Series Hybrid with Hubmotor



I am going to use this thread to work out some ideas for a series hybrid hubmotor system. I saw an example on this forum by Denny and I am just trying to work out the idea and maybe it will actually become a reality.

I am starting out with a Wilderness Energy brushed hubmotor:

I have had this kit for a few years on a Giant Sedona. I am running 36V 12aH lead acid currently.

The goal is to build a Series Hybrid.

A gas motor powers a generator which can be turned on to power motion and charge batteries. The motive force comes from an electric hubmotor which is powered by batteries or by a combination of batteries and generator.

So basically this is an electric bike with an add on gas powered generator that can be turned on to increase range.

The hubmotor is a 600W motor. The motor probably uses 600W only when starting from a stop uphill. At full battery charge (about 37.5V) the motor would draw 16 amps (600W / 37.5V) at full load. At 50% discharge (36.6V) the motor would draw 16.4 amps at full load. Note that cruising along at the normal top speed of 17mph will likely use much less than 16 amps, 16 amps is a short term, peak load.

Currently I use 3 batteries to reach 37.5V full charge voltage. I would like to increase this to 4 batteries -> 50.6V full charge voltage. I have heard good reports that the hubmotor and controller can handle up to 72V and remain reliable.

In order to choose an output voltage for the generator, I will look at the maximum recommended fast charge voltage, which is 2.45V per cell. With 4 batteries each with 6 cells, I get a fast charge voltage of 58.8V. This is my target voltage for the generator.

My next goal is to specify a gas motor and dc generator to provide about 58.8V and to be able to power a peak 600W, more or less, which is what the hubmotor is rated for. Note that average power use will be less. I also want to specify a lightweight generator system.
If link doesn't work search on BOSCH 1/2 HP 120V DC WIND GENERATOR MOTOR MILL LATHE

This dc electric motor has the following ratings:

120V at 10,000 rpm's
5.5 amps
Continuous Duty

Voltage and rpm's are roughly linear, so I would expect 58.8V output at 4900 rpm's. (12V per 1000 rpm)

The amp rating is lower than I would like, so it is possible that this motor will not provide enough power. However, in short term high load situations, the batteries will act as a buffer and will provide additional power beyond what the motor can put out, so I am hoping that the 5.5 amp value will provide more than the average current draw while riding the bike. At 58.8V, 600W of power would result in just over 10 amps peak load, so I may be OK with 5 amps for average load. If not, I might need to move to a 5 battery system at 73.5V. If 73.5V are needed, I would expect to measure 73.5V at about 6125 rpm.

So the generator is rated at 640W and it needs to run at about 4900 rpm to produce 58.8V. Now a gas motor is needed that will produce more than 640W at about 5000 rpm.

The Honda GX35 four cycle motor is rated at about 800W at 4900 rpm and about 950W at 6125 rpm. The power rating is adequate and the maximum torque for the motor is in the desired rpm range. This means the gas motor will be running efficiently.

The motor and generator would be connected with either a flexible or solid coupling. The motor throttle (idle screw) would be adjusted until the generator produced the desired voltage, which is 58.8V for a 4 battery system and 73.5V for a 5 battery system. To start the generator, a double pole double throw switch would connect the dc motor to the battery voltage and would also break the ground to the motor shut off, allowing the motor to run. The dc motor would start the gas motor by turning it. The gas motor would fire up and produce voltage to charge the batteries and power the hubmotor.

In operation, the bicycle could be used as a battery powered bike until it is noticed that there is a reduction in power due to batteries starting to discharge. Then the motor could be turned on either full time or during heavy loads such as starting from a stop or climbing hills, depending on how much additional range is needed. For long distance travel, the motor could be run full time.

The benefit of a series hybrid is that it works well for extending range and provides a guarantee that you will make it home if you travel further than your battery range. It is less suited for long distance travel but may work using full time power generation. Adding a gas powered generator turns the electric bicycle into a plug in hybrid vehicle. My current range with the 36V 12 aH battery is about 10 miles. Adding additional batteries would likely extend this range to about 14 miles. The gas generator system should at least double the range and perhaps would be powerful enough to provide as much range as the fuel tank allows.


Active Member
Apr 18, 2008
I like that. You obviously know much more about the engineering details of such a system than do I, but it is pretty much what I have envisaged in a couple of prior posts, including the Gas/Electric Hybrid thread. It just seems to me to give a lot of benefits. Certainly there are trade-offs (there always are) in terms of straight mechanical efficiency,overall weight of system, etc, but they seem to me to be outweighed by the advantages.

Not least is the ability too, at need, effectively run silent on pure electric power, and to tune muffler/exhaust systems to the lowest attainable sound output levels at the desired rpm range. Ive been looking lately at how one might go to a computer based system management regime in a small hybrid vehicle, with inputs from sensors on the throttle, brakes, and vehicle power system. It seems doable, relatively cheaply, utilizing something like the sub-laptops or expanded PDA's now available.


New Member
Jul 22, 2008
The problem with adding a gas engine and generator is the added weight and where are you going to put it on your bike?

If you saw this thread...

I am working on a similar system, but a pusher trailer, based on Denny's idea.

I am using an 36v/350W electric scooter, a Homelite 25cc gas trimmer engine and the Bosch wind generator motor. This will all be combined to make a pusher trailer.

The problem I have right now is that I took the scooter apart and disconnected all the electronics. I eliminated everything but the key switch, and throttle and tried to wire it all back to the controller. Unfortunately the scooter controller jas a number of auxillary functions built in and I can't get the controller to recognize the key switch and throttle. Plug everything in and the motor goes full speed. I am going to have to figure out a way to open the controller and rewire it or get a another basic controller.

I am open to suggestions as to what other controller I should try. I really only need one that has on/off and throttle.

As far as the gas engine and generator motor are concerned, the issues are coupling the shafts together and figuring out a way (bracket?) to mount the whole thing to the frame of the trailer.

Shaft coupling is tricky in that the shaft from the generator is slightly larger than the shaft of the gas engine. You have to make sure they line up and center on one another. I did find a shaft coupler online that can have two different diameters and will center each shaft but is it $33+. I will probably order it since it really it the right way to do it despite the cost.
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It looks like the GX35 only comes with a clutch, I need one with a shaft. Will have to call .



skyl4rk and mac....I am honored at your interest. Crude as it was, mine worked surprisingly well. The split couplings are the way to go from a dependability point of view. Sky, your take on the engineering is dead on....and on mine proved out pretty much as expected. Using a PM motor, you could use an engine with a governor to control voltage. On my *new and improved* version I went with a Honda GX25 with the idea that it was governed, but it wasn't. The GXH50 apparently is.

I ran mine for nearly 800 relatively trouble free miles until the Alaskan climate got in the way. I ended up getting moisture inside the hub, and blowing the controller. In a drier climate, it would probably still be going.

My other objection was hill climbing ability. Juneau is kind of stuck to the side of a mountain. With 48 volts, and the kind of loads I carry, hills can be a problem. Under less abusive conditions, it would work fine.

Anyway, thanks for the recognition, and good luck. Hope it works well for you



Here is my theory for motor control:

Use an Arduino programmable processor to "sample" the voltage level every second or two. The Arduino would then control a servo attached to the throttle of the gas motor/generator. Under load, when voltage is below target voltage, throttle up. If voltage is above target voltage, throttle down. That would be a pretty simple programming job on the Arduino, although it might take some finesse to stop motor pulsing.

There might be some value in knowing the no load battery voltage as well but at first I would just check the operating voltage every second and adjust the throttle accordingly.

I could probably use a relay to let the Arduino turn the gas motor/generator system on and off but that is something I would rather control manually, at least at first. I don't want the motor to start up when I don't expect it.

I'm waiting on a new carb for the weedwacker motor, then I can start ordering some more parts to make it happen, assuming I can get the weedwacker to run.

By the way, I was looking at scaling the concept up for a little more power, and there don't seem to be too many permanent magnet dc motors or generators in the 1kW and up range. Plenty of 3hp four cycle gas motors though. Their rpm range is not as wide as the GX35 so there might need to be some gearing between gas motor and dc motor/generator to reach the right rpm to generate the required voltage.

Maybe the cheap and easy solution is to use multiple 500W dc motor/generators, geared appropriately. For now I will just work on the bicycle series hybrid system and not try to scale it up to a streamliner or velomobile application.


Progress Report:

I have upgraded the system to a 48V 12aH lead acid system with powerpole connections. There is a battery tray in the rack so the rear rack is free for the generator.

I have not been able to get the weedeater motor to run for more than about 20 seconds. There appears to be some fuel supply problem which I think is in the gas filter. I put a new carb on it with the same results. So maybe next weekend I can get it working? Got to find parts, nobody around here seems to have the right fuel line and filter.

Pics will be posted soon.




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