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Maxwell Heavy-Duty Transport Ultracap Modules for Electric Buses in Milan

The HTM125 125V heavy-duty transportation module.

Vossloh Kiepe GmbH, a producer of heavy vehicle drive systems, has selected Maxwell’s 125-volt BOOSTCAP ultracapacitor modules for braking energy recuperation and torque assist in electric buses it is producing in collaboration with Van Hool NV for the Milan, Italy, municipal transit system.

The Vossloh-Van Hool buses for Milan are low-floor articulated vehicles equipped with ultracapacitor-based energy storage systems that absorb and store energy from recuperative braking for reuse in powering electric motors. The basic drive system can use energy supplied via overhead lines or a 100 kW Euro-5 diesel genset.

Vossloh hybrid drive for mass transit vehicles. Click to enlarge.

The city of Milan recently announced that it is taking delivery of the first 15 of a total of 70 vehicles it has ordered from Vossloh Kiepe and Van Hool.

Maxwell shipments are scheduled to begin during the current quarter on Vossloh’s order for a total of 300 BOOSTCAP HTM125 heavy duty transportation modules to be delivered by the end of the year.

This is version 2 of the HTM125. New revisions to the module include an improved junction box; PT100 thermistor for temperature monitoring; vent for over pressure management; ground stud for multi units common grounding; design improvements for increased long term reliability; improvements in mechanical stability; and improvements in isolation.

The HTM125 is the result of extensive work with Vossloh Kiepe and other leading transportation OEMs to develop a robust multi-cell module to withstand the harsh environments and extremely demanding duty cycles that are typical in heavy transportation applications. The HTM125 meets transportation industry requirements for watt-hours of energy storage and watts of power delivery per kilogram, and is designed to perform reliably through one million or more deep discharge cycles, or about 15 years of operational life for most vehicles. This is the first of what we expect to be several series production orders for heavy vehicles in Europe.

—David Schramm, Maxwell president and CEO

The HTM125 is based on 2.7V BOOSTCAP BCAP3000 Power cells. Nominal capacitance is 63F. Encased in a rugged, splash-proof, IP 65-compliant, aluminum chassis, it weighs less than 60kg and measures 315x425x744mm. Integrated monitoring capabilities and an efficient cooling configuration enable it to sustain continuous current of up to 150 amps with minimal temperature increase in high-temperature environments. Energy capacity is 101.7 Wh.

Up to 12 125-volt modules may be linked in series to deliver a total of as much as 1,500 volts. Maxwell also offers a complete line of standard 15- to 75-volt multi-cell modules.



101.7 Wh? Thats about the same class as a weak human on a bike. Maybe it's 101.7 kWh.


Can you find a human that generates 101.7 Wh in 10 seconds? Or can you find a city bus which needs to brake for 1 hour?

The Vossloh-Van Hool buses for Milan are low-floor articulated vehicles equipped with ultracapacitor-based energy storage systems that absorb and store energy from recuperative braking for reuse in powering electric motors.


Realist: Just curious;

where do 10 seconds and braking for 1 hour come from? I didn't see those numbers in the article.

Henry Gibson

It might be better for the ultimate customer of these buses to invest more EUROS up front for enough ZEBRA batteries to meet the power requirements. This would allow for plug in charging to save on fuel as well as charging at some stops. Most of the equipment for full electric opereation has already been paid for; why not have a plug in hybrid? A combination of ZEBRA batteries and Ultra capacitors might be a less costly alternative than full power ZEBRA batteries, there is less full electric operation. The energy in a full complement of Ultra capacitors is less than the energy in a lorry starting battery.

CSIRO has tested a combination Ultra capacitor cell and lead acid cell with great success for high power density from the capacitor and high energy capacity from the lead acid cell. The cell also regulates the voltage across the capacitor. Perhaps the FireFly foam technology combined with an Ultra-capacitor would make a lower cost plug in hybrid possible.

EFFPOWER has lead-acid bipolar batteries for hybrids nearing production that also must be seriously considered because of their simplicity and much higher energy density. Atraverda may also be nearing production of a lead-acid bipolar batteries suited for hybrid applications. Both companies should consider CSIRO and FireFly technology.

Two or more small flywheel systems might have a lower cost, greater electronic and construction simplicity, higher energy density, and higher power density. Two flywheel rail-trolley hybrids are entering revenue service in England. Some New York subway cars had flywheels at one time for power saving. Flywheel buses or trolleys that charged at stops were in revenue service decades ago in Switzerland. There were also hybrid flywheel locomotives operating in England on some non-catenary electrified rail systems to compensate for the necessary gaps in the electrified rails. These locomotives could stop in a gap and minutes later pull the train out of it.

The Ultra capacitor cells have the advantage of being made in large quantities and putting many of them in a large package is well understood. Ultra-capacitor systems with 101.7kWh. would be bigger than the bus. ...HG...


As with any capacitor
The numbers are imlpied. 150 amps at 125v with 12 (Max) in series suggests If system design alows (Clamp limit) 1,500 volts at 150 amps. Or possibly 1800amps at 125v instantaneous for a full set (battery).
My ability to quickly do the maths fails at this point.
However we are talking stored electrons in joules. The time frame is dependant on the flow through resistance (motor, controler or other limiting aspect). It is expressed in Watthours but can be delivered in seconds - or minutes etc as required. It can also charge up in seconds or the minutes dependant on the current provided by say braking or the (mentioned100kw generator.)


Ive often wondered what the ultracap would look like, and the specs.
The article gives a good feel for the technology.
Truly amazing.
One million or more deep charge disharge cycles.
Note provides tourqe assist for electric bus, Read assists battery set, not replaces.
There are many ways these can contriute to the whole system, probably the easiest thing to comprehend is to
'Imagine a max of 1,000 killowatts peaking from regenerative braking. Batteries won't look at that.


Arnold: Yes, I have a working knowledge of capacitors. But thanks. On blogs it is hard to avoid providing too much detail, or too little, because you don't know the training of readers.

My question was about the particular numbers. There seemed to be a disconnect between comment #1 and the rather rhetorical reply in #2.

Braking for 1 hour? Who said that? Looks like a straw man to me. Actually I would be surprised to see a bus brake steadily for five seconds (most city braking has very short duration).

The other number, 10 seconds, seems plausible. A bus might well accelerate for 10 seconds at times.

mumble, mumble, mumble ... the 101.7 WH works. A man can be rated at about 1/8 horse power.

Produce that in 10 seconds: 360 x 1/8 = 45 horsepower.

45 horsepower seems a reasonable output and about what a large city bus should need to complement the 100 kW from the diesel genset.

Sorry about the mixed unit estimating.

Reality Czech

100 Wh over 5 seconds (1/720 hour) is 72 kW or about 110 horsepower. That is a nice kick in the acceleration especially if you get it for "free".


where do 10 seconds and braking for 1 hour come from? I didn't see those numbers in the article.

It was just a response to:

101.7 Wh? Thats about the same class as a weak human on a bike.

Obviously this capacitor only needs to store the braking energy of the bus at low speeds - it's completely irrelevant what the average power of a human is.

But I guess, you've figured it out anyways.


Since ultra-caps can capture much more breaking power, supply larger energy boost to get a vehicle moving, reduce the size of the battery pack required, extend batteries life 2 to 5 folds and last 1 000 000 + cycles... etc why are they not used in all Hybrids, PHEVs and BEVs?


HarveyD: It is my understanding that the ultra-caps themselves are expensive. However I think there are other factors delaying them.

Caps are inherently variable voltage devices. The electronics to integrate them with batteries and a drive train isn't simple or free. Some say both problems, cost and complexity, have been overcome.

And manufacturers have reasons to take only one big step at a time. The first step was hybrids (batteries with the ICE.) Adding supercaps is a second step.

Usually operations should be optimized where expense is highest.

Municipal buses and delivery vehicles rack up high mileage, burn a lot of fuel, and make frequent stops. Supercaps can save more there than on mom's SUV.


Other than reasons cited by 'K', ultra-caps are bulky (low energy density) - just look at specs above:
315x425x744mm for a module of 101.7 Wh.
Looks like BEVs will need much more room for energy storage, and may need to look different, or to have reduced storage or passenger space, compared with current ICE cars with comparable range.
In order to have swappable batteries (as they plan in Israel) and/or modular/upgradeable batteries, battery area will need to be easily accessible.
If ultra-caps and/or ICE genset is to be added, more need for room.
So ultra-caps, swappable and modular batteries are more suitable for (larger) commercial vehicles - with the current state of technology.


Where's EEstor? out there somewhere with Winnie
the Pooh and Tigger, too?


K & MG;

Thank you for the info. I thought that a valuable trade off could be had with reduced battery size to make room for ultra-caps on a pure BEV.

Ultra-caps are inherently better than batteries with breaking power recouperation and for accelleration power boost + pontential extended battery life and e-driving range?

Just a thought...


HarveyD: you are on the right track and supercaps or ultra-caps, or whatever the surviving word proves to be, will be a fast growing industry for quite some time.

There is research to integrate the supercap with the battery in one case also containing control electronics.

To the customer the combination would possess the best traits of the battery portion and of the capacitor portion but not exhibit the undesirable traits of either.

I haven't been following the combo efforts. I think they will work but perhaps not succeed.

You deserve an explanation after that cryptic remark.

Capacitors may be useful indefinitely, perhaps beyond the life of the car. They can have value even after the battery or control have departed.

A battery probably will not last the full life of the components. It has a negative value at some point.

And the controls may very well be specialized for each use. They are unlikely to fail but have zero value for reuse.

You guess is as good as mine.


>There is research to integrate the supercap with the
>battery in one case also containing control

Why not just a Supercap module you can attach a
12 battery on to?
Then it doesnt matter if its LiIon or Lead Acid.
Me I am hoping for FireFly + Supercap. This should extend the life of the battery even more and keep the high current burst off the batteries.

Something like this combined with the new flywheel recovery system might be pretty interesting.



Bob: I don't see much sense in integration of caps with batteries. But the future often surprises me.

Usually combination devices don't prevail. One reason is they reduce the options on placement and weight distribution. And usefulness or style is often compromised by the pairing.

Did you ever see one of those pens that writes in four colors that you actually wanted to use?

When I wrote my prior message I was tired and sensed it wasn't well said. The text wouldn't flow nor good examples but I posted it anyway.

It happens.

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