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Charge to go: The EP Tender portable EV range extender


July 30, 2013

EP Tender range-extender system

EP Tender range-extender system

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While electric vehicle charging networks continue to evolve and expand, there is another approach to reducing range anxiety: taking the charger with you. The EP Tender range-extender system is designed to give an extra lease of life to EV batteries with the addition of a trailer mounted generator system.

Applying the same principle that we've seen in the ebuggy and Rinspeed Dock+Go concepts, the EP Tender portable charging system is essentially a mobile commercial generator that's been pared down for consumer use. The 200 kg (440 lb) system utilizes a trailer mounted 600 cc engine that resides in standby mode until a low charge signal is received from the vehicle. While the vehicle is being driven, the tethered trailer activates the generator, which in turn recharges the EV battery. The idea is that this charge on the go system could negate the need for lengthy charging stops.

Currently under development in France, the system can generate up to 22 kW of power during a cycle at speeds up to 130 km/h (80 mph). The company is also working on developing a 35 kW system using a 900 cc engine. Depending on the vehicle EP Tender reports a range of 600 km (360 miles) could be achieved using the charge to go system. But there is a hitch. As Autobloggreen points out, main stream EVs aren't necessarily designed to be charged while in motion, which adds an extra level of complexity to the equation.

EP Tender's portable charging system is still in its early testing stages, with similar systems for electric motorcycles and hybrids reported to be in the works.

Source: EP Tender

About the Author
Angus MacKenzie Born on the cold, barren Canadian plains of Calgary, Alberta, Angus MacKenzie couldn’t decide between marketing, automotives or an entrepreneurial path - so he chose all three. With an education in automotives and marketing, Angus has rebuilt the carburetor on his 1963 Rambler Ambassador twice, gotten a speeding ticket in an F430 once, and driven & photographed everything from Lamborghinis to Maseratis to various German and Asian designs. When not writing, Angus has for the past six years been Editor-in-Chief for elemente, an internationally recognized architecture/design magazine. All articles by Angus MacKenzie

those who use an electric car tend are likely to be unable to drive with a trailer . but I don't blame them . I couldn't drive this thing , as modern cars have windows the size of a quarter , and high up in the sky , so you have to get out , and push the trailer by hand or use the mirrors to guide a trailer that short (fast to turn) and low (nearly invisible) , narrow (already sideways when you can see it appear in the mirror) completely useless , stupid idea .

Károly Hőss

As a DIYer this is exactly what I've planned. But a Nissan LEAF would be ideal. I even wrote to Nissan suggesting that a couple of these, hireable from their new car resellers, would alleviate any need for a car swap/ 2nd car, for a single guy like myself, for when I want to do my annual country holiday in Australia! Common Nissan, do it and demand will soar!


This is a no brainer and something I have been asking EV manufacturers about for over a year. Portable generator + charge controller +trailer = unlimited range. This should be a standard product!

Max Kennedy

What is neat about this is that if one is just traveling around town and have a full charge, one can just disconnect the trailer and leave it at home. One could reconnect it for longer trips.

It think it would be really green if they develope a fuel cell version that would not any exhaust beside water. There are a lot of companies developing fuel cells. One in Europe is being made by Clever Trailer.


Better yet, rent the trailer from an (Esso?) gas station just before you head onto an interstate, and drop it off when you get to your distination. You would only need one typically for occasional long distance trips, and the rest of the time just stay all electric for in town use. Rent it here, leave it there! I should have patented this approach when I played around with the concept twelve years ago!


This is awesome. There are so many variations one could think of! A camping version engine trailer. A flatbed version for transporting. A woodgas version. Turbines? Stirlings? Done right, this could double as emergency generator for my house. For half a dozen houses in fact.

De-coupling the engine from the car will open up options like never before. Let's hope manufacturers standardize interfaces soon, couplers, hitches, connectors, voltages, and protocols. (I'm just dreaming...)


This seems to be more of a step backwards than a step forwards. If the idea is so good and necessary for range, why not have the generator built into the car?

Brian Shaffer

Anything electric is good as this is where everything is heading. For the next decade or so I'll be sticking with a standard combustion engine car and using electric bikes. Don't need a 2nd car plus with the amazing all weather gear available can do this year round. You can put a kit on a mountain that will give you 40 mph ( more if you like ) and travel 30 miles or more. PLus you can charge the batteries from any outlet at home work or school. Have yet to see an apartment or condo complex that offers electric car chargers. The bike can be done for under $2,000. Standard and electric bikes will do more to save the planet than electric cars.


What a great idea. Use up all that energy to pull your energy needs around with you. Why not make it so big that you will never have to stop for more energy ever again.


Koraly you are projecting your own lack of skill. Facts are most EV drivers are far better ones than others because EV driving takes skill and generally smarter.

I was wondering when these would start getting built. I disagree though that a trailer is needed.

There are few EV's that need larger than 10kw range extender for unlimited range., only large and fast ones and they only need 20kw. Lotus made one for 35kw in only 115lbs.

These can be built under 70 lbs and go on the trailer hitch like the wheelchair carriers do. I think all EV's should have these mounts, connection standard.

I drive my lightweight EV's so I only need a 4.5kw one that only weighs 45lbs. An OEM could do it in 30 lbs.


I am glad Chev have the forsight to design a charger into its Volt to help alleviate the anxiety of limiting how far a EV can travel. The charger in the Volt will automatically kick-in as it sense the battery level depleted. A small 9 gallons tank will provide an additional 350 miles distance with a single fill up. Thank you Chev for that innovation design...


Another example of this was the Long Ranger trailer with BackTracker Steering system developed by AC Propulsion many years back when Alan Cocconi and crew where running the show.

It had the advantage of a self steering system that made reversing a non issue. This also allowed it to have a vary short length. ( Normally the shorter the trailer length the more quickly things go wrong when reversing )

There is an old video of it reversing though a slalom at the bottom. http://www.tzev.com/2001_rxt-g_steering.html

Brian Smith

why not hook the generator to the trailer's wheels so it will charge as you tow it along the road?

Artisteroi Rlsh Gadgeteer

Now some things like a trailer have universal appeal this generator is a very good example and some thing I haven't seen yet is a solar trailer yes put a solar array on top of that generator trailer and a camper then you got a marketable product!

Bryce Guenther

If the price was low enough, I might go for this interim solution to the range problem. I would only need to use it once a year and it would be nice to have the option of unlimited range anytime.

Even when EV range reaches 400+ and charge time under a half hour, the infrastructure is lacking. That will come shortly after (1-3 years?) but for many the inconvenience means they will wait.

If switching fuels does not save money I won't buy. I don't want to "save the planet" I just want to save myself. These two goals should be compatible, and would be under a capitalist system, but no such system has ever existed in pure form, i.e., without government interference. A mixed system will not work because it is founded on the belief that economic freedom is not morally justified. Unfortunately, a mixed system of capitalism & socialism is found in every country.

Don Duncan

I drive a Nissan LEAF and love its performance and efficiency. A trailer like this would destroy all that. I might as well keep an ICE vehicle sitting around and well maintained for long trips. (which is what I do). Its a lot cheaper, I'm sure, and I've got an extra vehicle in case I need it. I would also dread trying to back up with such a short trailer even though I could do it. Hooking up a trailer to a LEAF would be like tying a cocoanut to an normally unladen swallow. It may as well be a ball and chain.

Rustin Haase

why not make the portable generator powered not by a gas engine, but by mechanical flywheel motion and the wasted energy that's not collected from forward motion of the wheels on the trailer??

John Carman

What a kludge - Tow your own power plant all because battery technologies are still unsuitable for electric vehicles needing anything but the shortest commuter range.

Want a real vehicle with real range: get a gasoline or diesel engine and forget the feel good electrics until batteries approach the needed energy density. Don't hold your breath for this or you'll turn blue.


While a detachable range extender offers some advantages, there is the trailer problem. Will it be stable in tow? The distance between hitch and axle is rather short. That distance makes it almost impossible to back up the vehicle. An option is a single-wheel trailer (the wheel swivels) with a 2-point hitch. Still some interesting driving dynamics involved whichever you choose, with the equivalent of 2 passengers hanging out there at the rear.

Bruce H. Anderson

Why on earth would someone 'be unable to drive with a trailer'?

I have a trailer hitch on my VW beetle, and used it to drive the 5000miles it takes to get down the AlCan highway. It was great.

I woud definitely do this. Likely would not buy it from a specialty vendor though. Easy enough to buy a small generator and build it yourself. I suppose of major importance is the question of whether all curent ecar software allows for charging while on the go. Its basically a giant cell phone as far as that part goes, and there are quite a few electronics that balk at charging and being used at the same time.


I agree with the potential 'towing a low trailer - issues'... Perhaps mounting it to the rear of the vehicle in a way where 'one' wheel lowers to make contact with the road when charging is required. The rear opening would become unusable but hey...trailer problem solved...?

Gerald Grey

This just totally cracks me up. By the time you figure out the (lack of) efficiency of the generator, the cost of same, and the pollution it puts out, the whole package comes out as crap, thus proving the absurdity of electric multi-passenger vehicles given current battery technology. Electric motorcycles, bikes, and electric city vehicles do make some sense, but this? This makes about as much sense as an environmentalist driving a 1971 Beetle (which puts out over 200 times the pollutants of a modern SUV). Fail.

Mike Barnett

Why not drive with 3 bank of batteries, and run on one, and when that goes low, switch to bank 2, and when on 2, bank one is recharging, and when bank 2 is low switch to bank 3, and when running on bank 3, bank one and bank 2 is recharging, and then when bank three is low switch to bank one, which is recharge, with this system you could go 10,000 miles and more, without taking on a outside charge, but then the Oil companies, would complain to the congress they pay off in campaign funds, and congress would find some dirty way to outlaw the process.


Regarding the issue of backing with a trailer: please have a look at http://www.youtube.com/watch?v=KtKmCIJC4Vo&feature=youtu.be As you will see this is a neat and efficient solution. Furthermore, the Tender is attached to the car in a single move by pushing a lever which both locks it mecanically to the car and connects the plug. EP Tender doesn't use the car's onboard charger: it générâtes DC current at battery voltage. www.eptender.com/blog will give you details on fuel efficiency and range (skip the French part). I'm looking forward to batteries which would be 10 times better (2025 at best?): 44kWh, 2kUSD, 130 pounds. That would make great and popular EVs... which would still from time to time need a RE for highway trips longer than 180 miles! Why carry a RE (assume 5kUSD and 300 pounds) 360 days a year if you use it only 6 days and maybe 30 hours !? Chevy Volt is a great technology achievement, but I wouldn't call it affordable. Put a RE in a Spark and the car has no market (payload vanished, cost through the roof). Allow a Spark to use EP Tender and it's market might be significantly increased ! Tom Gage did indeed do a great job with his earlier version at AC Propulsion. Kind regards Jean-Baptiste Segard, founder of EP Tender

Jean-Baptiste Segard

@Károly Hőss: Not much of a driver, are you? Or maybe you just own a wee too much fossil fuel stock? Anyway, what a great idea and it's amazing how many EV enthusiasts/owners had already thought of this (myself included).

Could mean trouble for Tesla — guess I better sell my stock! LOL

Fritz Menzel

Not sure if those suggesting hooking a generator to the wheels of the trailer are sincere, joking or trolling but I will quot my father's favorite response to this often heard sort of question at EV conventions. "That is a great idea in fact we do that already its called regenerative braking".

I would suggest EP Tender look in to lobbying for some sort of exemption to California's 55mph speed limit for vehicles with a trailers, 75mph is commonly the speed limit for cars with out trailers. Best of luck in bringing your product to market.

Brian Smith

Isn't that cheating?


Telsa, mass produce this for your car line & lisc for other makes? Radical. Id drive with this to extend range 200 miles.

Stephen Russell

An excellent idea but rather defeats the idea of having an electric vehicle at all. A better idea would be to put he engine and generator into the car to perform exactly the same function and then call it a hybrid!


A light removable engine/charger in the car would make sense as it would be easier to drive than a trailer and not have the weight penalty of a permanently mounted engine. If drive could be taken from the engine directly , it would be more efficient especially for long trips. How efficient is the Volt at extracting power from its petrol compared with a normal ic car ?

Stephen Colbourne

This is what I have been suggesting for Tesla for the last 4-5 years on their website. I figure that with such a trailer, you can go 500-1000 miles before needing to stop (which is plenty far). The idea being that when you absolutely NEED to go long distance, you tent a trailer that has a engine/generator, OR a fuel cell, OR a super conductor, OR.....

G Richard Raab

This depresses me. As others have pointed out this is total face palm stuff. Towing a combustion engine to provide your electricity! ?Lowering your road efficiency. Inefficiencies in conversion etc. I'm sure if we had the stats for the actual extra miles provided, and the amount of gas used, it'd be obvious that it would be less efficient and producing more greenhouses gases than a conventional car! Unless I'm missing something obvious here.

future me

Cannot believe some of the short sighted questions and points put forward here. Some counterpoints:

Having an engine permanently mounted in the car adds weight and takes away valuable space. This means that the vehicle has the inevitable penalty of larger brakes, stronger suspension etc for something that would not be needed for 95% of the time.

The engine can be designed to run at an optimum speed unlike the engine in your car which has to cope with a wide range of revs etc. Think of how comparatively efficient a constant speed generator is. The comment that it would be more efficient to run a conventional car is BS.

As to all the points about the hazards of such a small short wheelbase trailer - pffft. Yes, having owned a short drawbar trailer I realise it is easier to jack knife, but everything has drawbacks and tradeoffs. I'm sure they could build a version with a longer drawbar and if necessary poles or marker lights to make seeing the extremities simpler. Increasingly cars are being equipped with reversing cameras anyway. That is not rocket science. Once this trailer is hooked up you can no longer add the usual box trailer for the inevitable holiday junk - I forsee the range extender being built into a larger trailer anyway so people have got more space anyway.

just a few things to think about.

Marc 1

the BMW i3 coming out soon will do this combintation.


Thanks Marc 1 : excactly to the point.

Plse have a look at http://www.youtube.com/watch?v=KtKmCIJC4Vo&feature=youtu.be to see how I'm solving the issue of baccking with a short trailer. No risk of knife jacking!

future me: plse have a look at www.eptender/blog to see MPG figures (skip the French part...), and don't forget that using EP Tender 6 days in a year allows you to drive zero emissions for the other 360 days...

Jean-Baptiste Segard

I thought about something similar, but my idea arose out of how to take a several-day camping trip in an EV. I figured you could build a small pop-up sleep-in trailer with about the footprint of a teardrop trailer that contained a small generator and solar panels on top. That way, once you got to your spot and parked in the sun, it would just sit there charging for several days and you'd be ready to get home non-stop, the whole trip accomplished without plugging in and with minimal use of fuel.


Thanks Marc and Jean-Baptitse. The only point I hadn't really thought of was the increased efficiency due to running the engine at a constant speed. Buy that said, looking at the data from the blog, it is still less efficient in mpg than my modern trad car. Is closer than I imagined though, but still... However, the figures given are not real world figures, and do not seem to take account of some factors that would further worsen results (going on a long trip you may have heavier luggage and more occupants, etc etc). Real world statistics are definitely needed as simulated data is very unreliable IMO...

Good info though, I'll think on it more . . .

future me

It may not be practical. own Chevy Volt which more functional than an EV. But it has its own issues.

1) Range should have been more at least 100 miles.

2) It should have been designed for five passengers.

3) Should designed for FAST CHARGING at least 30 minutes max to full charge.

To Solve the issue of Range Anxiety the EV Manufacturers should consider to survive and over come the present issues the following suggestions should consider.

A) There should b Universal Charging System.

B) All gas stations should be equipped with SUPER CHARGER/FAST CHARGER.

roh vemula

" and don't forget that using EP Tender 6 days in a year allows you to drive zero emissions for the other 360 days..."

Was thinking, If this is the likely use scenario, this device is surely not the answer. Hiring a conventional car would offer a much better solutionfor those long journeys environmentally, financially, and is shared among many users by default.

I don't know what the retail price of the EP tender would be. But hiring a car with "unlimited" range and greater fuel efficiency would cost in total about £60 here in the UK (approx$100) for the year for those 6 days, and have ample room etc. If Ep tender is at the $1000 mark (Anyone know the projected retail price?) it would be a very expensive kludge, and maybe bad long term investment.

future me

600cc to start, 900cc to come... Wouldn't a drive wheel make more sense? The 600cc is more than enough for most EVs to maintain highway speed. A drive wheel to move along and an alternator to charge the batteries would be more than enough.


@future me: Yes, rental is the plan. Tenders will be rented by both private and professional users in order to spread load across the year, week days and week ends.

Retail price which is envisaged: 40 € per annum (to be a registered user and get the NFC card) 10€ per day, plus 7€ per hour of running the generator 10 € addition for one way trips.

In other words return trip London-Southampton area same day: 20€ 2 days WE to Norwich: 40 € Summer holiday to Lake district: 52 € in and 52 € out.

This cheaper, more convenient (door to door trip), and more environmentally friendly than renting a car.

And over time I hope there will be fuel cell Tenders on routes equipped with hydrogen distribution (but this will take time): your same EV will be able to use that as well.

Jean-Baptiste Segard

I haven't seen the added wind drag being addressed. As well as anything that charges by flywheel, drive wheel, or any other type of drag decreases the range of the vehicle, by adding drag. regenerative breaking DOES make sense, but only if the unit makes up for the added drag it creates! "There is no such thing as a free lunch" This saying is quite old, but still true. Simply towing a second battery pack would extend range, and could be charged before a trip on mains power, or a solar array. I would suggest that the waste heat created by any type of car, and the cooling effect of the car moving, would provide additional range as well, without the added drag. Waste heat from the sun on the windows and heating the cabin space, we currently use power to cool. We should be using that heat to recharge, anytime the vehicle is parked in the sun. And with thinfilm, and printable solar panels, there should be a solar panel standard on the roof and hood of every electric car, charging, and extending the range at any time the vehicle is out doors. With simple fiber optic skylights charging could continue even while in the garage< in addition to whatever charging system is currently (pun-intended) in-place, mains power, solar, or generator.


It is interesting to look at the "to and for" on this topic, but it seems that two obvious things have been overlooked. First, rather than towing a generator - tow an extra set of batteries if you are going to tow anything at all, this would take advantage of low cost electricity from the grid.

Second go have a look at a better solution in the form of battery exchange as developed by a company called Better Place. Click here to go to their website ht tp://www.betterplace.com/ I think this concept has the potential to make electric cars a reality.

(You might want to have a look at this: http://www.gizmag.com/better-place-liquidation/27682/ - Ed.)


Extending the overall powerplant by the addition of a compact generator erfectively makes the car a series hybrid. Imho series-hybrid is a much better system than the parallel-hybrids in use today, miles better (literally!) than electric-only and infinitely better than electric-only with a trailer. Nor is it a new concept: go back 100 years or so - http://press.porsche.com/news/release.php?id=642

D Francis

The trailer does nothing about the cost problem of stored energy EVs but it does solve the range problem for road trips without too exorbitant of cost. I think that the trailer's shape could be improved by making it an aerodynamic tail cone. This could easily be done by attaching the trailer rigidly to the car and using a single swivel mounted tire.

It might be cheaper just to have the engine on the trailer drive the trailers wheels fixed into a high gear using the cars motor to get up to speed.

While they are at it they may want to make the trailer larger and give it significant cargo space.

re; Jean-Baptiste Segard

That is just so cool.


I agree with Slowburn, when it comes to utility. If I'm pulling a trailer, and going somewhere distant, I probably have something to carry or taking people on a trip. The 'trailer' should be configured to be useful as well. -luggage? -bulky items (large cooler) Are there any flat opposed efficient engines that could improve the packaging?


There will be a small luggage rack on top of the Tender. Will typically allow for a large suitcase, or 2 golf bags, pram, bicycles, etc.

It is conceivable to build a larger Tender (especially for the US), but will naturally depend on the market requirements. I believe remaining lightweight is an important feature.

Compact and light range extenders prototypes do exist , but none has yet secured enough volume to be industrialised and reach a market compatible price. In the meantime, deriving the Tender from a small car's engine is a quite reasonable and suitable option (regarding emissions as well).

Jean-Baptiste Segard

I have owned a single wheel trailer for a few years now. I can not see the trailer at all except in the reversing camera (last 6 months) In the short term ERTs, extended range trailers, are the answer, For some one way rental would be best. For some a removable generator set on a single wheel trailer (that can be fixed to a larger trailer) is the best. But a standard fitting is most important. On to the trailer can be fitted batteries like a hybrid so the car can recharge and the trailer drives the car like a hybrid, (but not though the single wheel). Eventually all houses will have recharge points and some home owners will sell electricity to cars that have "run out" via a pole in the from yard. Service stations as we know them will cease to exist except for a very few, because recharges will be available from shopping centres where you buy "on road finger food" and petrol will be available from the local shop. Future cars will have so much technology built into them that people who quibble now about electric cars will be walking, or learning to drive again. Technology doubles every 8 years and we cant stop change.

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