Connecting a Generator to a Classic

[cabinrob]

One more kick at this topic....
I searched the forum and found a few posts about connecting a generator/mains to a Classic.
The general answer was to avoid it.

Well....  In my case I have no AC mains, but a generator as a backup to my solar system.
I currently do have a battery charger which I have occasionally used to charge my batteries - but it's output is limited to 30A or so.
I'm now eyeing my Classic 250 which can put out 79 amps.  To buy a charger with this performance - I'm going to spend as much as I would on a Classic!
Instead of buying a new expensive charger, it seems to make sense to just rectify/filter the generator output, feed that into my Classic and all is good.
So, instead of hearing "don't do it" - maybe we can get a bit more technical and understand exactly why not, or better yet, how to do it.
There was talk of making sure the supply was "soft."  Well, how soft is soft?  "Soft" isn't very specific.
Is there an optimal mode for the Classic - something other than solar?  Maybe Micro?  Maybe Wind with a table specifically populated to optimize performance with the generator.
Any technical feedback would be greatly appreciated!
rob

[TomW]

Rob;

I am just tossing this on the wall to see what sticks:

Wondering if you could get away with programming a Wind curve that maxes out at XX amps and YYY volts with XX amps well below The Classic's current limit?

Tom

[TomW]

Depending on efficiency demands you could stick a resistance in series with the batteries to "soften" the supply by limiting current. Pretty nasty power waste but could maybe work?  Not very elegant   I guess that comes from my vacuum tube and brute power origins in herding electrons:

I will shut up now.

Good Luck.

Tom

[ClassicCrazy]

Rob
Isn't the best solution to get an inverter that will also charge ? You connect your generator to the inverter and it will charge the batteries and also run your AC loads without the generator.

Do you just run pure DC loads and don't have an inverter ?

Larry

[BobWhite]

One more kick at this topic....
I searched the forum and found a few posts about connecting a generator/mains to a Classic.
The general answer was to avoid it.

Well....  In my case I have no AC mains, but a generator as a backup to my solar system.
I currently do have a battery charger which I have occasionally used to charge my batteries - but it's output is limited to 30A or so.
I'm now eyeing my Classic 250 which can put out 79 amps.  To buy a charger with this performance - I'm going to spend as much as I would on a Classic!
Instead of buying a new expensive charger, it seems to make sense to just rectify/filter the generator output, feed that into my Classic and all is good.
So, instead of hearing "don't do it" - maybe we can get a bit more technical and understand exactly why not, or better yet, how to do it.
There was talk of making sure the supply was "soft."  Well, how soft is soft?  "Soft" isn't very specific.
Is there an optimal mode for the Classic - something other than solar?  Maybe Micro?  Maybe Wind with a table specifically populated to optimize performance with the generator.
Any technical feedback would be greatly appreciated!
rob

I like your thinking!

Rob
Isn't the best solution to get an inverter that will also charge ? You connect your generator to the inverter and it will charge the batteries and also run your AC loads without the generator.

Do you just run pure DC loads and don't have an inverter ?

Larry

Just a thought, maybe trying to work with what is available for now and work into a perfect system as the time permits! Not everyone can set up everything today, sometimes it takes time and we work with what is available for today. JMT

Walt

[CDN-VT]

Rob what size of AC gen do you have ?
bank voltage & size also please .

VT

[cabinrob]

Some answers to the questions that were asked:
I do have an inverter (Xantrex 2000W Pure Sine) but I don't use it very much.
Most of my cabin runs off of 12V.  (Lights are 12V LED, fridges are both 12V, radios 12V, water pump 12V etc.)
I only turn on the inverter if I need to run things like power tools.
Battery bank is 10 x 6V Golf Cart batteries wired for 12V and giving me a capacity of about 1000 Ah.
I don't see that getting an inverter/charger is going to gain me much.  (Typically high quiescent currents.)
I do have an Iota 12v charger - but the output voltage is not high enough.  It starts charging with a reasonable current, but this soon levels off and I waste a lot of gas without much charging going on.
The generator is a 2500 watt gas unit.
For completeness, I have a Classic 200 (mis-typed in the original email.)  Solar has 6 x 305W panels set up as a virtual tracker.  One series string of 3 points SE, and the other series string of 3 points SW.
Needed a Classic 200 to handle the Voc at the cold temps that we get.
The added benefit of a Classic 200 is that the Voc of the rectified generator output would in the neighbourhood of 170V.  A Classic 150 wouldn't handle this.  The other interesting thing about this setup is that the panels give me close to the RMS equivalent of line voltage - so I anticipate diverting extra power to an electric water heater.

Back to my original thought - the Classic already knows about temperature compensated charging, and has set-points adjusted for my battery bank.  It is all wired and set to go - with the exception of the input.
Why get another charger?
I could max out the Classic 200 and would only load the generator at 50%.
Everything seems like a slam-dunk - except for the matching of the generator to the input of the Classic....

[Vic]

Hi .....rob,

There have been a number of Threads on this subject here,  and elsewhere.

MidNite  Engineering has said that this is NOT RECOMMENDED.

As Tom noted,  adding some reasonable resistance in series with the Vin to the Classic  will help 'soften"  the input,  but would waste a lot of power.

It was also mentioned here,  that having the battery voltage close in voltage to the external Power Supply would help protect the Classic  --  this was in the context of an external PS,  and your situation seems far from this recommendation.

Also,  a simple bridge/filter capacitor  would seem to present a poor Power Factor to the Genset.

One way that many MPPT CCs protect themselves is that they reduce the load on the input power source,  which causes the input voltage to rise.  In Solar Mode,  the Classic "knows"  that the input source IS from PV,  and this operating point WILL reduce the input power to the CC.  With your direct output from the filter to the CC,  this is not really the case.

It was also noted in that same Thread,  that running the Classic in Legacy P&O  would probably be the preferred Mode for these relatively risky approaches.

Your comment,  "I do have an Iota 12v charger - but the output voltage is not high enough.  It starts charging with a reasonable current, but this soon levels off and I waste a lot of gas without much charging going on",  probably indicates that the Bulk charging has finished,  and the Absorption voltage has been reached,  so the battery charge current will continue to taper.

This apparent battery charge current taper,  could also be that with FIVE STRINGS (!!!) of batteries,  that there could be poor current charge current sharing between all of those batteries,  and the one or two batteries that are hogging much of the current have gotten to Absorb ...  just  guessing.

Your thoughts are common ones,  at least about using an external PS that feeds an MPPT CC input.  Direct rectification for this input is also risky,  as there is NO isolation between the genset AC output,  and DC side of the system ...   this is often risky in its own right,  for Safety reasons.

To determine the approximate value of the "softening" resistor,  you could look at the Vmp verses Voc curves of the PVs that you are using,  and figure out about what is the output impedance is of the String of PVs that you are using,  this could possibly guide you.   BUT,  this will become a bit of a heater.

And so on,   GOOD LUCK !   Vic

[cabinrob]

Some comments:

There have been a number of Threads on this subject here,  and elsewhere.

MidNite  Engineering has said that this is NOT RECOMMENDED.
>> It's is still not clear to me why!    I hate being told that I can't do something - without a solid explanation.

As Tom noted,  adding some reasonable resistance in series with the Vin to the Classic  will help 'soften"  the input,  but would waste a lot of power.
>>Well, having a generator run at a light load is also just wasting gas - hence wasting power.

It was also mentioned here,  that having the battery voltage close in voltage to the external Power Supply would help protect the Classic  --  this was in the context of an external PS,  and your situation seems far from this recommendation.
>> My panels run at 130V.   This is far from the battery voltage.  No-one is telling me I shouldn't being doing this.  What exactly am I protecting?

Also,  a simple bridge/filter capacitor  would seem to present a poor Power Factor to the Genset.
>>Since I would be drawing less than half the rated power from the generator, this should be a non-issue.

One way that many MPPT CCs protect themselves is that they reduce the load on the input power source,  which causes the input voltage to rise.  In Solar Mode,  the Classic "knows"  that the input source IS from PV,  and this operating point WILL reduce the input power to the CC.  With your direct output from the filter to the CC,  this is not really the case.
>> Maybe "many" CC do this?  Is this what the Classic does?  As far as I can tell, the Classic is just a DC-DC converter.  I would think that it would be more sophisticated in managing currents than picking a different panel operating point in order to manage input current/power.

It was also noted in that same Thread,  that running the Classic in Legacy P&O  would probably be the preferred Mode for these relatively risky approaches.
>>Why not hydo mode?  Isn't a hydro generator just a, well, generator connected to a water turbine?  From the point of view of the Classic, how is this different from a gas generator.  It's just a "generator" being run by a gas engine instead of water. No?

Your comment,  "I do have an Iota 12v charger - but the output voltage is not high enough.  It starts charging with a reasonable current, but this soon levels off and I waste a lot of gas without much charging going on",  probably indicates that the Bulk charging has finished,  and the Absorption voltage has been reached,  so the battery charge current will continue to


This apparent battery charge current taper,  could also be that with FIVE STRINGS (!!!) of batteries,  that there could be poor current charge current sharing between all of those batteries,  and the one or two batteries that are hogging much of the current have gotten to Absorb ...  just  guessing.
>>I have measured the current going to my batteries - and the strings are quite balanced.  The issue is that the Iota does not have a high enough output voltage.  It does not understand temperature compensated charging.  I have boosted the output voltage to its maximum - and it's just not high enough.

Your thoughts are common ones,  at least about using an external PS that feeds an MPPT CC input.  Direct rectification for this input is also risky,  as there is NO isolation between the genset AC output,  and DC side of the system ...   this is often risky in its own right,  for Safety reasons.
>>I don't see how this is any different from having the same power/voltage/current coming from my panels???  I don't know the design of the classic, but I would have expected that it's basically just a transformer.  On the input side there is a power oscillator converting DC to high frequency AC.  This goes to a transformer and then on the output side there is a rectifier/filter.  I would think that other than a common ground there should be isolation???

To determine the approximate value of the "softening" resistor,  you could look at the Vmp verses Voc curves of the PVs that you are using,  and figure out about what is the output impedance is of the String of PVs that you are using,  this could possibly guide you.   BUT,  this will become a bit of a heater.
>>I would rather start from the input impedance spec of the Classic and then work back to my power source.  So far, I've only read about "soft."   This really does not help me.

>>For what it's worth, there may be another approach to this problem that could do away with resistor to massage the characteristics of the supply.  Most generators have some sort of feedback look to manage throttle vs. load.  As the load increases, the "throttle" of the generator is increased.  If one disables this feedback mechanism, then the generator will probably start to look a whole lot like a solar panel.  Just thinking out loud.....

And so on,   GOOD LUCK !   Vic
>> I was really hoping to avoid luck and maybe throw in some science/engineering!!!
>> I do appreciate the input.  Thanks.

[CDN-VT]

Some answers to the questions that were asked:

Everything seems like a slam-dunk - except for the matching of the generator to the input of the Classic....

Iotas are adjustable & have smarts , what is your model , I have 12,24,V in 45a&90&120A in 24V .

Gen set is it a screamer or the inverter style , if so inverter tap within , screamer use Iotas (2) on 220 split phase & balance gen , and croon amps in thru shunt bypassing classic , let classic finish & monitor .



VT

[Vic]

Hi .....rob,

There have been a number of Threads on this subject here,  and elsewhere.

MidNite  Engineering has said that this is NOT RECOMMENDED.

As Tom noted,  adding some reasonable resistance in series with the Vin to the Classic  will help 'soften"  the input,  but would waste a lot of power.

It was also mentioned here,  that having the battery voltage close in voltage to the external Power Supply would help protect the Classic  --  this was in the context of an external PS,  and your situation seems far from this recommendation.

Also,  a simple bridge/filter capacitor  would seem to present a poor Power Factor to the Genset.

One way that many MPPT CCs protect themselves is that they reduce the load on the input power source,  which causes the input voltage to rise.  In Solar Mode,  the Classic "knows"  that the input source IS from PV,  and this operating point WILL reduce the input power to the CC.  With your direct output from the filter to the CC,  this is not really the case.

It was also noted in that same Thread,  that running the Classic in Legacy P&O  would probably be the preferred Mode for these relatively risky approaches.

Your comment,  "I do have an Iota 12v charger - but the output voltage is not high enough.  It starts charging with a reasonable current, but this soon levels off and I waste a lot of gas without much charging going on",  probably indicates that the Bulk charging has finished,  and the Absorption voltage has been reached,  so the battery charge current will continue to taper.

This apparent battery charge current taper,  could also be that with FIVE STRINGS (!!!) of batteries,  that there could be poor current charge current sharing between all of those batteries,  and the one or two batteries that are hogging much of the current have gotten to Absorb ...  just  guessing.

Your thoughts are common ones,  at least about using an external PS that feeds an MPPT CC input.  Direct rectification for this input is also risky,  as there is NO isolation between the genset AC output,  and DC side of the system ...   this is often risky in its own right,  for Safety reasons.

To determine the approximate value of the "softening" resistor,  you could look at the Vmp verses Voc curves of the PVs that you are using,  and figure out about what is the output impedance is of the String of PVs that you are using,  this could possibly guide you.   BUT,  this will become a bit of a heater.

And so on,   GOOD LUCK !   Vic

.....rob,

This is obviously is something that you really want to do.

On this Forum,  MidNtie Engineering said that this is not recommended  --  a polite way of saying DO NOT DO THIS.

There may not be enough time in a day to discuss this in detail,  at least there is not enough time in my days.

I have no horse in this race.  You seem to have the answers,  so who am I to try to dissuade you from your task.

Vic

[ClassicCrazy]

My thought about building the ideal generator rectifier like you are talking about is that by the time you engineer to  put in all the parts and design to consider all the details  you will have something like what you see in an inverter charger or the better Iota chargers mentioned above. 

I am not an electrical engineer so couldn't begin to design such a device like you want. I watch tear downs of devices on EEV blog on  youtube and he explains in detail all the engineering and design details of devices - there is so much to consider.  Check out that youtube channel - he can spend 45 minutes just scratching the surface on what thoughts went into some electronic device.

And as mentioned above by vic - no matter how big your generator capacity is , for lead acid batteries once they reach the Absorb setpoint the current is going to taper back. If you want a battery pack you can quickly dump a lot of power into then you should consider getting lithium batteries .

Larry

[Halfcrazy]

So the "Real" issue is the stiffness of the generator, The Classic (like other MPPT controllers) is used to working with things it can through around in voltage. So if it tries to sweep all the way down to battery caboom goes the FETs.

That said I have a set up just like this, 120vac generator rectified and filtered slightly with capacitance fed through a resistor and then fed to a classic. I use Hydro mode with a low voltage limit of like 145 and current limit set to 40 amps. The Resistor acts as a shock absorber. I use 2 ohms and it seems ok.

Of course tech support will tell you this is "Unsupported" and will "Void your warranty"

I can agree with them on both parts as it can be a large consumption of time to walk some one through making it work and then they have the liability if the Classic blows up. So long story short, yes it can be done, Yes it is slightly tricky and YES it can blow up the classic if not done right

[TomW]

So if it tries to sweep all the way down to battery caboom goes the FETs.

Gotta love those Fire Emitting Transistors!!

[cabinrob]

Some responses to the responses!

Iotas are adjustable & have smarts , what is your model , I have 12,24,V in 45a&90&120A in 24V
>> I have a DLS-45.  In dual voltage mode - it puts out 14.2V and 13.6V.
>> In Smart mode: Bulk:14.8V, Absorb:14.1V and Float 13.6V
>>At  cool temps, these voltages are too low.  The Bulk needs to have the ability to be about a half volt higher than it is.


This is obviously is something that you really want to do.
>> Yes!
On this Forum,  MidNtie Engineering said that this is not recommended  --  a polite way of saying DO NOT DO THIS.
>>I can understand the liabilities that some have explained.  I also understand that MidNite may not want to spend time explaining how to do this.
>>I'm willing to accept the responsibility it I destroy my Classic.  I was hoping that someone on the forum had experience.
There may not be enough time in a day to discuss this in detail,  at least there is not enough time in my days.
>> Understood.  On the other hand, if others have done this - then the answer could be fast and simple.
I have no horse in this race.  You seem to have the answers,  so who am I to try to dissuade you from your task.
>>If I had the answers, I wouldn't be asking for help or suggestions!

My thought about building the ideal generator rectifier like you are talking about is that by the time you engineer to  put in all the parts and design to consider all the details  you will have something like what you see in an inverter charger or the better Iota chargers mentioned above. 
>> I think that people are over analyzing this problem.  The answer should be as simple and a few diodes, a capacitor and maybe a resistor.  If the Classic had a mode that does not sweep, we would be all done.
I am not an electrical engineer so couldn't begin to design such a device like you want. I watch tear downs of devices on EEV blog on  youtube and he explains in detail all the engineering and design details of devices - there is so much to consider.  Check out that youtube channel - he can spend 45 minutes just scratching the surface on what thoughts went into some electronic device.
>> I do have a background in electronics.  Really, really, it's not all that complicated....
And as mentioned above by vic - no matter how big your generator capacity is , for lead acid batteries once they reach the Absorb setpoint the current is going to taper back. If you want a battery pack you can quickly dump a lot of power into then you should consider getting lithium batteries
>>Correct.  But if the Bulk and Absorb voltages are too low, then one is not putting as much current into the batteries as one could.  This means that the generator ends up running longer than it should.  Even a completely unloaded generator eventually runs out of gas.  Where I have my cabin, it is very difficult to get fuel - not to mention the  philosophic issue of wasting something that you don't have to.


So the "Real" issue is the stiffness of the generator, The Classic (like other MPPT controllers) is used to working with things it can through around in voltage. So if it tries to sweep all the way down to battery caboom goes the FETs.
That said I have a set up just like this, 120vac generator rectified and filtered slightly with capacitance fed through a resistor and then fed to a classic. I use Hydro mode with a low voltage limit of like 145 and current limit set to 40 amps. The Resistor acts as a shock absorber. I use 2 ohms and it seems ok.
>> Thanks for the data.  I'm not sure where you got the 145V from?    The rectified, filtered, unloaded voltage should be around 170V.  But as you put a load on the output, you're going to come back down to the 120 RMS voltage.  I guess if you are lightly loading your generator, you could be at 145V?
Of course tech support will tell you this is "Unsupported" and will "Void your warranty"
>>I accept the consequences.

I can agree with them on both parts as it can be a large consumption of time to walk some one through making it work and then they have the liability if the Classic blows up. So long story short, yes it can be done, Yes it is slightly tricky and YES it can blow up the classic if not done right.
>>Having said that - I would think that Midnite Solar could get some marketing mileage out of being able to use their CC in such a mode.  Heck, they could even build the interface box and add a mode that does not sweep the input.  They might sell even more CC's.