outback FM80 MPPT

mshen11

ok this is an easy one. in the manual it describes the panel read as following:

INPUT - current and voltage of battery PV
OUTPUT - current and voltage reading of battery

does it really *instead* mean the input and output to the charge controller? i turned off the breaker between the charge controller and the battery and the output value did not get cut off. i also noticed the output value is always higher than the input value - even when the battery has no load

GreenPowerManiac

Re: outback FM80 MPPT

Sounds like the inverter doesn't care what's after the Output. Load or No load, it'll compensate until its limits.

BB.

Re: outback FM80 MPPT

I am not quite sure I understand the question--also, I know next to nothing about the Outback products...

But, when you say the output is higher than the input... Are you talking about Volts, Amps, and/or Watts?

Remember the equation:

P=I*V...

• Pout = the output Voltage * output Current
• Pin = Vmp * Imp -- or = Vpanel*Ipanel (mp is maximum power, when the charge controller cuts back to absorb/float--it is no longer drawing maximum power from the panels during the mid-day).
• Plosses = Every controller uses some power to run the electronics and turn on and off the "switches"... Those losses typically appear as heat from the heat-sinks/controller.

Pout=Ppanel + Plosses (where Plosses is around 5-10% of Ppanel)

• Pout should always be 5-10% less than Pin (or even a bigger difference when in float / near sun up or sun set).
• Vpanel should always be bigger than Vbatt (for a MPPT type controller) so the current will flow into the battery... Either by a couple of volts or a lot more.
• Vbatt should be pretty close to the battery voltage
• Ibatt is the current going into the battery--typically, Ibatt is larger than Ipanel (remember, with a MPPT controller, P=I*V--so When V is lower--such as at the battery, then Ibatt must be higher than Ipanel because Vpanel is also higher.

Clear as mud?

When you open the breaker between the controller and the battery--I have no idea what is going to happen... In theory, the controller should fault (it is the "battery bank" that really regulates the system voltage). No battery, most controllers are incapable of operating correctly (controller should over voltage/under voltage fault until the battery is reconnected).

-Bill

mshen11

Re: outback FM80 MPPT

ive been hanging around here for a while to understand what youve said

however my questoin is specific to the FM80 panel reading where there is an entry for 'input' and 'output'. the manual (at least to me) seem to be misleading/unclear - and my questoin specifically ask what output means (does it mean at the battery or at the output of the controller). i assume it it is the latter - if it is the case, it is poor wording on outback's manual to refer to throw "battery" into the description

crewzer

Re: outback FM80 MPPT

The "Output" is the voltage and current as measured at the charge controller's output terminals. Assuming a correctly calibrated system, the "output voltage" may be a bit higher than actual battery voltage while charging due to voltage drop (E=IR) in the wiring, circuit breakers, etc. between the charge controller and the battery bank.

The "output voltage" should be a fairly accurate indicator of actual battery voltage when there's little or no current flow in or out of the battery bank.

HTH,
Jim / crewzer

mshen11

Re: outback FM80 MPPT

thanks for the answer

i was assuming the output side (current) was higher because of MPPT?

Cariboocoot

Re: outback FM80 MPPT

Also note that the CC's system voltage reading can be fine-tuned under the optimization menu to be accurate to a DVM reading taken at the battery. I wouldn't bother, though; the Outback's temp compensated reading is probably more accurate than you get sticking a couple of probes on the battery terminals. It's usually not off by more than 0.2 Volts anyway.

From my own experience: these controllers will read an interesting variety of nonsense if not hooked up on the input and output. The only time you should be looking at it in its 'disconnected' state is when setting the system Voltage.

As for the MMPT function, in normal operation the INPUT Voltage will be higher than the OUTPUT Voltage, whereas the current will be the other way around. That's what MPPT basically does; converts higher PV Array Voltage into more usable charge current at the system Voltage.

crewzer

Re: outback FM80 MPPT

i was assuming the output side (current) was higher because of MPPT?

Yes and no.

The controller’s DC-DC down-conversion and the MPPT function really are separate functions IMHO. For example, the Blue Sky SB2000, with it’s 30 Voc input limit (including low temperature correction) is an MPPT controller that can take any “excess” voltage from a "12 V" (nominal) array and convert it to additional charge current to charge a 12 V (nominal) battery.

However, you can’t connect a "24 V" (nominal) array to the SB2000 to charge a 12 V battery. But, assuming sufficient difference between the array's operational Vmp and the battery voltage, the MPPT function alone can provide for an output voltage that's lower than the input voltage, and an output current that's higher than the input current.

IMHO, this feature alone is the true beauty of MPPT. It's best observed in the winter, when the cold array's operational Vmp is noticeably higher than in summer.

The legacy MX60, the FM80, the FM60, the XW, and other “big” MPPT charge controllers not only include the MPPT function, but also the ability to down-convert a relatively high PV array voltage (up to ~60 V nominal; typically 150 Voc max) down to 48 V, 24 V, or 12 V (nominal) battery banks, with corresponding increases in charge current.

HTH,
Jim / crewzer

mshen11

Re: outback FM80 MPPT

crewzer wrote: »

\
The controller’s DC-DC down-conversion and the MPPT function really are separate functions IMHO. For example, the Blue Sky SB2000, with it’s 30 Voc input limit (including low temperature correction) is an MPPT controller that can take any “excess” voltage from a "12 V" (nominal) array and convert it to additional charge current to charge a 12 V (nominal) battery.

However, you can’t connect a "24 V" (nominal) array to the SB2000 to charge a 12 V battery. But, assuming sufficient difference between the array's operational Vmp and the battery voltage, the MPPT function alone can provide for an output voltage that's lower than the input voltage, and an output current that's higher than the input current.

IMHO, this feature alone is the true beauty of MPPT. It's best observed in the winter, when the cold array's operational Vmp is noticeably higher than in summer.

The legacy MX60, the FM80, the FM60, the XW, and other “big” MPPT charge controllers not only include the MPPT function, but also the ability to down-convert a relatively high PV array voltage (up to ~60 V nominal; typically 150 Voc max) down to 48 V, 24 V, or 12 V (nominal) battery banks, with corresponding increases in charge current.

HTH,
Jim / crewzer

1. how does the controller know if you have a 24V (or higher voltage) system (as oppose to a panel w/ REALLY high voltage output

2. my experiment today i had 2A in 12V system (test run on cloudy day)... the output was always 0.2 to 0.4A higher. am what i have observed MPPT at work?

Cariboocoot

Re: outback FM80 MPPT

mshen11 wrote: »

1. how does the controller know if you have a 24V (or higher voltage) system (as oppose to a panel w/ REALLY high voltage output

2. my experiment today i had 2A in 12V system (test run on cloudy day)... the output was always 0.2 to 0.4A higher. am what i have observed MPPT at work?

The system Voltage (nominal Voltage of the batteries and inverter) is programmed when you set the unit up. The default value is 12 Volts.

The charge controller senses the array Voltage and adjusts accordingly. A nominal 12 Volt panel has a Vmp of about 17 Volts - more than even the recommended Equalization Voltage on a 12 V system. You don't get the full MPPT down-converting advantage is the nominal Voltages for the system and the array are the same.

Remember: System Voltage is the nominal rating for the batteries & inverter. Array Voltage is the nominal rating for the panels. (Maybe I'd better add that to the glossary.)
Gotta keep the terminology separate to stay unconfused!

mshen11

Re: outback FM80 MPPT

Cariboocoot wrote: »

The system Voltage (nominal Voltage of the batteries and inverter) is programmed when you set the unit up. The default value is 12 Volts.

The charge controller senses the array Voltage and adjusts accordingly. A nominal 12 Volt panel has a Vmp of about 17 Volts - more than even the recommended Equalization Voltage on a 12 V system. You don't get the full MPPT down-converting advantage is the nominal Voltages for the system and the array are the same.

Remember: System Voltage is the nominal rating for the batteries & inverter. Array Voltage is the nominal rating for the panels. (Maybe I'd better add that to the glossary.)
Gotta keep the terminology separate to stay unconfused!

yep i undrestand everything youve said...
like i said my setup is at 12V... i am making a decision to stay that way. but it seems like you are implying it would be better for me to have higher nonimal voltage? given my case should i be going higher? as previously stated my reasons for 12V is:

* shading
* higher voltage == more dangerous
* may not have multiples of x for 24V, 36V, 48V systems. right now i own 4 panels and need to digest that. may go to 6 or 8 - but i doubt the real estate will allow it
* have a breaker installed on the input side - limits to 48VDC

it would be nice to go higher than 12V because my distance is 40ft. i currently have 10AWG to cover that distance (kyocera 130TM panels).

but in my current seup i DO get *some* MPPT advantage, just not full. right?

RandomJoe

Re: outback FM80 MPPT

Anytime the input voltage is higher than the system voltage, you'll get *some* effect from the MPPT. It's just that there won't be a whole lot, as a 12V (nominal) system's voltage can rise to 14.2V or so at the end of bulk and during absorption and "12V panels" are typically rated for 17-18Vmp so not a whole lot of difference. But some.

You mention shading as a problem, but you do have four panels. You could connect the panels in two series strings, then parallel those two strings (2x2 config) - that would double your voltage, and cut the current on the panel side in half. If one panel is shaded, you will lose two panels' worth of current but then is that often occurring or is it just at one particular time of day? If the latter, then the reduced current losses may be worth it. If I remember right, this is the config (input voltage 2X output) where the FM80 is most efficient as well.

Another thought - do your panels have bypass diodes in them? If so, then shading shouldn't be that much of an issue even with them all in series - one panel shaded in a string, the current of the others flows through the bypass diode and no big deal. (Other than changing your breaker for something that can handle the higher voltage.)

mshen11

Re: outback FM80 MPPT

how do i tell if the ykyocera 130TM have bypass diodes (by default)?

so for MPPT - the greater difference between the actual voltage and the programmed nominal voltage on the controller, the more the MPPT can squeeze? in this case you are suggesting i do 24V panels (2x2) and still program the outback as 12V nominal voltage?

as a demonstration of my understanding, given the two situations under IDENTICAL conditions (same sun, same location, etc...):

1. 2x2 24V system
2. 4x1 12V system

the outback would generate more current under situation #1?

Cariboocoot

Re: outback FM80 MPPT

mshen11 wrote: »

how do i tell if the ykyocera 130TM have bypass diodes (by default)?

It would be fairly safe to say that there are no commercially produced solar panels these days that do not have bypass diodes.

so for MPPT - the greater difference between the actual voltage and the programmed nominal voltage on the controller, the more the MPPT can squeeze? in this case you are suggesting i do 24V panels (2x2) and still program the outback as 12V nominal voltage?

as a demonstration of my understanding, given the two situations under IDENTICAL conditions (same sun, same location, etc...):

1. 2x2 24V system
2. 4x1 12V system

the outback would generate more current under situation #1?

The 'rule of thumb' for getting the best MPPT effect is that the nominal Array Voltage should be 2 times the nominal System Voltage. A 12 V system charged by a 24 V array, 24 V system charged by a 48 V array, et cetera.

There is a limit, however. Obviously you can't run an array with a Voc above the input limit of the charge controller. The MX60's maximum is 135 V, so 8 "12 V" panels in series would be trouble because the actual Voc of the "12 V" panel is 17-18 Volts: 8 * 17 = 136, whereas 8 * 12 would be only 96. The other issue is, the further from system Voltage the array is, the less efficient the charge controller becomes. So there's a sort of "happy medium" at 2X system Voltage.

But then you never know the fun of having a 72 V array attached to a 12 V system and hearing the controller "wake up" in the middle of the night and try to charge when the moon is full!

Windsun

Re: outback FM80 MPPT

Cariboocoot wrote: »

The 'rule of thumb' for getting the best MPPT effect is that the nominal Array Voltage should be 2 times the nominal System Voltage. A 12 V system charged by a 24 V array, 24 V system charged by a 48 V array, et cetera.

While generally true, there are some exceptions, such as where you have very long wire runs. We have one customer that lives in a canyon, so his panels are mounted over 300 feet away up on the ridge, so it makes more sense for him to run near max voltage and lose a couple percent in controller efficiency.

Cariboocoot

Re: outback FM80 MPPT

Windsun wrote: »

While generally true, there are some exceptions, such as where you have very long wire runs. We have one customer that lives in a canyon, so his panels are mounted over 300 feet away up on the ridge, so it makes more sense for him to run near max voltage and lose a couple percent in controller efficiency.

Indeed! It's one of those "fine tuning" issues. It would perhaps be more accurate to say the array Voltage measured at the charge controller's input, rather than a straight-off-the-spec-sheet Voc calculation.

But you have to start with the basics, then work out the details. Here I was just trying to explain the general principal.

crewzer

Re: outback FM80 MPPT

but in my current seup i DO get *some* MPPT advantage, just not full. right?

Maybe. Your 520 W STC array will typically generate ~400 W or so. The FM80 is ~95.5% efficient at that power level with a 12 V system, so some of any initial MPPT gain is lost as a result.

as a demonstration of my understanding, given the two situations under IDENTICAL conditions (same sun, same location, etc...):

1. 2x2 24V system
2. 4x1 12V system

the outback would generate more current under situation #1?

Again, maybe. Check the attached graph and you’ll see the controller efficiency is higher with a lower input voltage from the array, suggesting situation #2 would be preferable.

However, the array current in situation #2 will be higher, and you’ll therefore probably see higher voltage drop – and therefore power loss – in that configuration compared to #1.

HTH,
Jim / crewzer