I wholeheartedly applaud your decision to make your own energy. All that being said, a dedicated generator is probably your best bet for a number of reasons. Still, if you want to make your own generator, you could.
Yes, running 24 volts is basically more efficient than 12 volts.
Make certain that your lawnmower engine is up to the task of running a decent sized genset. And an EFI engine is more efficient than pure carburetor so if you can find one go for it.
For batteries I would recommend either a deep cycle lead-acid battery or a LiFePo4 battery. The latter, though expensive, will store a lot of energy and stands up to many charge-discharge cycles. BTW, a LiFePo4 battery might just cost more than the rest of your build combined. And yes, a Prius battery should work just fine (it is likely a LiFePo4).
This is an interesting project and I am intrigued. There are some challenges to overcome but we can work on those.
Sounds like experimentation, and it should be perfectly OK if you can keep all of this in test mode, for learning purposes only. You might possibly combine this with recycling, if you visit the right locations for parts (generator repair shops, possibly junkyards, possibly the dumps).
Produce mechanical energy - build a generator out of whatever is laying around, and experiment with producing (mechanical-electrical) power. Use "broken halves" of gensets, where engine is dead but generator and other parts are still reasonable; match that to a dead genset where engine is reasonable, but other parts are fried.
Store energy - from genny to batteries (battery charger), and experiment with different sizes, chemistries, voltages. Start w/ 12v and experiment with what you can power (no inverter needed, w/ 12v DC devices like lights, etc). Step up to 24v later, and/or start adding inverters to power AC devices. Figure out the wiring for both DC and AC modes (wire size, panels, fuses, etc.)
Produce solar energy - Find some used solar panels, mppt and other parts, and try to add that alternative power source into the mix.
Production system - safety systems, backups to the primary parts, etc. Keep it as external to the house as possible, as a fire in the utility shed only affects the shed, but the same gear in a house might be more disastrous in scope.
I would tend to not move any of this DIY gear from test to production, as the diy gear may not hold up to safe production use. But at this point, you've learned so much that you now know what you are doing in all areas, and know how to find the best deals and combinations of equipment, with the goal being to buy or build quality stuff that will provide power for the family (without endangering them). You are "assembling" a quality system out of lego-like components, and saving on the cost of buying a pre-assembled system from vendors.
For all the details, visit forums like "diysolarforum.com"
What you are basically talking about making is a DC generator. The voltage does not really have anything to do with efficiency. I would say that if your plan is to burn gasoline in a small engine to produce power, then just buy a generator and an AC-powered battery charger. You will lose efficiency in the conversion from AC to DC, but your system is going to be a lot easier and much more flexible. Really though, there are much better ways to produce off-grid energy than by burning gasoline.
I suggest you look into a small battery system with some solar panels to get you started. If you dont get enough sun, then you can add a back-up generator with a charger.
To size your batteries you are going to need to figure out how much power you are hoping to use. Look at all the nameplates on the items, they will tell you the Amps, which you multiply by the Volts to get the Watts. Multiply the wattage by the hours you want to run it, and you get watt-hours. Add everything up, and then you have a baseline for your daily needs. Remember that inverters draw power just idling, so say it has an idle draw of 20 watts, that is 480watt-hours per day. Batteries are rated in Amp-hours, so multiply that by the voltage to get the watt-hours. For example a 225AH lead acid battery at 12v has 2700wh of energy storage. Lead acid batteries are terrible, so you can only draw them down about 50% unless you want to be replacing them every year. So really the 225AH lead acid would only be able to give you 1350wh before it needed to be charged back up.
As for using hybrid batteries, this will likely be quite complicated, and not very cost effective. Early Prius batteries were Nickel Metal Hydride, and the newer ones are some sort of lithium ion chemistry. I do not remember which one. They are high voltage, I want to say around 200v, so you would need to break the whole thing apart into smaller modules and then reconfigure it to a voltage that will work with available inverters and charge controllers and such. Also, despite having a very high power output capacity, they store very little energy. I think the usable capacity of a gen2 prius is only like 1500wh. Lead acid batteries are very tolerant of abuse, but NiMH and lithium have more exacting charging requirements. Some lithium chemistries also suffer from thermal runaway phenomenon, so they need sophisticated battery management systems to keep them from bursting into flames from being overcharged.
I would suggest you get your feet wet with some cheap flooded lead acid batteries, and then go from there.
Here are a few cheap dc generators I put together. The one is a pressure washer that was given to me and the other is an old hatz diesel engine. The pressure washer is nice to run around the farm with. It is used to peak out battery sets and to start tractors. The hatz is the backup system for the solar/windmill system for our barn. The alternators are 10si gm alternators from Napa. 50-60 bucks for one is what I give.
The best place to pray for a good crop is at the end of a hoe!
Yes 24V is more efficient that 12V. Here is why. You have 3 forms of power loss as heat in a typical alternator. Loss in the armature winding+voltage regulator, loss in the stator winding and loss in the diodes. When you double the voltage the loss in the armature winding goes up as the voltage goes up. Now this is more than counter balanced by the fact that when the voltage doubled the current the stator winding had to carry for a given power level was cut in half. So you break even or slightly more than break even on these 2 pieces.
Where the real gain is made is in the diode voltage drop losses. In forward mode you can consider the diode beginning to turn on at .7 volts and fully on at 1.4 volts. If the alternator is 12 V then it is likely putting out between 14.2 and 14.7 depending on the regulator. But the winding has to put out 1.4 more volts at peak to cover for the forward voltage drop. So for now lets call the regulator setting 14.6 just to keep the math easy. That gives a winding voltage of 16. Now I am going to greatly over simplify the power lost in the 6 diodes by treating it like it is one big diode flowing power all the time. For this purpose the numbers over time will be nearly identical. Now if I am moving 100 amps that voltage I am loosing 140 watts as heat from the diodes out of a total potential of 1600 watts. But if I double the output voltage the current needed to produce that 1600 watts falls in half. So now my current is 50 amps thru that same 1.4 volt drop for a 70 watt loss in the diodes. So you have an 8% diode loss in the first case and a 4% diode loss in the second with the higher voltage.
Now a comment on the Delco 10SI alternators shown in the last picture.(12SI is very similar and most part will interchange) Because of how the components in them can be configured these are an incredibly flexible alternator. That can easily be rebuilt with just hand tools. You can get voltage regulators for 6(after market only), 12, 24 and 48 volt outputs(12,24, & 48 available in SE too). The swinging ag style brushes are a more durable answer but need a rear end housing to match. You can get an armature winding with more wire in it that takes slightly lower currents to turn on.(needed for max output.) Windings come in 30, 42, 42SE, 62-63, 72 and 105(this one needs the heavy duty armature) amp output possibles. The SE stature winding has more wires in it for lower rpm turn on(will over heat if run at max power for any real length of time and go up in smoke) . For belt drive if you are stepping up voltage and current there is a bigger front housing bearing and armature for high load situation.(it is used on New Holland combines) Because they are totally common, cheap and can be completely disassembled with hand tools and all the combinations they can make these would be one of my first choices for something like this.
To calculate your energy needs, there is a program called "HOMER" which is put out by the National Renewable Energy Lab (NREL). You can play with all the figures and calculations with it including your power generation, expected load and size of battery bank you'll need. Of course they stopped making the free one and now expect you to pay for it. They do have a web based version that's free, government at work.
From there you can see what your budget allows for.
I have to agree with what others have said - consider it an experiment and don't plan on relying on it as a full time power source. And make your life easier, get an alternator with an internal regulator. And the RPMs of the alternator is also important. They put out maximum power within an RPM range. In your car, assuming it's an ICE based system, your system voltage with always be around 13.5 - 14 volts when the engine is running - expect that with your setup.
The thing with doing this is that you'll want to feed the charge into a deep cycle battery and then draw it off for the inverter. Also, inverters have a range of voltage it will use, usually as high as somewhere around 13.5 volts. With the alternator whirring away, you may wind up over that high end voltage. And once the power generation is shut off, you have what potential you have stored in the batteries to power your devices.
A very important note is this: Wet cell deep cycle batteries will be ruined in a few months if you go below a 50% charge. Size your system so that you use no more than 30% of it's capacity when not generating power. So many people over the years have asked me about why their battery bank is toast after a few months. They don't realize that deposits build up on the plates when they're discharged. Sometimes putting an equalizing charge works, sometimes not. There is a fluid that I use in the cells that prevents much of that which is a commercial product. I've got one deep cycle from 2006 that still works fine using that.
Also, plan on everyone complaining because it will be loud. Like standing next to a lawn mower for hours and hours.
I've had some success with what you're talking about but still, you'll be fiddling with it a lot. I'd like to suggest that if you really want to succeed, assuming that you abandon that particular setup, get a small inverter generator. There is only really one company that makes the engines for them, Ducar. It's the box and features that each company puts around the engine. My Ryobi 1800 watt inverter generator runs for 15+ hours on a gallon of gas at around a 30% load. I use it to charge the battery bank (430 amp hours) during the day and power the tiny home.
The Ryobi I have works at 57 db and in the generator house it sits in, 20 db.
Hi Bruce. I have found it will run off of any oil as long as the fuel is clean. Sometimes I get in a hurry and leave dirt in the fuel. This causes the injector to stick. It is easily removed and cleaned. The nice thing about hand start diesels is I can hear the injector ping if it is working. So far I have used waist cooking oil, waist engine oil, bio diesel and diesel. They all run fine in it. I do not let bio or cooking oil sit in the tank. I only gave 300 for 2 identical hatzs from a sale one day. They just needed cleaned up.
The best place to pray for a good crop is at the end of a hoe!