why generators are bad battery chargers

[Mwilliamshs]

http://solarhomestead.com/why-120240-volt-generators-are-not-good-battery-chargers/

I'd typed up something very similar about sine waves and single phase power and googling a diagram brought me this article. Great tech.

 

[TheViking]

I used to read up on solar technology and haven't read the article but a shine wave inverter should help clean up the process.

 

[dwh]

I used to read up on solar technology and haven't read the article but a shine wave inverter should help clean up the process.

No, it won't.


First of all, an inverter takes DC and makes AC. Since the generator is already making AC, where would you hook up the inverter?

Second, synchronous generators are already producing sine wave, so you don't need a sine wave inverter to get a sine wave - even if there was some way to hook it up, which there isn't.


Inverter generators like the Honda eu series or the Yamaha ef series spin an alternator to make 3 phase, sine wave, "wild AC" (frequency not regulated), rectify it to DC and then feed the DC into a sine wave inverter to get sine wave AC. That's actually a really dumb way to make sine wave AC, because it's horribly inefficient to go from AC to DC and then back to AC - and it costs twice as much to build the machine - but it's still worth doing because the frequency regulation of the AC no longer depends on the alternator spinning at a constant speed, so the inverter generators can do a neat trick of throttling back when there is less load.

In terms of watts per gallon of fuel, inverter generators are actually a lot less efficient than a similar sized synchronous generator, but they can end up saving fuel anyway because they can throttle back when the load is light, and the synchronous generator can't - it has to be governed to turn at a fixed RPM to produce AC at a fixed frequency of 50 or 60 hertz.

Since most portable generators almost never run at full load, using an inverter design generator - even though it is very inefficient - can save fuel just by being throttled back most of the time. Of course, it'll cost twice as much to buy the machine...so in terms of dollars, you might not be any better off.

And if your load was constant and the generator spent most or all of it's time running at or near full load, the inverter generator would use more fuel than a similar sized synchronous.



To get back on topic - generators are not "bad" battery chargers. They are inefficient and expensive battery chargers. That is not the same as "bad".

In the world of off-the-grid green solar purists who are constantly in pursuit of the Holy Grail of that last percentage point of efficiency; inefficient and expensive - and OMFG! It uses FOSSIL FUEL! - is certainly considered a bad thing.


But is it? Really?


I can take my camper's 100ah battery from full dead to full charge in about 12 hours using a small synchronous generator burning about a gallon of gasoline. Even when gasoline was 5 bucks a gallon, that meant I could do it once a week for two years for 500 bucks in fuel, plus 200 for the generator, which would be worn out in 1200 hours of run-time. (I know this for a fact, because that is exactly what happened when I did it. I had to replace the generator about a year ago.)

At less than 5 bucks a gallon, the cost is even less.

How much would I have to spend on solar to be able fully recharge a dead 100ah battery in a single day? More than 700 bucks. So, even at 5 bucks a gallon, over a 2 year time span, the generator is cheaper. Eventually, over a long enough time span, the solar would end up being cheaper.

And even if I wanted to spend the money for a solar system that could do the same job a small cheap generator can do - would it even fit on the roof of my camper van? No, it wouldn't, so it's not an option anyway.

Oh sure, I could jack around with portable solar panels, and pack and unpack them and set them up every time I stop and pack them up when I move and re-align them to the sun several times a day...but...nah, screw that. Seems like work to me. I'm trying to avoid work, not make more for myself.


So is a generator a "bad" battery charger? Nope, not at all. It's just inefficient. But for the way I use my camper, it's still cheaper than solar, does a better job, and I get to park in the shade.

I consider that to be a "good" thing.


And it sure beats the holy hell out of driving an alternator with a big truck engine.

 

[dwh]

And oh, BTW...there is an error on that page linked in the OP:

It says,

"If you think about it:

All electricity is made as 3 phase power.
All utility made electricity is generated as 3 phase.
All electricity in the automotive industry is generated as 3 phase
All industry operates using 3 phase electricity."


The error is in bold. SOLAR does not make 3 phase power. I would have expected an article-writing solar aficionado to know that.

 

[Mwilliamshs]

Yeah he should've said AC is 3 phase because a DC generator (not alternator) is also not 3 phase, just like solar panels. True DC power generation is the anomaly.

 

[SoCal Tom]

I'm skeptical of the article anyhow. The battery charger I use at home is plugged into my 15A 115V wall socket. Last time I checked it was single phase, 60HZ. My battery charger is designed to work off of 60HZ single phase power, not 3 phase. I'm not an electrician, but my understanding of A/C power was that there are two sides to the power, pos and neg. The pos sine wave is opposite the negative, so the difference is always 115V. Do I have that wrong?
Tom

 

[Mwilliamshs]

Yes Tom. Single phase power is what your house has. It's generated as three phase and 1/3 of it is sold to you. Industrial/commercial customers use three phase because it's far more efficient. With a single phase generator like in the article you're fueling the generator for 3 phases but only using 1. Then you take 1/3 of the power you've burned has to generate and convert it from AC to DC via a battery charger at a loss.
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A 3000 watt, 120 volt generator with a 30 amp breaker (bigger than most) like my Onan which is specifically designed for RV/Marine use can run less than 110 amps of battery charger based on what was available 2 years ago when I was shopping battery chargers. This is not a compact, lightweight portable Honda EU generator but a heavy duty, commercial grade permanent mount generator hard-wired to the power distributionion panel
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I've done some digging on the subject of using a 110v generator +battery charger. The 16.7A current available from the Honda 2000w generator can power at least one 70A capable charger I've found, the Schumacher INC-700, rated input 110v and 15A. I found several in this range that it could not power, as they're continuous input is more like 18-20 amps. This is a very nice industrial charger I'd consider using long-term if I was to go to the generator route.
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So if I went with a generator instead of an engine-mounted alternator, I'd max out at 70 amps charge capability.
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On the Onan, that 3kw of power requires 4.02 hp, and consumes 10228.63 BTU/hr (8.9% of 1 gallon of gasoline) plus mechanical losses.
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A 130amp alternator can be spun at max output with 2.58 hp, or 6564.64 BTU/hr (5.7% gal) plus losses.
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A compact generator, if it could run 130 amps of charger, would take up more space and weigh more when combined with the charger(s) and cords than my 2nd alternator (bolted to the engine) and would require it's own fuel supply or to be plumbed into the van's fuel system, negating its compact and portable benefits.
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To recharge a DC battery, a DC source (alternator, solar panel, etc) is most efficient. Period.

 

[pugslyyy]

That's really not a very good article at all. It seems to suggest that it is impossible to create a DC output from an AC input.

My $0.02 -

A generator is great for bulk charge, not so good for absorption, and a complete waste for float. If my batteries are getting low, I'll crank the generator in the morning and let it run until the charger drops out of bulk and into absorption, then let solar take it the rest of the way. With a good generator and high output charger it doesn't take long at all to get to 80% (absorption).

You can also get the same benefit from an alternator / solar combo, pretty much if it is set up right.

 

[Mwilliamshs]

That's really not a very good article at all. It seems to suggest that it is impossible to create a DC output from an AC input...

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From the article: "First Problem: Your generator is producing 120 or 120/240 single phase which must be reduced to 12/24 or 48 volts with the use of huge and inefficient transformer."
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That "transformer" is AKA a battery charger which takes AC in and puts DC out.
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I'm not saying the article is perfect or unbiased. Good tech is like good food, does you no good if you don't digest it.

 

[Haf-E]

Transformers aren't really huge an inefficient - especially if the same transformer is used for the battery charger and for the inverter - as is done in all kind of modern units. Most transformers operate well over 90% and some are in the 95+% efficiency.

There is a number of misleading statements in this article - such as half of the ac source's sinewave output being wasted since it has a negative voltage - that is not correct as nearly all chargers use a full wave bridge type rectifier that allows both the negative and positive halves of the cycles to be used. Also, while traditional chargers do not use the complete AC sinewave - many now days do by the use of high frequency switching techniques and power factor correction methods.

It is true that for many people who already have a nice sinewave inverter, a DC output engine-alternator generator would be a better choice than an AC output generator - but they are hard to buy off the shelf and typically use a belt drive which needs to be maintained. They also can't be used as a back-up / alternate power source for loads which the inverter can not operate or while the battery-inverter system is shut down for repair or if it has a failure.

While solar might be more expensive - the lack of noise, oil changes and refueling allowed by a large solar array is certainly a nice option to have. Being able to park in the shade is also nice - but not always something essential - such as here in the PNW where we like to be in the sun when possible or in areas where trees are not frequent (like baja) - which is why most vehicles have shade awnings.

If you need to have an AC unit operating all night - then a generator is almost always required - but other than that a large solar array and high performance battery-inverter system would provide a nicer experience if you can afford the up front costs.

 

[DiploStrat]

Some Comments

While I don't have a generator on my rig, for all of the usual reasons, I have considered it for the following reasons:

-- Generators are good at meeting large AC loads in real time. Air conditioning is the prime example, but microwaves and other ovens fall into the same category.

-- A generator that consumes less fuel than your vehicle engine when idling is good for bulk charging. N.B. To save fuel, run your genset when your batteries are most discharged and will take the greatest charge. Shut off the generator when the charge rate drops.

-- A genset gives you a nice backup when camping for extended periods without driving or without sunlight. In this scenario one of the Honda or Yamaha variable speed generators might save you a lot of noise and fuel.

A solar kit excels in providing the long hours of acceptance charging. Obviously, the more sun the better, but performance is a bit better than you might think as your acceptance charge rate may be 20A or lower.

Building a camper without a generator requires:

-- A healthy alternator(s) and wiring.

-- A big battery bank.

-- A larger solar kit.

-- A larger inverter/charger - stupidly large if you want to run AC air conditioning.

All of these are doable, but there are cost, weight, and size considerations. Nice things on the horizon:

-- Reliable lithium batteries

-- DC air conditioners.

 

[dwh]

If you need to have an AC unit operating all night - then a generator is almost always required - but other than that a large solar array and high performance battery-inverter system would provide a nicer experience if you can afford the up front costs.

It's not just the up front costs. Other problems are: Being able to afford the weight penalty, having adequate roof real estate for a large solar array and spending enough hours in the sun.

Cost is only one factor, and often not even the primary factor.

 

[pugslyyy]

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From the article: "First Problem: Your generator is producing 120 or 120/240 single phase which must be reduced to 12/24 or 48 volts with the use of huge and inefficient transformer."
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That "transformer" is AKA a battery charger which takes AC in and puts DC out.
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I'm not saying the article is perfect or unbiased. Good tech is like good food, does you no good if you don't digest it.

Actually, transformers don't "transform" AC into DC. A transformer simply increases or decreases the amplitude of an AC output with respect to the input.

You then use a diode rectifier to convert the AC into DC and then smooth it with a couple of inductors and/or capacitors. It's really not that complex - the article grossly misrepresents the process of converting 120VAC->12VDC.

This is a better illustration (230VAC->5VDC but you can get the point from it)

 

[Mwilliamshs]

Pugslyy, you're right. I was simply pointing out the attempt the article made at explaining the AC->DC process so as to say the article didn't claim it was impossible, as was the suggestion to which I replied.

 

[dwh]

So if I went with a generator instead of an engine-mounted alternator, I'd max out at 70 amps charge capability.

True, but what's the time factor? How much time will the alternator spend at a charge rate above 70a anyway?

On the Onan, that 3kw of power requires 4.02 hp, and consumes 10228.63 BTU/hr (8.9% of 1 gallon of gasoline) plus mechanical losses.
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A 130amp alternator can be spun at max output with 2.58 hp, or 6564.64 BTU/hr (5.7% gal) plus losses.

This comparison is a bit misleading, because the number for the Onan appears to include the energy consumed to run the entire machine - including the engine.

To do that comparison and make it fair, you'd need to include in the truck/alternator numbers, not just the fuel used to turn the alternator, but also the fuel used to run whatever part of the engine isn't being used to drive the alternator. And the power steering and water pumps, which, unlike the a/c compressor, are going to be running whenever the engine is running.


Charging [EDIT: should say, "Bulk charging"] from a truck engine alternator is certainly a great way to charge a house battery bank...as long as the engine is already running and doing something else, like moving the truck. Then the added load of the alternator doesn't much matter - there's enough horsepower, and the engine is humming along anyway.


But sitting in the mountains, going nowhere fast, running the truck's engine at high idle for hours to bulk up the house bank? More efficient than a smaller genset and a halfway decent charger?

I guess it just depends on that "time" variable; big engine running for less hours, or smaller engine running for more hours.

I know in my truck, I'm way better off - in terms of watts into the battery per gallon of fuel - using the smaller engine for more hours to get the job done.


(My problem has been that I can't find a generator with a small enough engine. Absorb takes hours and hours. I don't need a bigger engine for faster bulk time - I need a smaller engine for that long slow time in absorb. I've gone through three gennys on this truck, getting smaller every time, and I'm pretty sure right now I've got just about the smallest available (63cc), and even so I could actually go smaller. But the problem there, is that just about the only thing smaller (50cc) is either the Honda or Yamaha 1000w inverter generator - neither one of which is really more efficient over time than the tiny synchronous I have, and both of which cost 10 times as much but will last only 2-3 times as long.)


But I also have a split-charge relay, so when the engine is up and running, and the alternator is spinning anyway, it certainly does get used. Not that it matters all that much - the voltage regulator just holds 14.5v at any RPM above idle, so once the house battery surface charge reaches 14.5v...not a lot going on there in terms of charging. Basically just a long slow absorb at low amps.

To recharge a DC battery, a DC source (alternator, solar panel, etc) is most efficient. Period.

From an electrical perspective, that's certainly true.

Not always true through from an internal combustion perspective.