Finally, reduced maintenance costs on a battery-dominant hybrid vs. an engine-dominant hybrid are a very real prospect. Both types of hybrid systems will give ample reductions to traditional high-dollar maintenance items, such as brakes, and have operating modes which are much easier on diesel engines. One of the big maintenance differences will be the lifetime of the hybrid battery. While the top two integrators of heavy-duty hybrid drives have done an admirable job with their products currently in the marketplaces, the design life of the lithium battery in the BAE System’s drivetrain has been advertised at six years, as has the nickel metal hydride battery of the Allison system.
While the math on lifetime maintenance costs is absolutely in favor of the engine-dominant hybrid bus vs. traditional diesel bus, these hybrid batteries are still quite expensive to replace. In fact, the basic calendar life of both these battery chemistries is greater than 10 years. The shorter lifetime of these batteries is actually a casualty of the basic engine-dominant approach to hybrid design. It was mentioned earlier that batteries are more efficient when the ratio of power being moved in and out of the battery vs. the total battery capacity is low. This is called C-rate. Running a battery at the high end of its C-rate capability will shorten the life of the battery, while running a battery at low C-rates is much more benign. By nature of its fundamental design, an engine-dominant hybrid system wants to have the smallest possible battery capacity, and push the highest amount of power in and out of the battery, resulting in high C-rates which stress the batteries and cause them to reach end of life earlier.
In a battery-dominant approach, because the battery is two to four times bigger, the C-rates on power being pushed in and out of the battery are much lower, resulting in dramatically lower stress on the battery. Today’s available lithium chemistries are in fact bringing the prospect of a battery-dominant hybrid bus having a single battery, which could conceivably last the 12-year life of the vehicle.
The downside to the battery-dominant approach is of course a much higher first-cost vehicle. However, in the United States’ subsidized transit market, fleet operators are motivated to pull operating costs forward into the first cost of the vehicle. In this regard, lifetime operating costs become the primary budgetary concern of the operators and battery-dominant vehicles will most certainly have strong appeal.
One potential difficulty with the battery-dominant approach for OEM’s and public fleet operators alike is the Buy-America provisions in the Federal Transit Administration subsidies. If Buy American is important to your agency, it is important to understand that while many of the battery packs for transit use are “assembled” here in the United States, the cells these packs are built from are actually manufactured overseas, in most cases Asia.
With the smaller battery packs on the current engine-dominant hybrids, the foreign manufacturing content has not bumped up against Buy-American limitations. However, transitioning to much larger battery packs on battery-dominant hybrid city buses, or even pure electric buses, could begin to alter the content equation, as very few providers are yet assembling lithium battery packs with cells that are also made here in the United States.
Mark Aramli is the transportation commercial manager for International Battery.