Introduction: Off Grid LPG (propane) Powered Battery Charger

About: Fixer, Finder, Fabricator. I teach engineering to high school students, at St Marys Secondary College in Nathalia VIC Australia

If you live off grid, sooner or later you will have a problem with the house batteries going flat. Maybe the solar panels cant keep up because it been cloudy for a few days, or its the middle of winter and you don't have enough daylight hours. Or someone left the TV on all night, what ever the reason, chances are you will have to charge your batteries with a petrol (gasoline) generator at some stage.

Many off grids set up use a 110 or 240 volt generator plugged int a battery charge to do this, but this set up is expensive and not particularly efficient.

This Instructable covers the build of 2 two battery chargers one with a Honda clone engine and another with a Kawasaki lawn mower engine. Also shown is how to convert a car alternator to produce any voltage between 12- 60 volts DC. which is Ideal for charging the house batteries.

Also I will show you how to convert the petrol engine to LPG (propane) or natural gas, which in my part of the world is less than half the price of petrol.

Step 1: Parts and Equpment You Will Need.

You will need a few parts, and you can you a second hand alternator to keep the cost down. I would recommend either buy a new engine (they are cheap enough) or use an engine that is known to be in very good condition, as a worn out engine will be unreliable and likely give problems when it is dark cold and wet....

  • 6 hp engine
  • 40 amp speed controller rated at 50 volts or more.
  • Digital Voltmeter Ammeter rated 100 volts 100 amps

  • 9 volt battery and battery snap.
  • Small push button switch
  • Automotive alternator. Rated voltage doesn't matter but the more rated amps the better.
  • Dual fuel carburetor, if you wish to run your engine on gas.
  • 5 or 6 inch pulley
  • Vee belt
  • Wires, electrical terminals, tape, heat shrink etc.
  • 25mm RHS steel to make the frame
  • Nuts bolts and fasteners
  • Hack saw, welder and general workshop tools.

Step 2: Battery Charger One. the Vertical Shaft Engine.

I would not recommend this approach of using a vertical shaft engine off a lawn mower, as it is more complicated to mount the motor and alternator to the frame.

However the people I made it for live in "foot rot flats" they picked up this engine for $50 and it looks to be near new.

The plate that mounts the engine and alternator was designed in CAD and cut out with a CNC plasma cutter, which is probably the hardest part of the project. A steel frame with legs made to stiffen the 6mm plate and legs were fitted to give some ground clearance and keep the belt out of the dirt.

Everything was then test fitted, and the frame was then sanded and painted and then reassembled. A pulley and belt were fitted then the contraption was almost ready to test run.

Step 3: Setting Up the Voltage and Curret Control.

If you are running 12 or 24 volts you can certainly wire up the appropriate voltage alternator with the original regulator and that will charge you batteries, but there is a couple of problems doing it that way.

  • First, automotive charging system are not designed to charge the battery to 100 % typically they will only charge a battery to around 80% or 14.2 volts and deep cycle battery should be charged at 14.6 - 14.8 volts
  • There is no way to control the load on you engine, a small engine will struggle to drive a large alternator if the battery is very flat.
  • 24 volt alternators can be expensive and difficult to find.
  • If you have a 36 volt or 48 volt system, its not going to work.
  • That said using the original regulator will make your charging system Idiot proof and you can buy after market regulators that will do 14.7 volts.This is what we fitted to the Honda engine charger.

To get around these problem is quite simple, just fit a motor speed controller to the rotor of the alternator. The speed controller shown in the photos was a bit over $10 and will work on 12, 24 32, or 36 volt systems. Its rated at 50 volts, so it may be damaged if been used on a 48 volt system, but Im sure there are other speed controllers you could use if you wanted to get more voltage from you charger.

To fit the speed controller you just need to..

  • Remove the alternators internal regulator (if fitted)
  • The M+ and M- on the speed controller are wired to the brushes, polarity does not matter (this is what powers the rotor)
  • The Power + and power - go to the battery been charged. Its probably worth fitting a 20 amp fuse just in case someone hooks up the batteries around the wrong way.
  • You must get the polarity correct on the power side of the speed controller.

Step 4: Fitting the Volt and Amp Meter

The volt and amp meter can be fitted next, but it didn't come with a wiring diagram. A quick internet search was able to help with that problem, and the schematic is above.

I would recommend that you power the meter from a 9 volt battery rather that the battery been charged, as it could be damaged by a voltage spike, or over voltage.

A push button can be used to give a readout which will save on the 9 volt battery life.

Step 5: Operating Procedure

Understanding how the system works and how it could be damaged, will save a lot of repairs, burnt wires and swear words. The charger is definitely not idiot proof, it can be easy damaged and over charge or flatten you battery so you need to follow the start up and shut down list.

First make sure the speed controller is turned off by turning the knob fully counterclockwise!

  1. Connect charger to the battery, double check the polarity or the speed controller will be damaged.
  2. Start the engine and set to desired rpm.
  3. Turn on the speed control by turning the knob clockwise, it will click.
  4. The speed control is very sensitive, slowly turn up the speed controller to load up the engine.
  5. The voltage and current can be checked by pushing the button.
  6. The voltage needs to be regularly checked and adjusted as necessary, there is no automatic voltage control.
  7. To shut down turn the speed control knob counterclockwise until it clicks
  8. The engine can then be shut down.

You will notice in the video the charger is easy able to over power the battery, and put out 50 amps. A car battery was used in this test, the bigger the battery the more amps you can put in without going over the battery's maximum charger voltage which is usually around 14.8 volts

Step 6: Converting to Gas

Using LPG or propane as fuel will reduce the running cost, but another advantage is that the engine oil stays clean as it not contaminated by unburnt petrol. This will increase the life of the engine considerably.

The Honda clone engines are a easy to convert as the gas carburetor was designed to bolt straight on, and work first time. The Kawasaki engine however is a completely different ball of string. Although it does kind of bolt on the engine, the throttle is on an awkward angle and the governor works back to front, compared to the Honda engine. You can see in the photos the difference between the carburetors they are almost a mirror image of each other.

A small lever was made with aluminum and glued on to the throttle with a type of super glue called rapid fix. The rod to the governor arm had to be modified and bent but the result was very good. The engines RPM is stable, the engine starts and runs as it should.

The Kawasaki engine was never fitted with a throttle that the operator could adjust, just a silly engine brake and kill switch. Its off a lawn mower, so if you let the handle go it would kill the engine and a brake would engage to stop the engine also. This nonsense was disable with a cable tie, and as the engine will only be running on gas to turn the engine off, you just turn off the gas.

Step 7: Battery Charger 2. Horizontal Shaft Engine

Horizontal shaft engines are much easier to set up, a simple frame can be knocked up with length of 25mm square box section. If you use four pieces laid out as shown in the photos, the belt can be can be adjusted by moving the motor. Some short legs, a mount for the alternator and a handle or two and the frame is ready for paint.

This charger had the after market regulator fitted, as the batteries are 12 volt and in poor condition. As the battery bank needs replacement the voltage can swing wildly so an automatic regulator made constant adjustment unnecessary .

This is not a problem if the battery bank is in good condition.

Step 8: Testing.

The first test run of the Kawasaki charger caused a bit of excitement, as the students had tipped the engine over the engine oil had got inside the cylinder and exhaust, so there was clouds of thick smelly smoke which blew into several classrooms.

Needless to say the cooking class did not approve.

Once the smoke cleared the engine ran sweet and only require a small adjustment to get the engine running at the desired rpm.The Kawasaki charger gets used on a 24 volts system if there has be a day or two of rain or cloud. Its cheaper to run and way quieter than the diesel generator they had been using, and charges the batteries quicker.

The Honda engine had no such drama, and fired up on LPG on it second pull. And has been used every night for several months on a dying battery bank. Previously the owner use a petrol 240 volt generator connected to a 12volt charger and it cost him around $50 a week to run. He now spends around $30 a month on LPG. Some of the reduce cost is due to the cheaper fuel, and some of it due to the engine running a a low rpm and increase efficiency (no losses converting 240 volts to 12 volts).

The photo is of the student who built the honda charger, handing over the project to a person in need who lives without power, running water, or proper sanitation.

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