Using power from an alternator

Discussion in 'Off Grid Living' started by Robert1982, Feb 6, 2014.

  1. Robert1982

    Robert1982 Monkey

    Aspiring off-gridder here looking for some feedback!

    I am looking into the idea of creating a 12v DC generator, and using it to do two things: charge car batteries and hook up to a high powered (1000 watt inverter).

    Question #1: How do I know the batteries will not be overcharged? Is there some type of component built into alternators which prevents the alternator from overcharging the battery? Or is that built into the vehicle somewhere else? Do I need to be concerned with overcharging? Should I use a solar charge controller that is good for 24v (since the output of alternators exceeds 12v) to control the batteries from overcharging?

    Question #2: If I hook up an alternator that is producing 16v up to a 12v inverter, I am afraid of damaging the inverter. How can I stabilize this voltage so that it is safe to run into a 12v inverter?

    I am thinking about taking a simple small engine and hooking it right to a powerful alternator such as one out of a full sized pickup truck. Then using the power generated to either charge a battery (or a group of batteries that are wired parallel) or I could hook it up to a 1000 watt inverter to produce some strong juice for running some more demanding applications.

    Any wisdom would be much appreciated!

  2. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    Welcome to the Monkey Tree, Robert.... Answers we can give......

    1. Most Car type Alternators will NOT overcharge a 12 VDC Battery, simply if they did, everyone would be killing off the batteries i the vehicles. That said, there are Off-Grid Charge Controllers, that do a much better JOB, of Maintaining a Full Charge, on a 12 Vdc Battery,
    than the standard Car Alternator Regulator. The are EXPENSIVE, but if you go this route, you can ALSO use them to regulate Wind, Solar, and or Hydro as well... Look at the OutBack Power MX Series, Charge Controllers, for an example of the Industry Flagship.
    2. Most common Vehicle Alternators, will "Barely" output 1 KW. Do the Math (1000Kw /12 Vdc = 84 Amps) and if you tried to actually draw 83 Amps from one, for any length of time, It would MELT the Copper Windings into a Pile of Copper Slag, in short Order. The Minimum you would need is a 120 Amp Alternator, that was designed to output that "Continuously" and even then you wouldn't be pushing much into the Battery that sits in between. Again, do the Math. ( 120Amps * 12 Vdc = 14.Kw)
    3. Voltage Input of Inverters is a very critical Parameter that MUST be taken in, when designing such a system. Many of the "Chinese Junk" and Low Cost Inverters around today will Fry, if the DC INput Voltage gets anywhere above 15 Vdc. Where the better Inverters and the Best Inverters can do up to 18Vdc on the DC Input. Now, as Noted above, If you had an OutBack Charge Controller doing your Battery Charging, They being MPPT type Chargers, can take up to 80Vdc Input, and convert the difference, into extra Amps, available to the Battery or attached Loads, like an Inverter, without raising the Battery Buss Voltage above a User Settable Maximum Voltage Limit.

    If this were my system to design, I would get a 120 Amp 24 VDC Alternator, and use an Outback MX-80 MPPT Charge Controller to charge a 24 Vdc Battery Bank of sufficient Size, for my DC Loads. I would then look at either a Trace 2424, or equivalent OutBack Inverter, for my DC/AC Conversion. Math (75Amps *24Vdc = 1.8 Kw) this gives you a bit of "Overhead" in the Charge Controller, and with a Bank of T-105s, or L16HDs, connected in Series, gives you plenty of Room for High Power Loads with the Alternator Charging, and sufficient Battery to run LED, or CFL Lights, a Laptop, and a few other small Loads while the Generator is NOT running.

    The critical Design Choice in these kind of systems is:
    1. Inverter DC Input Voltage ... Rule of Thumb (1Kw or less 12Vdc, 1 Kw-2.5Kw 24 Vdc, 2.5Kw -4Kw 48Vdc)
    Once you decide that.....
    2. Then you figure your Loads for the Battery Bank, when NOT Charging, and number of hours between Charge Cycles. That determines the AmpHours of Battery you need for you Battery Bank. Rule of Thumb (You should NEVER discharge your Batteries below 60% of Full Charge, if you want them to last 8-10 Years)
    3. If you change your mind, or want to GROW your System, using this design Criteria, then all that is required is a Higher DC Input Voltage Inverter, and then Add more batteries in Series to your Bank String to increase the Battery Buss voltage to match the New Inverter. Then reprogram the the Charge Controller to match the New Battery Buss voltage.

    Easy Piezy.... All it takes is MONEY...... ..... YMMV.....
    kellory, hank2222, swampbilly and 5 others like this.
  3. techsar

    techsar Monkey+++

    Robert, Welcome! Keep in mind that ratings vary by manufacturer and not all "1000 watt" inverters will carry that amount of load for an extended period. As Bruce mentioned, the cheap Chinese ones tend to be unforgiving of voltages outside, or for that matter, near the limits of their specs. This holds true for output current also.

    Look at operating temperature ranges too. I have one inverter (a cheap psw SunForce) that will shut down when temps get into the mid 20's.

    You also mention "more demanding applications." Startup current on motors can often draw two to three times the nameplate rating, so a 600 watt "thingy" could require 1800 watts to start... just something to figure in before you start spending $$$.
  4. Robert1982

    Robert1982 Monkey

    Wow! Some really useful information here. Thanks!!! I'm taking notes!

    So if I hook up an alternator to a battery(s), and my inverter to the battery(s) as well, will it damage my inverter to use it while the alternator is spinning as well? I think it would be the same as hooking up an inverter to the car battery while the vehicle is running. Let's say the alternator puts out 16 volts with the engine revved up; will those extra volts be absorbed by the battery, or could they pass right along to the inverter and burn it up?

    alternator (charging) -> battery <- inverter (also hooked up at the same time the alternator is charging)

    I am trying to prepare in steps. Eventually I want to be fully dependent on solar and wind power. Until I have a collection of panels and a wind generator or two, I may need to have a way to boost or fully charge up the batteries. That is where I thought I could grab a used small engine, a used alternator, a belt and bingo! I have a way to recharge my batteries on top of the panels and wind energy. Then I am not under high pressure to hurry up and buy 2k worth of panels and such. I can add on as I go, and the more I add to the system, the less I need to run the generator.

    That brings me to another question. Would I be ok in setting up an array of panels to a charge controller, then firing up my alternator once in a while? Could the juice from the alternator harm the charge controller?

    For that matter, is it okay to use more than one charge controller at the same time on the same battery(s)? If I buy a basic charge controller along with a couple panels to start, then add another charge controller to handle a few more panels, can I hook them all up to the batteries? I would likely be using a 12v system and the charge controllers are hooked up parallel to the battery pack.

    a few solar panels -> charge controller 1 -> battery pack <- charge controller 2 <- a few more panels

    I know that 24v and 48v systems are more efficient, but I'm likely going to start out with a 12v system. I already have several inverters I've picked up over time, and I'm more comfortable with 12v system wiring. I will likely go with 24v or greater in the future, because I understand I can link some of the panels serial to boost the voltage, and also change the battery wiring to increase voltage. (Somebody correct me if I'm wrong here!)

    Any insight or wisdom is much appreciated!!
  5. ghrit

    ghrit Bad company Administrator Founding Member

    You're going to have to know more about the alternator you intend to use, also more about the charge controller. In automotive applications, there's a voltage regulator (and rectifier to change the ac to dc) usually inside the alternator itself that will shut off charging when the battery is satisfied, that is, at rated voltage. The regulator may or may not be happy with the charge controller and vice versa. In effect, you are asking if the alternator and charge controller can operate in parallel when the sun is shining or wind is blowing. The question is whether the charge controller will be able to see the alternator output and work with it.

    As far as the inverter goes, you won't hurt it by running the alternator to charge while drawing power via the inverter. Unless, of course, you somehow get the battery voltage higher than the inverter likes. There should be a voltage limiting circuit on the inverter dc input if the inverter is sensitive to over voltage.

    Further deponent sayeth not, I have some doubts that charge controllers will be happy in parallel. It could be that some are compatible operating that way, but you will have to know for sure.

    One last item: There is no way to wire up a 12v charging system to charge a 24 or higher voltage system without some fancy switching that will take the batteries out of the circuit during charge.
    swampbilly likes this.
  6. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    @ghrit you need to look at the Outback MPPT MX Series Charge Controllers... They can be setup to charge ANY Battery Buss Voltage less that 60Vdc, from ANY DC input Voltage less that 120 Vdc, with the caveat that you do NOT exceed the maximum Output Amps of that version of the Unit. This is due to the Switching Power Design, of the Units. ......
    swampbilly likes this.
  7. William Warren

    William Warren Monkey+++

    I suggest you visit this link: Basic to Advanced Battery Information from Battery University, where there is a lot of very good information about batteries. Regular car batteries aren't the most effective choice for "float" service, and you'll do well to plan a system with battery capacity and type well matched to the inverter demands and the charging/discharging/float duty cycle they're most likely to serve.

    Also, keep in mind that car alternators aren't that efficient: they don't have to be, since even four cylinder car engines have a lot of unused horsepower and can tolerate inefficient electrical generators. If you are thinking about using a "pup" setup (e.g.,, it's important to plan on getting less-than-ideal efficiency from the engine. In other words, you won't get as many watts-per-gallon from a pup setup as you would from a purpose-built genset that is designed for 12V or 24V output. OTOH, pup generators can be assembled from junked power equipment and junkyard alternators, so they cost a lot less up front, and that will make a big difference if you're only using one an hour or two per day.

    A 1,000 watt inverter isn't going to do much for you: A single space heater would strain a 1 KW inverter's capacity, and two 15 amp (120 volt) appliances will chew up 3600 watts. A refrigerator will need more, since the inverter has to have a "reserve" capacity in order to start motors, especially large ones.

    The first step is to buy a kill-a-watt or similar device, and use it to measure the power requirements of devices you know you have to have, such as the house heating system, the refrigerator, the water heater (if it's electric), a few lights, and (if needed) a well pump and/or sump pump. Once you have that list, you can price inverters, battery banks, and charging methods that will match your usage.


    William Warren
    hank2222, swampbilly and kellory like this.
  8. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    No but depending on Price, it looks very interesting...
  9. Tevin

    Tevin Monkey+++

    To add to what has already been said...

    Automotive alternators run in the 60-130 amp range. To make this easy, let's say you are in the middle at 90 amps.

    A 90 amp alternator will push your 1000 watt inverter, but there is very little headroom on that. I am a big supporter of getting something larger than you know you will need and operating it at a modest capacity instead of getting a device that is just barely enough and running it "balls to the wall".

    Also, one horsepower equals 745 watts. Therefore, 1000 watts is about 1.33 horsepower. Where all this becomes meaningful is in the selection of your engine. Keeping in mind inefficiency and my "don't run it balls to the wall" philosophy, you would need a 3-5 HP engine to run your alternator.

    Twelve volt inverters run at about a 10:1 current ratio. That means a device that requires one amp at 120 volts will pull 10 amps from your twelve volt battery. The laws of physics are absolute and there is no wiggle room on this.

    Of course, you will not always be running the inverter that hard but it's something you need to be aware of. Resign yourself over to the idea that you will probably need to run the engine to support the batteries when pulling high current.
  10. Gator 45/70

    Gator 45/70 Monkey+++

    Just to throw this in as an idea.

    Georgia_Boy and kellory like this.
  11. William Warren

    William Warren Monkey+++

    Bicylce-driven battery chargers are nice for a couple of reasons:
    1. Chances are you already have the bike
    2. Spare parts are easy to get
    I like the way he goes for the permanent-magnet motor instead of an alternator, and it's a good idea, since alternators aren't as good at generating electricity.

    William Warren
  12. BTPost

    BTPost Stumpy Old Fart,Deadman Walking, Snow Monkey Moderator

    Just so you know..... Even the BEST Bicycle Athlete can only muster about 100 Watts, on a continuous (3-5 Hours) Basis... He would be much better off, to use a MPPT Charge Controller, than that Watt-Gobbler thing that he advocates. Put the Over-voltages to use, rather than throwing them away in the Charge Controller Heat. Those two 60 watt Light Bulbs he hooked up to his Inverter, would drain that battery prettyDARN Fast. 100 Watts over 5 hours is still 500 WattHours, and that doesn't include the losses encountered in Battery Charging. (15-20% Depending on Battery type) Not the smartest way to keep a Battery Charged, in my Opinion..... YMMV.....
    Tevin likes this.
  13. Tevin

    Tevin Monkey+++

    The guy in the video made two h-u-g-e oversights:

    1. He specifies using a 250 watt DC motor for your generator. The implication is that you will get 250 watts of actual energy out of it. BTPost explains it perfectly (and his numbers are generous!). There is no way in hell any mortal human is going to generate that much juice by pedaling a bike.

    2. As the video demonstrates, by using a DC motor the pedaler must self-regulate the RPM to stay within the proper voltage. This is very awkward and random. On top of that, they would not be able to increase output even if they did have the muscle power. It would be far more practical to use a car alternator: They already have a regulated output so the voltage is the same no matter how fast or slow you are pedaling. Stronger riders don't purposely have to hold back. You can also completely eliminate the homebrew regulator thingy video guy uses on his.

    The idea of bicycle generators gets kicked around in prepper circles a lot. I generally have a negative view on them, not because there's anything really bad about them, but because their capabilities are often oversold. Bike mills are ok for charging cellphones and other small time stuff, but thinking it will pull you through Armageddon (as the dude in the video suggests) is just nuts. Also, there is no "free energy". Charging a battery requires the same amount of energy no matter whether the fuel source is the sun, gasoline, wind, or candy bars. For prepping purposes, carefully consider if the time and calories spent spinning a bike wheel could be better used doing something else.

    All said, I think bike generators are worth it but only because they are cheap and easy to make. You really have nothing to lose by having one around. But don't fall into to foolish trap of enlisting it as your primary source of electricity, or think that you can use a bike in place of more practical generators.
    ghrit likes this.
  14. Altoidfishfins

    Altoidfishfins Monkey+++ Site Supporter+

  15. Duck

    Duck Monkey

    I made a 12 volt generator a few years back as an experiment.

    It wasn't intended to be anything I'd actually use or depend on. It was strictly experimental, so I didn't bother with things such as making the alternator mount adjustable or adding an idler for belt tension.

    I had an old worn out 3 HP Briggs & Stratton engine and a 78 amp Delco alternator with internal VR. I lag-bolted it to a piece of cedar 2x12 and just did the bare minimum with scrap angle-iron & an acetylene torch for the alternator mount.

    An alternator won't work without first having some juice to energize the field coils. So I attached a car battery. A house type light switch turned the field electromagnets on & off. The pulley on the engine was designed for electric motors up to 1/2 horsepower but again ... it was experimental.

    An alternator will work regardless of which direction you spin it, but the cooling fan is designed to run a specific direction so in order to maintain clockwise rotation for proper cooling of the alternator, I mounted the engine & alternator diagonally on the plank so there would be room to have the pulleys facing opposite directions.

    I started the engine, let it run 'til I could take the choke off all the way, then flipped the switch for the field coils. The engine dogged just enough I could tell it was working.

    I hooked up a motorcycle headlight with test leads and heard the engine load slightly increase. I tested it with a trolling motor but didn't have the prop in water.

    So I put some lawnmower wheels on one end & a piece of 4x4 on the other so I could pull it around by the handle on the battery. I took it out on the river on my homemade raft. I brought along a deep cycle battery but didn't hook it up.

    I headed upstream with it, driving away from the launch point using the marine battery but then switched to the generator once I was away from other people wanting to use the ramp.

    The engine struggled with the 40 lb thrust Minn Kota trolling motor on speed "3" of 5. When I turned it to "4" the engine was overwhelmed and died. I would've thought the alternator's mechanical load on the engine would drop once the RPMs got too low, but it actually stalled the engine. So more than 3 HP (or whatever that old worn out engine was actually producing) isn't enough to run a trolling motor. Some website I found for reference said that particular trolling motor would draw 40 amps on speed "5".

    If I were to make another one, and I'm kind of considering it, I'll be using a bigger engine.

    Here's a picture of the experimental generator:


    I just looked at this thing today. Both the alternator and the engine are seized, even though it was in the barn for 10 years.
    Last edited by a moderator: May 15, 2014
    Sapper John and BTPost like this.
  16. kellory

    kellory An unemployed Jester, is nobody's fool. Banned

    Welcome to the monkey tree.
    Your generator reminds me of a "quick and nasty". (Something I will throw together to do a job, it works, and I don't care how it looks, it is short term) some of my " Q&Ns have been working for years. ;) there are also threads on this type of homemade generator, here at the monkey. I would suggest you try the search box at top right, you might like some of the other designs.
  17. Duck

    Duck Monkey

    I threw that in there mainly to show what DOESN'T work, .. LOL. Not enough HP, don't use that type of pulley, etc.

    I'm planning a new one & I'm sure I'll find some ideas on here.
    kellory likes this.
  18. kellory

    kellory An unemployed Jester, is nobody's fool. Banned

    I like renewable resources. Solar, water power, wind, things that once built, simply work. The less I need to pay for, the better.
  19. Sapper John

    Sapper John Analog Monkey in a Digital World

    Welcome to the Monkey tree @Duck, thanks for the share!
    Duck likes this.
  20. Tevin

    Tevin Monkey+++

    Wow, that's a cool setup.

    A 3hp engine should be enough to push the alternator. I can't see an electric trolling motor being too much of a load. I'm going to guess maybe the engine was not right.

    A car alternator will produce about 1000-1400 watts depending on what you've got. Since 735 watts = 1hp, a 3 horse plant should be enough. Adding a bigger engine is fine, but you are still limited by the capacity of the alternator.
    Duck likes this.
  1. Alanaana


    Uploaded by: Alanaana, Dec 4, 2023, 0 comments, in album: Alana's_Adventures
  2. Alanaana


    Uploaded by: Alanaana, Dec 4, 2023, 0 comments, in album: Alana's_Adventures
  3. martha_mill
  4. DKR
  5. SurvivalJester
  6. Dunerunner
  7. Asia-Off-Grid
  8. Asia-Off-Grid
  9. Asia-Off-Grid
  10. Asia-Off-Grid
  11. Asia-Off-Grid
  12. Asia-Off-Grid

    Generator FAQ 2001

    Generator FAQ, By Steve Dunlop. [img]
    Posted By: Asia-Off-Grid, Jul 19, 2018 in category: Energy
  13. troybillett
  14. Jaywest
survivalmonkey SSL seal warrant canary