Bulk - What Does It Mean - for Solar Charge Controllers?

[MikeCanuck]

Sunking, thanks for your newest sticky "Bulk Absorb Float Equalize: What Does It Mean".
This was very helpful as I was reviewing and trying to "absorb" all the stuff you, PNjunction, and others here were trying to help me in previous threads understand about how a lead acid battery gets charged, maximum smoke, etc.

So I think I get now how a on-grid charge controller interacts with the battery.

But I want to make sure I get how a solar charge controller interacts - esp in the Bulk stage through a PWM charge controller....

Sunking wrote:
"Bulk mode in a solar charge is not CC. It is Constant Power. What Power?. Who knows, depends on how strong the sun is...
...In Bulk at a few brief seconds around noon it was 10 amps for a second. What is it 30 minute after noon?. Something less than 10 amps. If it is hazy maybe 5 amps. All your controller can do is transfer the maximum available power at the time. It cannot do any more than that. So that makes it Constant Power, not CC..."

In another thread, PNjunction wrote:
"PWM" is really just a charging waveform that is different from a steady-state output such as from a typical linear power supply. The pulses (on/off) are so fast, you usually can't see it with average consumer voltmeter gear. In reality, the pulse varies from a very low state (such as in bulk when there is essentially NO pulse, and then it ramps up the pulsing very very rapidly for absorb."


So I just want to make sure I understand - HOW does a PWM versus MPPT solar charge controller get the varying power to the battery in the Bulk stage?

PWM: does a pwm charge controller simply and LITERALLY connect the panels DIRECTLY to the battery in the bulk stage and just lets whatever power pass through from the panels to the batteries until the absorb voltage is reached?
Or is it actually doing pwm "pulsing" but those pulses are maybe very long duration and each pulse has a different amperage/wattage or something?

MPPT: I'm guessing MPPT controllers definitely does not directly connect the panels to the battery in the bulk stage, since it has to do stuff like DC to DC conversion etc prior to sending the power to the battery, so does it use pwm to do that? Or does it simply send whatever the result is after it does its MPPT magic and it is "logically" as if it was connecting the panels directly to the batteries and no PWM is taking place in the bulk stage?

thanks alot again for the learning guys...

[Sunking]

You are welcome.

Easy Peazy. PWM is nothing more than a Switch that turns On/Off real dang fast of thousands a time per second. In Bulk Mode a PWM controller connects the panel directly to the battery. In other words the PWM modulation is 100% ON all the time. A Solar panel is a Current Source and when you connect a panel directly to the battery, the panel is a constant current source of unknown current. Depends on time of day, clouds, time of year, humidity blah blah. Exact same thing with a MPPT controler being constant Power. Difference is MPPT will be more current (tus more power) than the panel alone. A 100 watt with PWM only delivers 67 watts at very best at the end of the Bulk cycle, Mppt 94 to 98 watts through the whole cycle.

Absord, Float and EQ are constant voltage modes. Both PWM and MPPT controllers are PWM when in those 3 CV modes. At that point the PWM modulation is less than 100% which turns a Current Source into a Voltage Source.

Read this for Laymen explanation of PWM

[MikeCanuck]

OK got it for Bulk (I think)..

So in Bulk there is no PWM(rapid on/off) happening at all, the switch is always on and the panel is directly connected to the battery.
And, the voltage is not being limited in anyway by the charge controller at this stage if I understand the sticky because that depends on what the battery will accept based on this formula, right?
A charging Battery Voltage = Battery OCV + (Charge Current x Ri)


Now for Absorb

Sunking wrote:
"Once the battery OCV reaches 14.3 volt magic happens. We automatically go into Absorb mode. Nothing really happens, nothing switches or does anything. Simple Ohms Law is still at play. As soon as the battery OCV reaches 14.31 volts charge current starts to taper off. At 14.31 volts Charge Current = Charger Voltage – Battery OCV) / Ri. So 14.4 – 14.31) .01 Ohms = 9 amps. As the battery saturates and reaches 14.4 volts all current stops because the Charge Voltage and Battery OCV are EQUAL"

So, when a PWM controller goes into Absorb (constant voltage stage),
Is it still simply directly connecting the panels to the battery, but now switch it on and off rapidly/pwm?
What is the purpose of the pwm - is it to limit voltage or to limit current?
I'm guessing it is limiting current based on the formula above?
Then how is "constant voltage" achieved since simple ohm's law etc is still at play, so voltage should still be determined by the battery as in the bulk stage, no?

I was also wondering something about MPPT controllers - in the Absorb stage, does the MPPT part stop completely and it only does PWM exactly like a PWM controller, so the benefits of the MPPT part is not used in the absorb stage? Or is it doing both MPPT and PWM in the absorb stage?

[Sunking]

OK got it for Bulk (I think)..

So in Bulk there is no PWM(rapid on/off) happening at all, the switch is always on and the panel is directly connected to the battery.
And, the voltage is not being limited in anyway by the charge controller at this stage if I understand the sticky because that depends on what the battery will accept based on this formula, right?
A charging Battery Voltage = Battery OCV + (Charge Current x Ri)

Close enough you got a grasp on it.

Now for Absorb

Sunking wrote:
"Once the battery OCV reaches 14.3 volt magic happens. We automatically go into Absorb mode. Nothing really happens, nothing switches or does anything. Simple Ohms Law is still at play. As soon as the battery OCV reaches 14.31 volts charge current starts to taper off. At 14.31 volts Charge Current = Charger Voltage – Battery OCV) / Ri. So 14.4 – 14.31) .01 Ohms = 9 amps. As the battery saturates and reaches 14.4 volts all current stops because the Charge Voltage and Battery OCV are EQUAL"

So, when a PWM controller goes into Absorb (constant voltage stage),
Is it still simply directly connecting the panels to the battery, but now switch it on and off rapidly/pwm?
What is the purpose of the pwm - is it to limit voltage or to limit current?
I'm guessing it is limiting current based on the formula above?
Then how is "constant voltage" achieved since simple ohm's law etc is still at play, so voltage should still be determined by the battery as in the bulk stage, no?

I was also wondering something about MPPT controllers - in the Absorb stage, does the MPPT part stop completely and it only does PWM exactly like a PWM controller, so the benefits of the MPPT part is not used in the absorb stage? Or is it doing both MPPT and PWM in the absorb stage?

OK I see where you are getting off track. In my discussion pertains to AC chargers. AC chargers are a stiff source of power. Example one that plugs into a wall socket has as much as 2000 watts available 24 hours a day. In an AC charger, charger current is limited by the charger circuitry. Solar that is not necessarily the case. In a Solar system power or charge current is limited by the panels, not so much the controller itself. In a Solar system you just make sure you buy a charger that can handle the amount of current the panels can supply. If your panels can generate say 50 amps, don't buy a 40 amp controller, you want one that can handle at least 50 amps or MORE

Think of this. I take a 100 watt solar panel, 80 amp controller, and a 12 volt battery. We discharge the battery so as to force the controller into BULK. You can pray and cuss all day, the most charge current you will ever see is 7.6 amps from a MPPT controller, or 5.5 amps with a PWM controller. You can cuss all day. with a 100 watt panel you wil not get any more power. Why? Because the panel is a soft source with limited capacity.

Once we switch to a CV mode, I do not care if you call it Absord, Float, Equalize, or a great big pair of (.)(.) it is a CV mode. In a commercial AC charger with a stiff source, when in CV mode will be at whatever voltage you set it to. Current will be less than Bulk and falling. But with Solar, maybe, maybe not. If the sun is getting weak, and the panels cannot deliver enough current to hold the voltage, by default the voltage folds back and you are back into CC mode at something less than BULK. That current will not be your normal BULK current with noon day sun, because the sun is to weak.

Summary, set Bulk = Absorb = Float to whatever voltage is required to get your specific gravity up to 1.277 at the end of the day. That forces your controller be it a PWM or MPPT to stay in BULK from sun rise to sun set and pump every possible bit of power it can into the batteries.

In the winter time, most likely there is no voltage high enough to get fully charged unless your panel wattage is over sized. You can set it to say 16 volts on a 12 volt system and your battery never gets fully charged. You would never see your battery voltage make it to Gassing Voltage which is where it needs to get to possible get fully charged. In Summer time assuming you have enough panel wattage, you might very well back th evoltage off to 13.8 to 14 volts and see a specific gravity of 1.277.

Use your hydrometer to set the voltage, not your owners manual. Your owners manual was written assuming you are using a commercial AC charger with all the time in the world to get fully charged and run through all three stages. That does not apply with solar.

Hope that helps.

[MikeCanuck]

Ya, I wanted to discuss again that maximum smoke part of the sticky, but still trying to get educated on how exactly each stage in a 3-stage Solar charge controller works and how a PWM controller versus an MPPT controller works at each stage

My best guess based on what I've learned so far is in the Absorb/Constant Voltage stage, in a PWM charge controller - I'm guessing it's not the charge controller that is keeping the voltage constant, but it is the fact that as the charge controller is tapering the current using PWM, if current didn't go down, the voltage on the battery would keep rising. But if current keeps lowering based on the most that the battery will accept at that voltage, then voltage would remain constant. Is this in the ballpark?
Then the issue is IF the panels can keep up the necessary current needed to compete the absorb phase, which leads to maximum smoke, yes?

--

OK maximum smoke, I keep mulling over that one.
I found an old post on this forum from a member in Finland named Anttistaatti.
He had a link to his blog which I found interesting because makes everything from scratch including the charge controller:
https://sites.google.com/site/diysol...argecontroller

Here he built a "linear shunt regulator" as his charge controller.

So instead of getting an expensive charge controller and then dumbing it down in winter so that it basically stays in BULK mode all the time, then FLOAT and skipping Absorb, would it be cheaper, simpler to just build one of these "linear shunt regulators" and be done with it?

--

But then again, if when a solar charge controller can't get enough current to maintain Absorb/CV and drops back to Bulk/CC, doesn't that automatically achieve Maximum smoke automatically?

[sensij]

. A 100 watt with PWM only delivers 67 watts at very best at the end of the Bulk cycle, Mppt 94 to 98 watts through the whole cycle.

This is fine from 10000 ft, but as your understanding improves, you'll see that there are conditions in which MPPT and PWM can have similar performance in bulk, and there are times when PWM can perform much worse than the 67% suggested. The basic relationship that Sunking is describing is based around the idea that a panel with a Vmp (maximum power point) of 18 V will lose 1/3rd of its power when operating at 12 V, assuming the current is constant in either case. The problem is that a panel's Vmp is not a fixed number, it varies throughout the day as a function of temperature. In the middle of a hot day, the Vmp might actually be very close to the battery voltage, leading to very little loss in the PWM system. In a very cold day, Vmp could actually be higher than 18 V, leading more like 40% loss from the PWM.

With respect to the CV charging stages, think of it like this.... during the ON portion of the pulse, the panels are connected directly to the battery. If the battery can take the available current, the only voltage change will be the slow OCV increase as the battery gains charge. However, as the battery approaches full and it can't take so much current, the extra current from the panels turns more rapidly into voltage rise, approaching the Voc of the panels. The PWM controller is monitoring the voltage and shuts off the ON pulse before the voltage can climb so high, and with some standard control circuitry can anticipate the ON/OFF ratio under normal conditions to maintain the voltage at the desired level with just a little bit of ripple between ON and OFF.

The dirty secret is that MPPT controllers actually operate in PWM for the CV stages, so the benefit to MPPT is really just in the bulk stage. There is no point to generating maximum power if the battery can't take it.

[PNjunction]

I can't resist this one.

Let's make a human pwm controller. Remove your existing one. Instead, place a knife-switch in series with one of the panel leads. Grab a voltmeter. And a lawn-chair to babysit.

Close the knife switch. Leave it closed until the battery terminal voltage rises to 14.4v say. This would be the bulk stage.

You know that if you leave it closed long enough, the battery terminal voltage will try to rise eventually to the panel's rated voltage, somewhere from 17-22v for a nominal 12v panel. Obviously not good.

Absorb - now that you have reached 14.4v for example, with the reflexes of a hummingbird, or a morse-telegrapher from the 1800's on his 8th cup of coffee, quickly close and open the knife switch fast enough to maintain what LOOKS like a steady 14.4v on your voltmeter with no jumping of the needle or display.

So even though 18-22v is trying to spike, it is being done so fast enough, that the apparent voltage at the terminals is only 14.4 due to your fast knife-switching until your wrist breaks.

Thought I'd throw out this human experiment in case you find yourself on Gilligan's Island, or perhaps a marine environment needing to get back to shore, but your CC has smoked. If you have a voltmeter, and keep an eye on it, at least you can get up to absorb.

Worst case, you might be able to emulate an even less efficient ping-pong controller by opening the switch when it reaches absorb, and letting it fall to say 13.8v, and then closing the switch again until it reaches 14.4v, and repeat over and over. Good to know for a Gilligan's Island event.

[MikeCanuck]

Thank you all ! I am inch by inch clawing my way up the mountain to enlightment and One Day will achieve the end-to-end state of understanding of exactly how panels-solar charge-controller-batteries actually work!!

Sensij wrote:
However, as the battery approaches full and it can't take so much current, the extra current from the panels turns more rapidly into voltage rise, approaching the Voc of the panels. The PWM controller is monitoring the voltage and shuts off the ON pulse before the voltage can climb so high, and with some standard control circuitry can anticipate the ON/OFF ratio under normal conditions to maintain the voltage at the desired level with just a little bit of ripple between ON and OFF.
+
Pnjunction's knife illustration

Thanks!! Yes I am getting it....Slowly but surely it is absorbing....
One day I hope to be able to correlate things to Sunking's formulas etc, one day...


Sensij wrote:
The dirty secret is that MPPT controllers actually operate in PWM for the CV stages, so the benefit to MPPT is really just in the bulk stage. There is no point to generating maximum power if the battery can't take it.

Ah ha - this is what I was suspecting/asking about. So just to be sure: If I had 100V of panels in series connected to an MPPT charge controller connected to a 12V (however unlikely) battery, are we saying that the MPPT part is totally turned off during Absorb and it simply directly connects the panels to the battery via PWM on/off rapid switching?


--

Any thoughts on using Mr.Anttistaatti's "linear shunt regulator" for maximum smoke?
https://sites.google.com/site/diysol...argecontroller
Set the dang thing to 16v for a 12v battery in winter like what sunking was saying earlier and be done with it, yes?

And yes, on my list of next toys will be a temp comp hydrometer so I can understand that part of it. Just don't like the idea of playing with freezing cold sulphuric acid in the winter, this may be more a summertime pastime...

[Mike90250]

....
Ah ha - this is what I was suspecting/asking about. So just to be sure: If I had 100V of panels in series connected to an MPPT charge controller connected to a 12V (however unlikely) battery, are we saying that the MPPT part is totally turned off during Absorb and it simply directly connects the panels to the battery via PWM on/off rapid switching?......

Not always. My Morningstar MPPT-60 will use MPPT in absorb & float if needed, it doesn't happen very often, but I have caught it doing it,

[Sunking]

The dirty secret is that MPPT controllers actually operate in PWM for the CV stages, so the benefit to MPPT is really just in the bulk stage. There is no point to generating maximum power if the battery can't take it.

Sensi that is not accurate. That is only true once the current level drops below Imp of the panel input current.

Couple of examples.

Say you are running 1000 watt panel. 4 250 watt panels in series with a Vmp = 120 volts, Imp = 8.33 amps. When you first switch go into Absord the batteries are still 80 amps of current. As the battery voltage approaches Source voltage, the current begins to taper. The controller will remain in MPPT mode until charge current equals or less than panel current of 8.33 amps. Catch is if the system is properly designed, by the time you reach PWM mode your batteries are already fully charged. Assuming a properly sized system a battery with 80 amps of charge current is a 800 AH battery and Absorb ends when charge current reaches 24 amps.

Another example. Same 1000 watt system in Float doing nothing. Turn on a load up to 1000 watts and all that power comes from the panels assuming the conditions are right, like high noon on a bright sunny day.

Take away is a MPPT only goes into PWM mode when current demands are equal to or less than panel Input Current.

[inetdog]

And once you get into the range where the battery current (the maximum it is appropriate for it to draw at that point in the charging profile) drops below the available panel current it really does not matter to the battery whether you use a PWM control function, a linear regulator wasting power or an MPPT control function deliberately operating away from the MPP. The batteries still get the same charging current.

But the linear regulator, unlike the other two, will be dissipating power internally instead of just not pulling it from the panels and so has to have a way to safely get rid of that heat.

[Sunking]

And once you get into the range where the battery current (the maximum it is appropriate for it to draw at that point in the charging profile) drops below the available panel current it really does not matter to the battery whether you use a PWM control function,

True Dave but not the full picture. Refer to my example