AC or DC generator? | DIY Solar Power Forum

thermopolis

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• Jan 7,

• #1

Hello all, I may not be posting in the correct forum however, I have a question for you all. I am thinking about getting one of the Kohler off grid generators to backup and supplement my solar system. i just cannot decide if AC or DC would be best. I have the SOL-ARK15k along with 10 EG4LL 48v batteries and 32 Panels at 455 watts each. My goal is to kick the electric company to the curb as soon as possible. I am in arkansas.

12VoltInstalls

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• Jan 7,

• #2

Does the solark have a charger?

Because as much as I like to maintain DC if possible 120VAC is more practical.

BentleyJ

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• Jan 7,

• #3

There is a small efficiency advantage when charging batteries with DC directly from the standby generator as compared to rectifying AC to DC either via the built in charger in the Sol-Ark 15 or a stand alone AC to DC battery charger.
The problem is if you only have one inverter and it fails, a DC generator cannot be used to directly power your house and fully charged batteries are of no use unless some of the lights and appliances are also DC.
In a case where there are 2 inverters in parallel then a DC generator would make more sense because its very likely at least one inverter would still be functional and be able to utilize charged batteries.

wdwtx2.0

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• Jan 7,

• #4

I would go with the AC genny.
You can use it for LOTS of things. The DC, not so much.

timselectric

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12VoltInstalls

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• Jan 8,

• #6

I would go with the AC genny.
You can use it for LOTS of things. The DC, not so much.

I still haven’t done it but I have a “GM one-wire” automotive alternator with a v-belt pulley. A 3.5HP-ish cheapo (Harbored Fright) would drive that just fine. Rated output is 95A but at moderate pulley speed 80A probably expected.
Basically a dependable < $250 DC generator by the time you add a cheap battery and some switches, cables.

But as far as a backup generator I’m not sure I’d want exclusively DC.
The Champion will run my welders and bigger tablesaw as well as provide 120VAC direct if I need it. With 3-4 120VAC-12V battery chargers on hand the “emergency” bases are covered. Having a 12V generator only you just have need and convenience covered.

hugemoth

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• Jan 8,

• #7

I still haven’t done it but I have a “GM one-wire” automotive alternator with a v-belt pulley. A 3.5HP-ish cheapo (Harbored Fright) would drive that just fine. Rated output is 95A but at moderate pulley speed 80A probably expected.
Basically a dependable < $250 DC generator by the time you add a cheap battery and some switches, cables.

But as far as a backup generator I’m not sure I’d want exclusively DC.
The Champion will run my welders and bigger tablesaw as well as provide 120VAC direct if I need it. With 3-4 120VAC-12V battery chargers on hand the “emergency” bases are covered. Having a 12V generator only you just have need and convenience covered.

The problem with using a GM 1 wire alternator is that it will attempt to put out maximum power to push the battery to ~14 volts. If the engine doesn't have enough horsepower it'll bog down and quit. Unlikely a 3.5 hp engine will run a 95 amp alternator at max power. If you use an alternator with an external connection to the field coil you can control the output with a resistor or a circuit that limits output based on volts and amps as needed.

DIYrich

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• Jan 8,

• #8

A generator is naturally A/C (spinning coil). Additional circuits required to convert to DC.

If you have a hybrid car, it can deliver about 3.6kW of high voltage that you can step down to recharge batteries. Hybrid cars make the best DC generators. Easy to refill the gas. Don't have to worry about weekly tests. Don't have to worry about storing gas and it going bad.

PreppenWolf

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• Jan 8,

• #9

Go AC and buy a Chargeverter from Signaturesolar. This gives you redundancy of use.

sunsurfer

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• Jan 8,

• #10

For the price of those Kohlers, I think I would go find an original (non-indian clone) lister CS 6/1 diesel and just mount up a direct drive axial flux gen head. Run it through a charge controller and call it a day. Co-gen would also be handy. You could also mount up an AC head, air compressor, etc.....

fpgt72

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• Jan 8,

• #11

I have done old cars quite a bit over the years, old tractors as well. Generators need to have so much RPM before they really will do anything.

Had an old MG that would not charge at idle, get it up to about rpm and it would charge just fine. It would also over heat at idle. The car was just not made to sit and idle.

Not sure if that means anything in the current world, but there it is.

Oh boy I am on a roll today.....current world.....current....get it.

Wonder if I am going to get banned for all the bad puns.

hwy17

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• Jan 8,

• #12

If I was shopping in the price range of the Kohler 48v I would want it.

Not for efficiency as much as simplicity.

sunsurfer

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• Jan 8,

• #13

I have done old cars quite a bit over the years, old tractors as well. Generators need to have so much RPM before they really will do anything.

The lister runs at 600 RPM and makes 6 HP. Cant image why that wouldn't be enough for doing some work. A lister 8/1 will get you 8HP at 800 RPM.

Low RPM is good for high efficiency due to the increased time for expansion within the cylinder, not to mention more time between rebuilds.

12VoltInstalls

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• Jan 8,

• #14

Unlikely a 3.5 hp engine will run a 95 amp alternator at max power

“Full power” probably will not be achieved anyway, but 3.5HP is ~2.5HP+ not running quite full tilt; including ‘waste.’
95A is ~W, and 2.5HP is 2kW, so it ‘should’ have the gumption and headroom to run that alternator.

hugemoth

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• Jan 9,

• #15

“Full power” probably will not be achieved anyway, but 3.5HP is ~2.5HP+ not running quite full tilt; including ‘waste.’
95A is ~W, and 2.5HP is 2kW, so it ‘should’ have the gumption and headroom to run that alternator.

Maybe new alternators are more efficient than they were when I was building these things 20 years ago? I still have one BTW, a 60 amp Ford type alternator driven by a 5 hp Honda engine. V-belt drive, manual adjustable resistor to control field current. The engine is maxed out at rated RPM to achieve 60 amps.

12VoltInstalls

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• Jan 9,

• #16

Maybe new alternators are more efficient than they were when I was building these things 20 years ago

A GM 1-wire is a ’s part.
Pulley size affects output as well.

New alternator will be essentially the same efficiency. Probs 50%

The load (alternator) plus heat loss and phasing is the only efficiency factor- the load presented to the gasoline motor itself is the sum of the two. 5HP is ~W so your 60A (~1HP/~750W) less the heat loss of the running motor (not huge), cooling fan, rectifier, and a smaller amount of energy for the belt. An s as alternator may not stall the rotor but rather bog the rpm down to the point where output W is in equilibrium with the input power.
I have been warned that 3.5 may not render acceptable results but it’s cheap, others have done it, and I don’t plan on daily usage.

RCinFLA

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• Jan 9,

• #17

Amperage rating on an alternator is based on a given amount of cooling provided.

Most common mistake made hooking up an auto alternator to a small engine is failure to provide enough air cooling to alternator.

Some alternators have a fan behind the pully to force air through the alternator. Some rely on auto radiator fan and engine block heat sinking to cool alternator.

Vigo

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• Jan 9,

• #18

I think the ~60% efficiency of the alternator is the thing people miss when considering building small engine alternator ‘dc generators’. But luckily for us there are NUMEROUS portable generators that use some form of ~200cc honda clone that all make about the same engine power, and we can look at their ‘running watts’ rating to get an idea what should be possible. For example, i have a Champion gen with a 208cc which is rated ~ running watts.

hugemoth

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• Jan 9,

• #19

Some builders mount the alternator so it turns backwards in order to make the unit more compact. The alternator doesn't care which way it turns except that the cooling fan is designed to suck air through the case, not blow. Overheating happens unless the load is a small percentage of the rated output. The best option IMO is to use a 110v AC inverter generator driving a battery charger. N

Noname142

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• Jan 9,

• #20

As others have mentioned, AC for sure. it's less efficient.. but useful and practical for so much more than just charging your batteries. You have to think about this investment over the period of "your lifetime", and how much technology changes in batteries over time. 12v, 24v, 48v, whats next? will your DC generator be able to do whatever is next?

AC is here for the forseeable future, and the resitance to change from 120/240 in the US means it will likely be around for the rest of yours and my lifetimes. You can always buy a new battery charger that charges from AC because it wouldn't be dependent on one particular brand, or dependent on the company still supporting your generator model. (ie, to sell you a conversion kit from 48v to.. whatever.. 96v.. etc.

However, battery chargers for whatever the new voltage standard is, will be readily available to buy from multiple suppliers/manufacturers that all take an AC source.

I'd take the hit in efficiency, for the future upgradability/expandability.

sunsurfer

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• Jan 9,

• #21

who says you can't have both AC and DC?

sunsurfer

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• Jan 9,

• #22

Here is how my inverter generator charges my batteries;

First - the motor runs the permanent magnet radial flux generator producing a high voltage 3 phase AC about 200-350v depending on RPM.

Second - its rectified to DC and sent to the Inverter.

Third - its converter to 120v 60Hz AC and sent to my charger

Fourth - the AC is converted back to DC to charge my batteries with a 75amp switching power supply.

Fith - my house inverter converts the DC back to AC

Sixth - varies depending on the device but likely more conversion going on. Example; mini split converts the AC to DC and then back to AC to run the compressor

Seems like their is to many middle men in the picture. I think total efficiency from my gasoline is right around 3-6% on a good day. Especially considering the 60-70% loss just with the motor alone right at the start.

So from gas to cold air it is - AC to DC to AC to DC to AC to DC to AC

12VoltInstalls

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• Jan 9,

• #23

AC is here for the forseeable future, and the resitance to change from 120/240 in the US means it will likely be around for the rest of yours and my lifetimes

There’s countries using DC again?!!

And it’s not ‘resistance’ to changing from split phase to 230VAC explicitly, either. There’s 120 years+ of device history and products to consider if there is an actual move to go euro. People don’t like the idea of waking up one morning and having virtually everything they use go obsolete.

who says you can't have both AC and DC?

And every led light is DC

sunsurfer

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• Jan 9,

• #24

And every led light is DC

I mean AC and DC generators. Why not have both in one unit? I guess if we are being honest, does a DC generator even exist? I mean something that rotates to make power that is truely DC and not just regulated/rectified AC?