DIY generator on bike trailer

Yes, such a setup can be made to work, but it is NOT practical! WHY?--As an example, suppose that you use an electric motor with a 2 horsepower rating. Such a motor will require over 1,500 watts average, and as much as 6,000 watts when starting under heavy load, if full acceleration capabilities are desired! A generator capable of 6,000 watts generally weighs around 300 pounds (about 140 Kg) and you would add the weight of the trailer and fuel to this! This example, with only 2 horsepower, would not give great performance, considering that your weight, the trailer weight, the fuel weight, and the bike weight, would be added to the generator weight, giving a total weight likely to exceed 550 pounds, or 250 Kg! Much better performance could be achieved by using the gasoline motor to drive the vehicle directly, or using a smaller generator to charge a suitable lithium battery, thereby allowing use of a generator about 1/3 as powerful, and much lighter. (The battery would accomodate the peak electric demands, and the generator would replace that power, and accomodate the average power used.)Each time energy is converted from one form to another, part of it is lost! Motors, controllers, generators, wires, batteries-are NOT 100% efficient, so you always get out less power than you start with/put in.--Bob

Will it work? Yes. As pointed out, it is not efficient, though, due to the multiple energy form conversions.

If you are looking for significantly longer range, it is worth considering. The best way to do it is to have it charge your batteries once they get to a certain point, so that it is not running all the time, but rather is only running periodically for a short time, leaving you a quiet and mostly electric vehicle.

You could also set it up so that once you exceed a certain amount of current draw from the battery, that the generator starts and begins supplementing it, so for faster speeds or hills you don't run your battery down as much.

The basic idea of what you're doing is a "series hybrid", and they are apparently tough to do in an efficient manner, based on how few of them I see around the web that work well (efficiency-wise).

You might take a look at the Packrat Workshop, as he has built trailers to do this (as have people on Endless Sphere, I think, but I can't remember). http://packratworkshop.com

Turning the front fork around probably makes steering and handling pretty bad at low speeds. :) Generally more trail (up to a point) is better for low speeds, so depending on what speed you ride at you are almost certainly better off turning it back around.

As for charging the SLAs, you'll need an input voltage about 40-45V to charge a 36V pack. The current draw of the pack during charging will probably need to be regulated so it doesn't exceed the max charge current listed on the side of the batteries. The "cyclic" charge voltage range should also be printed there, so you can setup the regulator to be in that range.

Now, since the charger would be charging the batteries *while* they're running the motor, you have a challenge: keeping the higher charging voltage from reaching the controller, while still charging the batteries. Might not *have* to do this, but it depends on the max voltage your controller can take, and the charging system output voltage. If the controller can take up to 48V or more, it should be fine as long as you have a regulator on the generator that limits it to below that.

One simple way to do this is to use two sets of SLA batteries, one in charge mode and one in run mode, with a toggle switch between the two that moves the charger to the set not running the motor and vice-versa. But SLAs are heavy, and doubling your battery weight won't be fun. (my bike weighs about 150 pounds, a third of that for batteries alone, with 15-20 miles range).

If you're only going to use one set of batteries, you might simply need to put a few high-current diodes (possibly with heatsinks on them) between the battery and the controller, with a relay that automatically shunts around them when the charger is not running.

If you're not up to making a regulator/charger circuit, then another way to do this is to make an *AC* generator that runs an inverter powerful enough to run your existing SLA charger, and use it that way. It's heavier, and less efficient, because of the extra stage, but it is easier to put together.

What size SLAs are you using? If they're the typical 12v/12Ah, it might be much simpler (and lighter) to just go up to 12V 18Ah batteries, which depending on the efficiency of your drive setup (friction?) might give you the range you need without anything more.

Another option if your controller can handle it, and your charger will do it, is to go up to 48V, and simply limit your throttle usage to keep it at the same speeds you run at now. Less current will be used from the batteries to get the same power output, and the range will increase because the SLA's Peukert effect problems will be lessened. (The faster you draw power out of an SLA, the less of it you can take out in total, much worse than with other chemistries).

Doing that with your existing batteries might not be enough, but doing it with the next size up probably would be more than sufficient to get you enough range.

It would seem that if you simply installed some better batteries, such as 12 cells of the "Thundersky" 40 Ampere-hour lithium-iron type, you would be able to meet your range requirements, and power system weight (batteries) would be about 45 pounds total.(this might be equal or less than your present batteries?) If your bike uses 36 volts at 20 amperes for "cruising speed" you would be able to travel at that speed for about 2 hours before killing the battery! (For better battery life, you would cut that time/distance to no more than 2/3 of what would be possible) Example: Bike criises at 20 mph, using 20 amperes-battery range would be 40 miles, maximum! Reduce this to about 26 miles for better battery life.--These batteries would cost about $700.00 US.--Bob

I have a thought. The controller on one of my bikes is for a 24v but I run it at 36. could I use a 24v battery pack and set my generator throttle for 30v. That should be pretty safe and if the bike runs directly off the generator it should be fine to. If it needs to draw more amps it could pull from the batteries I guess. What about that.

Just run the generator circuit and the battery circuit in parallel to the controller.

Theoretically that should work, as long as the generator puts out enough current. The only real way to find out is to try it, then fix whatever glitches come up. :)

One thing you should monitor is what current the batteries are drawing from the generator, and what voltage they're actually at. Try not to let the charging current exceed the max current listed on the side of the SLAs, or you may boil out electrolyte from them that can't be replaced (since they're "sealed"). That shortens their life a LOT. (been there, done that, have three lead doorstops in the other room)

You may need to first run it on just batteries for part of your trip until they've run down enough to need a recharge, so they don't get overcharged, if the generator puts out enough power for most of your travel to run on just that.

To be perfectly honest, I don't have seven hundred dollars to spend on batteries. I build my own bikes from junk, because I can't afford the two grand for a fancy E-bike. Thanks for the advice guys. I'll see what I can come up with.

I know exactly where you're coming from. :)
http://lh5.ggpht.com/_zHr5qzOZQ9g/SrAJR9G1YBI/AAAAAAAACXQ/PXabncBl8OE/s512/dsc01766.jpg
That's my bike, started around the beginning of this year and still in progress, but thoroughly operational. :)

I suggested the battery sizes I did because they can be found as junk, in server-sized UPS units (battery backups for computers); that's where mine came from. Usually they have four 12V 17-18Ah SLAs, sometimes eight of them. Often the batteries are still good if you find one that was taken out by a storm. I live in the desert city of Phoenix, AZ, so we get a lot of lightning, and not enough protection is built into any of the electrical systems to take a close hit, or even some of the power outages/cycling that happens, and even a really good UPS may blow up in a storm. Keep an eye out and/or cultivate contacts with electronics salvage places, computer repair shops, battery recycling places, etc., and you may find free or close to free batteries waiting for you!

Freecycle.org and Craigslist are also good places to watch for offers or even to post your own want ad for junked UPS units or batteries.

If you can't find 18Ah, get whatever smaller ones you can and parallel them. This is bulkier and heavier, and each one may have a different actual capacity, is the biggest problem set.