Battery Bus Range – It’s All in the Math

With the advent of the battery-electric bus as a mainstream member of the public transit fleet, a new glossary of terms is evolving, especially as it relates to range. Due to the infancy of the technology there have been no clearly defined terms established for definition of range. To date each manufacturer of battery-electric buses have chosen to define range from their respective perspectives. This in turn tends to leave the transit public in a bit of a quandary as to a knowledge of how far a bus will actually go on a single charge in actual revenue service while not overtaxing the batteries in order to maximize battery life.

The following will be an attempt to identify the related factors and quantify those factors using the only current established industry standard, that being STURRA “Altoona” test data. At Altoona the comparative evaluation data is based on using 37.64 kWh of energy per gallon of diesel fuel. This is then translated into equivalent mpg by dividing the kWh of electricity used per mile and dividing the result into 37.64 kWh/gallon of diesel fuel.

For instance, if the Altoona result of 2.0 kWh/mile is divided into 37.64 kWh/gallon of diesel the result is 19 mpg diesel equivalent. Since Altoona is the industry accepted laboratory for bus evaluation, it is our suggestion that Altoona Data be the baseline for evaluation of range and energy consumption. We would point out that many factors such as temperature, passenger load, weight of bus, topography, driver habits, HVAC usage, etc. affect the actual range of a bus. Testing at Altoona is conducted year round so many times testing is conducted at times when neither air conditioning nor heat are in use. These HVAC factors should be taken into account in final calculations and evaluation of bus range. As a rule of thumb, AC and heat will reduce range by approximately 20 to 30 percent, depending on application, in similar fashion to a conventional bus test.

We would in addition point out that not all the entire energy in the battery pack can be used. A healthy range of use is from 20 to 95 percent state of charge. Generally it has been our experience to not charge beyond 95 percent SOC in order to allow capacity for regenerative braking energy early in the drive cycle. Conversely, discharging the battery below 20 percent greatly reduces the life of the battery. That said, a battery system has 75 percent of usable capacity or in the case of a 320 kWh packs as currently used by the two leading manufacturers of extended range buses, approximately 240 kWh of useable energy. 240 kWh / 2 kWh /mile equals a range of 120 miles. In some cases such as spring and fall fuel economy is much better. As an example, if the operator is able to achieve 1.5 kWh/mile then the range is extended to 160 miles.

Determining Battery Capacity

Once the quantitative range issue has been addressed, one must determine the battery capacity approach to one’s respective transit application. Keeping in mind that batteries are heavy and expensive, two approaches to range solutions have evolved among various battery bus manufacturers.  One solution is placing a large number of batteries on the bus in an attempt to provide a range adequate for a complete day of revenue service. Keeping in mind that a conventional transit bus operates from 150  to 250 miles in one day, an adequate capacity of batteries becomes a challenge if one is to stay within federal bridge weight laws unless a lighter bus body is developed to offset the added battery weight. In many cases, with current battery technology, full service of the route may require two buses for each route, one charging while the other is on route, keeping in mind that charge time for a complete charge is 2 1/2  tio 4 hours depending on battery chemistry.

We would point out that there are situations in transit where buses operate 3 to 4 hours in the morning and 3 to 4 hours in the evening in order to handle rush hour passenger traffic, commonly referred to as “trippers."  In such a scenario, a midday charge can be conducted if necessary and service would be no different than a conventional technology bus.

The second alternative to range is to employ a fast charge technology. This approach takes into account that most transit buses travel 11 to 13 miles in one hour, repeat their route every hour or so, return to a common point and have a five minute layover. At the layover point, a fast charge station is located. This fast charge station consists of a charge head attached to a robotic arm which is in turn mounted on a horizontal beam which extends from a vertical pole or is suspended from the ceiling of a transit stop awning. When the bus approaches the charge station, a computer-assisted docking occurs which is transparent to the driver. An hour of energy is then restored in approximately five minutes while passengers are exiting and boarding the bus.

Once the charge is complete, the head automatically retracts, the dash-mounted monitor informs the driver that the charge is complete and that the driver is free to reenter revenue service. In this type of application there is sufficient energy on board the bus for three hours of service should one or two charge events be skipped. The actual charger for this application is a free-standing 500 kW charger which can be located up to 200 feet from the actual charge pole.

One charger can service 6 to 8 buses per hour depending on route scheduling. The battery chemistry used for fast charge applications is Lithium Titanate which is heavier and more expensive than the lithium ion based chemistry employed in extended range applications. The positive of the fast charge chemistry is that it will withstand up to five or six times more total discharge cycles and has no memory.

In an hourly charge scenario, 30 percent of total usable capacity is just that, allowing for many more charge events as compared to lithium ion chemistries which see any charge event as a total discharge. Fast charge buses carry approximately 100 kWh of battery capacity as compared to approximately 300 kWh for an extended range battery-electric bus. For a more complete understanding of battery chemistries refer to batteryuniversity.com, which reports that lithium ion-based batteries are good for about 2500 total discharge cycles, while Lithium Titannate is good for more than 10,000 total discharge cycles. If Lithium Titanante is exercised in 30 to 40 percent of its capacity then the battery life is increase dramatically because of the lack of memory.

A final battery characteristic that requires consideration in route and chemistry selection is battery life. It is industry standard to consider battery pack replacement at approximately 80 percent of the available energy as compared to a new battery. In other words, if a battery electric bus will go 100 miles on a single charge when new, when the bus range reaches 80 miles a battery replacement would be in order. If the route the bus was configured for is 80 miles then one can understand the need for replacement. Conversely, if the route is 60 miles then the batteries can be used beyond the 80 percent figure.  

Battery Bus Operations

This then brings us to our suggested glossary of terms to define battery bus operation:

Nominal Range is defined as the total energy within the battery pack divided by the Altoona reported mileage. In reality, as stated earlier, approximately 75 to 80 percent of the available pack energy in kWh can be used repetitively day after day. While the battery packs can be charged to 100 percent state of charge (SOC) and discharged to about 10 percent SOC, repeated stressing of the batteries in this fashion will dramatically reduce the life of a battery. Based on laboratory test results we do not suggest charging above 95 percent SOC nor discharging below 20 percent SOC.

Actual Range is defined as the realistic range of a battery-electric bus in revenue service taking into account all variables such as passenger load, bus weight, speed, grades, temperature, hospital loads such as heat and air, dwell times etc.

Smart Range would be defined as a conservative calculation allowing for a 10 to 20 percent cushion from actual range to account for route delays, such as road work, accidents and inclement weather. This would be the suggested calculation to employ in route planning as to length of practical routes and layover times when determining whether fast charge or extended range battery chemistries should be deployed on a specific route.

Extended Range Battery-Electric Bus refers to a battery-electric bus with the ability to operate for an actual range of 80-plus miles between charges.

Fast Charge Battery-Electric Bus refers to a battery-electric bus that has the capability for a system to automatically restore an hours’ energy usage in under ten minutes with the ability to operate for a total range of approximately 30-35 miles on a single charge.

Once the industry accepts this or similar terminology, a comparative evaluation of battery-electric buses supplied by various vendors can take on a character that is quantitative in order that routes can be planned in a real life format.

Fleet Charging

The final issue that needs some significant scrutiny before a final decision is made as to vehicle choice is energy required for charging. It is one thing to charge 3 to 5 battery buses but when we expand to fleets of 100 buses the utility dynamic for extended-range buses changes dramatically. A bus carrying 320 kWh of energy will need restoration of approximately 250 kWh of energy in 2 – 4 hours depending on chemistry. Most buses are out of service at night for six to eight hours. At 250 KWh of energy per bus needing to be restored within that window will require a minimum power requirement of 5 Megawatts but in reality 10 to 15 megawatts to allow adequate flexibility in charge routines. The alternative for a 100 bus fleet is fast charge where 14 to 16 fast charge stations are distributed throughout the route system causing no concentrated major energy demand and at a price significantly less than a CNG charge station required to support a similar size fleet.

Dale Hill is the founder of Proterra Inc.

More in Bus