Introduction: DIY Plug-In Hybrid Car

About: Ordinary guy with no special skills, just trying to change the world one backyard invention at a time. See more at: On Twitter - @300MPGBen and at
This Instructable shows how I built a Plug-In Hybrid Vehicle.

A plug-in hybrid is a car which can run off a combination of electricity (stored in batteries) charged from a wall outlet, and some other fuel, such as gasoline.

This vehicle uses ONLY the battery-electric system to start with, and has manual control over when the other energy source is used, in this case, a propane generator.

Using only electric power, and THEN switching to a hydrocarbon fuel only when needed, is sometimes also called a Range-Extended Electric Vehicle, or EREV.

Think of this car as a poor-man's Chevy Volt or Plug-In Prius.

I already converted this Geo Metro to run on electricity. You can read all the details about that at my ELECTRIC CAR CONVERSION INSTRUCTABLE.

This Instructable will detail adding a second power source to the vehicle to extend its range.

I've been driving this car for about two years now as electric. The only thing I don't like about it is that I find that there are a few too many times where my destination is just a little outside my range, or there is no access to electricity at the destination to be able to recharge while I was there.

Adding the second power system to the car allows me to make trips that I would otherwise need to use a gasoline vehicle for.

In a nutshell - the generator makes AC electricity that powers the traction pack battery charger. The charger passes DC electricity into the batteries. By constantly "topping-off" the batteries, they are kept more full and allow the car to drive farther. The generator is NOT powerful enough to drive the car directly from. However, the generator can continue to run while the car isn't using any battery power, such as rolling downhill, or at a stop light.

So, the smaller, but steady power of the generator provides additional range, while the torque and energy of the electric motor and battery pack give the car good power for acceleration and hill climbing that the small generator engine would NOT be able to provide.

Also, engines running at a steady speed at fairly wide open throttle are MUCH more efficient than one running lightly loaded or at varying speeds (such as in a standard gasoline car.)

See more of my clean transportation projects at

Step 1: Tools and Materials

To get started, we'll need tools and materials for the project.

Also, this project involves AC and DC electricity, Batteries, Generators, Flammable Fuels, Welding, and Power Tools. Always observe all safety precautions. Wear safety glasses, work gloves, and ear protection. Never use tools or techniques you are not comfortable with.

Essentially, this is an electro-mechanical project.

For materials, we will need:
Electric Car
Circuit Breaker
Electric Outlet
Angle Iron and misc metal (check the scrap metal pile!)
Some plywood
Propane tank
Propane Primary and Demand Regulators
Propane hose, hose clamps, and threaded pipe
Muffler and pipe
Electric power cable (6 ga or thicker)
Heat Shrink and misc wiring connectors
12V relay
The Mechanic's manual for the car
3/8ths inch bolts, nuts, washers, lock washers, and threaded rod

For tools, we are going to need a bit of everything:
Ratchet and sockets
Wire Strippers
Work Light
Drill & Bits
Battery Charger
Welder (although bolt-together construction would also work)
Tubing cutter, scissors, or knife
Volt Meter
and assorted other home workshop tools.

I'm a pretty big fan of my Craftsman Cordless Tool set. Two drills/screwdrivers, a work light, a reciprocating saw, circular saw, flashlight, and power stapler all run off the same interchangeable batteries.  I have two chargers and four batteries.
It's great to always have the right tool handy and not have to trip on an extension cord!

Step 2: Generator

The heart of this project is a generator, which creates electricity for the propulsion system.

A friend of mine is an RV mechanic. A while back, I got an RV generator from him. It was non-functioning and pulled out and replaced with a new one. He told me that if I could fix it, I could have it.

He was also kind enough to loan me the repair manual!

Since it's an RV generator, it's already approved for use in vehicles, has a remote start feature, and uses a 12V starter with negative ground (just like on any old car!) The generator is a brushless, computer-controlled, variable speed generator, sometimes called an "outrunner". (Because the permanent magnet rotor is on the outside of, and revolves around the stationary internal stator.)

It's a 3400 watt generator - pretty powerful for how big it is.

Going through the manual, I followed all the troubleshooting suggestions, checked the spark, air filter, and oil - all the typical things you would do for a small engine.

Not included with the generator when I got it was the muffler and the propane regulator. It took me a while to find the exact right regulator. It's a fancy Garretson brand, "demand-type" regulator. The regulator prevents any propane from flowing through to the generator UNLESS there is vacuum on the output end. When the generator runs, it creates vacuum, which opens the valve and allows the right amount of propane from the tank to the generator.

I also needed a primary regulator - the one that goes right on the LP tank. It required an 11 inches of water column regulator, which could provide at least 100,000 BTUs. Those are pretty common. I picked one up at the local hardware store for $23. The Garrettson demand regulator was mail-ordered and cost $50, plus shipping.

Much of the time spent on the project was just to get the generator working right. It requires pretty specific regulators, and the fact that it was partially non-functional when I got it meant I had to really learn some troubleshooting. However, a FREE generator was part of the inspiration for this project as the cost was right.

The other thing I like about this generator is that it runs on LP gas! While I would prefer to run the car on electricity (from renewable sources, whenever possible) some hydrocarbon fuels are better than others. The propane burns really clean. This is a four-cycle, single cylinder engine. (Much better than 2-cycles! Ick!) Gasoline is smelly, nasty, and volatile. Also, it goes bad.

In the Chevy Volt, if you never use the gasoline generator, the generator will eventually run automatically, all by itself, just to run some of the gasoline through to help keep it from going bad in the tank. You don't need to do that with propane.

Step 3: Mounting Bracket

The generator is small enough to fit in the back of the Geo Metro. However, it is designed to hang down between mounting rails in an RV. The flywheel, oil filter, and muffler all are suspended BELOW the bottom of the generator. You can NOT simply set this generator flat on its bottom. It needs to hang.

So, what I needed was a way to mount the generator in the car AND have space below.

The spare tire well is perfect for the lower space required. Now, I just needed a way to span the back of the trunk, so the generator could hang down into the "below the trunk" area.

Looking through my recycling/scrap pile, I found that I had some pieces of old bed frame. It's a very sturdy material. It cuts well with an angle grinder with a cut-off wheel, and it welds OK. It is pretty difficult to drill a hole in though. You will want some high-quality metal-cutting drill bits.

I test-fit the generator to see exactly where it would go in the back. I also measured the depth of the generator and made sure it would clear the sloping "shoulder" of the spare tire well.

The bracket is a capital H shape. I measured the distance across the back of the trunk, and cut two angle irons that size. Then I cut two cross pieces the length of the generator (16 inches) to go between the cross rails.

After double-checking the measurements, I checked for squareness, and then welded the four pieces together.

I then fit the generator into the bracket, between two sawhorses, so I could see how it would hang, and check for any other issues. Good thing I did! The bottom of the generator has an odd shape to it. I had to use the angle grinder to trim just a tiny bit off one part of the bracket for clearance.

I could then mark the center of the mounting flange holes - two on either side of the generator. I pulled the generator out of the bracket and drilled the holes. First, start with a small drill bit, then work your way up to larger sizes, the last one being 3/8th inch.

The bracket also got 3/8th" holes at it's ends where it will bolt into the car.

Once all the holes were drilled, the bracket got a coat of primer and a coat of black paint.

Then, the bracket goes in the car, and gets bolted in. The generator drops right into it and is also bolted in with 3/8ths" bolts, washers, lock washers, and nuts.

Step 4: Remote Start

One neat feature on this generator is that it has a REMOTE CONTROL PANEL connection on it. That panel is an option for this generator, which allows you to turn the generator on and off from the front of the RV. It also supports an "ON" indicator light and an hours meter.

However, I didn't want to spend money on a special order RV part, (you wouldn't believe how much money they want for it....) and I really just need to be able to kick the generator on.

Fortunately, the repair manual has the schematic for the remote control cable.

By grounding the proper pin on the remote cable, the generator will kick on the starter.

I ran a 14 ga wire from the spice off the ignition to a 12V relay. The leg opposite of that goes to ground (the car body.) When I turn the key the relay is activated. The other two pins on the relay are the wire from the remote cable, and also the ground. When the relay connects the remote cable wire is grounded, and the starter is remotely activated.

A new battery power cable needs to be run to the generator for the electric start. This will provide the 12V+ cranking power for the starter. The 12V negative is through the frame of the generator to the frame of the car, and back to the 12V accessory battery.
The cable needs to be heavy enough to support the amperage required for starting. Since this is only a 220cc single-cylinder engine, I can use lighter cable than would be required for a typical car engine. I looked at the cable on the generator that goes from the starter solenoid to the starter. It was 6 gauge. I used the same weight cable to run 12V power from the battery in the front of the car to the rear of the car, up through one of the drain plug holes in the spare tire well (through a rubber grommet) and to the starter solenoid.

Another great thing about this generator, is that it has a 12V charging circuit. After the generator is running, it charges the 12V battery used to run the starting motor. In my application, this is really handy. On my EV conversion, there is NO ALTERNATOR to charge the battery (because there is not engine to run the alternator.) Nor is there a DC/DC Converter - a device that converts one level of DC electricity to another - for example, 72 volts from the main battery pack to 12V for the headlights, radio, etc. Instead, I simply had a small 12V charger right on the battery. It would trickle charge the 12V battery while the car was plugged in. Most of the time, this was fine. But in cold, dark, winter weather, the 12v battery didn't always perform as well as I would want it to. (For example, dimmer headlights than I would want.)

Now, the generator will take care of the charging the 12V battery. 

Step 5: Circuit Breaker and Outlet

With the generator working, I now need a way to get power OUT FROM the generator.

Even though the generator already has it's own built-in circuit breaker, it seemed like a good idea to have a separate one.

In the RV repair manual, it always showed the power from the generator going to a power center or main breaker box in the RV.

I bought the least expensive breaker box I could at the hardware store, and a single 20-amp breaker. I also picked up an outlet and box to mount it in.

I screwed the breaker box and electric outlet box to a piece of plywood. I then mounted the outlet in the box and installed the cover.

In the car, I mounted the electrical panel vertically to save space. This also faces the electric outlet to the front of the car. For additional rolling testing, I can have the Kill-a-Wall right there to see how many watts the charger is pulling.

The plywood is mounted upright with steel angle brackets, and a triangle of plywood on the end for cross-bracing.

The power cable from the generator feeds through the top of the box, with the hot wire going to the circuit breaker.

I did some early testing on the work bench to confirm everything was working right. An electric space heater makes a swell electrical load.

Step 6: Propane Tank and Mount

The propane tank mounts to the left of the generator.

It needs to be securely bolted down. The only really nice solid mounting points were already used by the generator mounting rack to hold it down to the car frame.

So, I removed two of those bolts, and replaced them with threaded rod. Threaded rod comes in 36" standard lengths. I cut one of those in half to make two 18" long pieces. That's long enough to reach from the car body to the top of the LP tank.

To cut the rod, I threaded on two 3/8ths" nuts and marked a line at the middle of the rod. I put the rod in the vise, but protected the threads by wrapping it in cardboard. I cut the rod with the cut-off blade in the angle grinder. After the cut, I unthreaded the two nuts off the CUT end of the rod. This makes sure the threads keep nice, instead of getting all mangled, and allows nuts to go back on later.

The threaded rod went up through the car body and had a washer, lock washer, and nut on the bottom. Above the floor of the car, it got a nut to prevent the rod from falling back through.

I then measured between the two rods and marked the length. Another piece of bed frame is cut to the measured length plus enough room to drill a hole in either end of the angle iron for the threaded rod to go through.

The bed frame goes through the handle and another hole in the top ring of the tank. Since it's angle, it completely clears the valve.

Adding top nuts and tightening them holds the LP tank securely in the car.

Step 7: Charger and Regulator

Both the charger and the demand regulator are mounted to the right of the generator.

The charger is a big square box, but needs to have adequate ventilation around it. The regulator is an odd shape, and the mounting holes (two on the bottom, two on the back) are in a strange orientation, making that challenging to mount as well.

This isn't a big car. The suspension mount on the right gets in the way as well, so I built a bracket that raises the charger to make plenty of room for both the charger and regulator.

This bracket is basically just a wood box, make of two pieces of plywood and some 2x4s.

Like under the LP tank and electric panel, the base of it is plywood, cut to fit inside the angle iron frame.

Both the regulator and charger have 1/4"-20 mounting holes. I marked and drilled two holes where I wanted the regulator to go. I put it on an angle to match up how the fuel line would run to it. Two bolts run up through the bottom of the plywood to hold the regulator in place.

I traced out another piece of plywood to match the dimensions of the charger, and then added an inch and a half off the back. That makes it easy to run wood screws through the top of the plywood into 9" tall 2x4 vertical spacers (legs) that connect the top and the bottom of this wood box.

1/4" holes are marked for mounting the charger. Drill the holes, and run 3/4" long 1/4-20 bolts through the plywood to hold the charger.

Wood screws go down through the plywood top into the 2x4s and UP through the bottom plywood into the 2x4s.

The entire contraption - plywood box, regulator, and charger are all put down into that corner of the car. Then I ran a couple of self-tapping screws through the wood into the generator support bracket.

I cut two pieces of hose to fit from the propane tank to the input of the demand regulator and from the output of the demand regulator to the generator's fuel input line.

Both ends of both hoses get a stainless steel hose clamp, tightened down snug.

I like my Handi-Cutter for cutting air hose and the like. It makes nice clean, easy cuts. Far better than a utility knife for this sort of thing.

Step 8: Muffler and Exhaust.

Converting the car to an EV simplified a lot of things. It totally eliminated the fuel, exhaust, and cooling systems.

With conversion to a hybrid, those now still need to be taken into account.

The propane tank is much smaller than the original gas tank on the car was, and easily fits next to the generator.

This is an air-cooled engine, so there is no need for coolant or a radiator. An integral fan pulls air through the top and exhausts it out the bottom to cool the engine.

That only leaves the exhaust system. Exhaust needs to be routed to outside the car. The generator also did not have a muffler when I got it. Getting the "right" muffler for this generator would mean special ordering one for big bucks through an RV dealer.

I figured that the engine was about the same size as a basic riding lawn mower. So, I just bought a riding lawn mower muffler. That muffler uses a 1" NPT connection. A 3/4" pipe is nearly the exact same size as the exhaust pipe on the generator. Also, this car has several drain holes in the bottom of spare tire well. A 3/4" pipe fits nicely through any of them.

Remember how I originally test-fit the generator in the rear of the car before designing the mounting rack? That was partly to figure out where the muffler would go.

I threaded a 3/4 to 1" adapter into the muffler, and a 3/4" pipe into that. I pushed that up through the spare tire well from the bottom, and then screwed on a 3/4" coupler. That lets the muffler hang, and then tightening it and the muffler to each other clamps the whole unit onto the sheet metal of the car. A 3/4" pipe then goes up to the generator exhaust.

Step 9: The Finished Car.

I will still need to collect data on total electric energy and LP consumed based on various range trips.

In electric only mode, this car has already clocked in pretty consistently at about 130 Miles Per Gallon Equivalent.

Running as a hybrid, it will NOT be as efficient, but I will be able to use the car on many more trips that I would have otherwise driven a gasoline vehicle.

I also like that the generator provides charging for the 12V accessory battery and has some possible use as a winter heater as well. (This car does NOT have an electric rear defrost, but the generator is right under the back window....)

Running in Hybrid mode, the car IS louder than a typical gas car would be, (from the drivers point of view. It's really not that loud from outside the car. )  I may explore completely boxing in the back, including soundproofing.

I also like that I have complete manual control over the generator. In commercially produced plug-in hybrids, the driver doesn't have any control over when the car switches over to run the generator. I like that I can decide when to run it, based on my battery charge and how far away my destination is. 

Overall, I am very happy with the finished project.

Swing by my blog for info on my other clean transportation projects.

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