Tests using the same standardized test cycle resulted in ranges of 103.9 and 109.0 miles on one test vehicle, and 94.2 and 102.1 on another. Further testing is ongoing, though with an average of approximately 100 miles per charge, I can say comfortably that a 40 mile or 67% increase in range has been achieved. What has caused this nearly 2x increase over the expected improvement in range?

With the vehicle and motor in this application, 72V puts the motor at the low end of its efficiency. This becomes clear when looking at the efficiency of different operating voltages as a function of motor RPM:

Don’t Forget About System Efficiency 1

Figure 1 Motor Efficiency Comparison

As much as a 15% improvement in motor efficiency can be had just by upping the system voltage. When driving a set course with pre-defined speed and acceleration, only a certain KW (power output) is needed. As only that level of power is needed, the resulting improvement in efficiency translates directly to additional range. In addition to greater efficiency, the motor torque for a given RPM is achieved earlier in proportion to the higher voltage. With a greater low-end torque, even less energy is required to operate the vehicle, translating to further improvements in efficiency and thus range. Finally, with a higher system voltage, lower current is needed to achieve these power levels, resulting in reduced system losses (principally wasted power in the form of heat, directly proportional to current flow).

All of these items translate to additional range- not just the increase in energy capacity. It’s important when designing a system, or a component part of a larger system, to consider all the possible ways in which it and the system affect the efficiency for the end product you are designing.

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