DIY "Inverter Generator"

Posts: 73

posted 4 years ago

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Hello Everyone,

I have been tinkering with the idea of building a diesel backup generator to charge my battery bank and serve loads (via my inverter) when other power sources are unavailable.

My first thought was to simply purchase a 110v diesel generator and a battery charger. Then I got to thinking about how much I like my Honda EU3000 inverter generator (gas) and wondering if I could copy the concept in a DIY project.

My thought is to couple a DC alternator to a 12.5 hp diesel engine and build a throttle controller that would vary engine RPM based on load. My first though is to monitor battery voltage and increase throttle as voltage decreases.

With the batteries as a "buffer" the response time of the controller would not be critical.

I could even add a timer function that would prevent the generator from firing up during sleep hours unless a critical low battery condition existed.

What do you all think?

Any thought on conventional auto alternator or permanent magnet alternator?

Any input is appreciated.

S.

Troy Rhodes

Posts: 632

posted 4 years ago

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Automotive alternators are middling in efficiency, IF you drive it at the correct rpm, AND if you get the belt and pulley setup just right.

Likewise, diesel engines can be quite a bit more efficient than gasburners, if you give them the right sized load. Most diesels are at their most efficient when loaded to about 70-80% of their total capability.

Here's a nice article that summarizes that idea:

https://www.ckpower.com/keys-to-running-your-generator-efficiently/

How many kilowatthours per day are we talking? Then we can do some basic sizing of components and so on.

Once we know what your daily needs are, and how many hours you want it to run, we can optimize the size of the prime mover and the generator head, whatever that is.

What kind of diesel engine are you contemplating?

Steve Smyth

Posts: 73

posted 4 years ago

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Troy Rhodes wrote:Automotive alternators are middling in efficiency, IF you drive it at the correct rpm, AND if you get the belt and pulley setup just right.

Likewise, diesel engines can be quite a bit more efficient than gasburners, if you give them the right sized load. Most diesels are at their most efficient when loaded to about 70-80% of their total capability.

Here's a nice article that summarizes that idea:

https://www.ckpower.com/keys-to-running-your-generator-efficiently/

How many kilowatthours per day are we talking? Then we can do some basic sizing of components and so on.

Once we know what your daily needs are, and how many hours you want it to run, we can optimize the size of the prime mover and the generator head, whatever that is.

What kind of diesel engine are you contemplating?

Hey Troy,

Thank you for the reply.

My choices of engine are a bit limited. I am looking at the little Kubota 482z 12.5hp water cooled 2 cylinder.

My instantaneous load can be as high as 6kw but only for brief periods. What I am holing to accomplish is something similar to the gas "inverter generators". I do understand that the engine will be at its best efficiency at a particular speed & load. In "real life" the load varies considerably.

Our total consumption varies significantly depending on weather and activities. Late Summer can be the highest at as much as 70kwh in a day. In the Winter we have storms that typically leave us without utility power for days at a time. When utility power is out we will consume 2-5kwh in a day.

I ran an AC generator (8kw) that was powered by the same engine at a constant 1800rpm. It was a great genny but I did not like the fact that it ran at full speed whether the load was 70 watts or 7000 watts. Having used the Honda for the last 10 years I have fallen in love with how quiet and efficient it is at varying loads compared to a constant speed generator.

Given a battery bank and inverters that will support my 6kw (peak) load I think the simplest path for me to follow is to provide a variable output DC source that will help feed the inverters (& other 12/24v loads).

Where I am really lost is:
Which alternator technology may or may not work well in my application?
What parameter should I be basing my throttle setting on? I have considered using battery voltage. The numbers may be off a bit but as an example: If V=13.7-13.9v do nothing. If V<13.7v then increase throttle. If V>13.9v then reduce throttle.

Troy Rhodes

Posts: 632

posted 4 years ago

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From a strict efficiency standpoint, even the wonderful and quiet honda inverter generators are most efficient at one speed.

Cheapo gas generators almost all run at a screaming 3,600 rpm, and are never "efficient". But they are least bad when loaded to at least 50% capacity. They are terrible at low load, and 3,600 rpm.

IF...you have a battery bank to supply the peak loads, THEN there is nothing wrong or inefficient with running the jenny at a constant speed, nicely loaded. It makes the control system super easy.

If you DON'T have a battery bank, then the throttled inverter generator has big efficiency gains when you have small loads. That's where the honda shines. I've never opened one to poke around, but I'd bet you a dollar the computer monitors load (watts or amps) and adjusts throttle response based on load, plus voltage sag. Based on X load, it makes the primary setting of the throttle(most likely a look up table, if X, set Y throttle), and then does minor tweaking based on maintaining the voltage at a precise 120V. An arduino could do that sorta pretty easily, but you just left my area of expertise.

The trick is to have a generator that can run at 80% load and doesn't sound like a screaming banshee.

Eric Bee

Posts: 107

posted 4 years ago