The Reality of eBus Flash-Charging
Tom Lombardo posted on October 02, 2019 |

Technology company ABB recently announced a partnership with King Long Motor Group to develop an e-bus fleet for China’s public transportation market. The companies will use ABB’s Trolleybus Optimisation Système Alimentation (TOSA) flash-charging system, which they claim is the world’s fastest electric vehicle charger. I continue to see misleading headlines that talk about an e-bus that “charges in seconds,” so let’s cut through the hype and see what they really mean by that.

Figure 1. A TOSA e-bus. (Image courtesy of ABB.)
Figure 1. A TOSA e-bus. (Image courtesy of ABB.)

Pickup and Top Off

A typical e-bus could require a battery capacity of over 300kWh in order to complete its daily rounds through a city on a single charge. That’s a very heavy and expensive “fuel tank,” so engineers have found creative ways to decrease the weight and cost of e-bus batteries. One method is to use a smaller battery, fully charge it at the depot overnight, and briefly top off the charge at select passenger stops along the route. That’s the underlying principle of ABB’s TOSA, but ABB isn’t the first to have that idea; Utah State University rolled out a similar system in 2013. TOSA, however, takes this concept to the next level by delivering much higher bursts of energy at each stop. These big power surges could cause a serious issue for the grid, but ABB’s innovative solution solves that problem and delivers a bonus.

Three Kinds of Charging

The idea behind TOSA is that the bus has a lightweight battery with a small capacity—perhaps around 50kWh—that can be charged in three ways: a single low-power slow-charge overnight at the depot, occasional medium-power fast-charges at terminals, and frequent high-power flash charges at passenger stops.

The depot chargers draw only about 45kW, making them easy to incorporate into the building’s existing infrastructure. Depending on the battery level when the bus returns to the depot, it can be fully recharged in an hour or less. The 400kW chargers are placed at terminals where buses typically spend three to five minutes. These stops can add 20 to 30kWh of charge to the bus, nearly topping off the vehicle completely at each terminal stop. The flash-charging stations are capable of delivering 600kW of power, adding two or three kWh of energy to the bus in just 15-20 seconds.

Grid Impact

Let’s look more closely at those flash chargers. While they’re delivering only about 3kWh of energy—less than 10 percent of the total battery capacity—drawing that much power in a few seconds puts a heavy load on the grid,  a factor that grid operators call “demand.” (To learn more about demand and the difference between kW and kWh, see this article about EV smart charging.) The innovation behind TOSA is to give each flash-charging station an energy storage device: a stationary battery that acts as a buffer between the grid and the bus. This battery can slow charge from the grid, reducing demand, and then deliver high bursts of power for short durations.

Figure 2. A TOSA flash-charging station. (Image created by Tom Lombardo.)
Figure 2. A TOSA flash-charging station. (Image created by Tom Lombardo.)

Notice the bidirectional connection between the grid and the charger shown in Figure 2. This allows the stationary battery to support the grid by delivering additional power during peak-demand hours and providing frequency regulation. Utilities are adding grid-level battery storage units anyway, so why not get double duty out of them?

The only thing that puzzles me about the TOSA concept is that the terminal feeding stations, which deliver 400kW, don’t have batteries. (The TOSA specification only refers to batteries in the 600 kW flash chargers.) Tesla’s third-generation Supercharger draws only 250kW, and it includes a battery to minimize the demand on the grid, so I’m curious as to why ABB didn’t include a battery bank in its terminal feeding stations.

No Need for New Batteries

One environmental criticism of electric vehicles is that after 10 years or so, the batteries are no longer viable, so they become part of the waste stream. That’s not quite true, though. When we say that a rechargeable battery has reached the end of its life, it means that the battery can only hold about 80 percent of its original capacity. So, in mobile applications, where range and weight are important factors, the batteries are no longer desirable. However, many used EV batteries are finding second lives as stationary energy storage devices, where they could function for another decade before being recycled. This makes the TOSA bus concept fulfill the three Rs of waste management: reduce battery size, reuse old batteries, and recycle them when they’re no longer usable.

Engineers love to devise solutions to problems, and it’s extra nice when one innovation solves multiple problems. TOSA buses have been operating in Switzerland for more than a year, logging more than half a million kilometers and keeping over a thousand tons of CO2 out of the atmosphere while demonstrating proof of concept. With outcomes like that, we don’t need overhyped headlines touting “a bus that charges in seconds.”


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