What happens to excess power when batteries are full?

Will an MPPT charge controller supply power to the battery based inverter when the batteries are fully charge and there is plenty sun.

Comments

  • littleharbor2littleharbor2 Solar Expert Posts: 1,132 ✭✭✭✭
    Yes, provided there is useful sun shining on the array the controller will allow whatever amperage through to the inverter by way of the batteries as is available. If your array is capable of producing enough power to meet the inverter's load requirements then the batteries remain in a fully charged state. If the inverter is drawing more amperage than the panels are producing the difference will come from the batteries.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • mcgivormcgivor Solar Expert Posts: 2,499 ✭✭✭✭✭
    edited May 2017 #3
    Welcome to the forum @geoffreykaila
    As the battery becomes fully charged, the current will taper down to a value which overcomes the small losses in the float condition and the inverter standby load if turned on. If a load is introduced to the inverter the array will divert current to the inverter, as long as the load current dose not exceed the array output, the battery will remain in its fully charged state. If however the load is greater than the array output, the battery will make up the difference thereby discharging in the amount of the deficit the array cannot support. Using the otherwise wasted power is what is referred to as opportunity loads, such as using a washing machine when the battery is fully charged and there is still sun available. Care must be taken to ensure the loads don't exceed the arrays ability by too great a margin, each system is different so getting to know what your system can supply is of paramount importance, watching the battery voltage during such loads can give an indication, if the voltage begins to drop, then the battery is being discharged. If you have a clamp on ammeter the current from the array and the load current can be measured to ensure they are close. Passing clouds will sometimes draw current from the battery but if the arrays output is sufficiently sized it will replenish what was taken out, one has to study the system over time to learn it's  limitations and maintain a ballance.  
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • Raj174Raj174 Solar Expert Posts: 650 ✭✭✭✭
    The simple answer is if you don't find a way to use it you lose it. The potential power production is wasted.
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
  • jonrjonr Solar Expert Posts: 1,187 ✭✭✭✭
    edited May 2017 #5
    With a MPPT charge controller, array current and load current should not be compared (because the voltage isn't the same).  But as long as battery terminal voltage is at or above float voltage, the array/charge controller is supplying the entire inverter load.  Any energy not used by batteries or loads ends up as heat in the panels.
  • mcgivormcgivor Solar Expert Posts: 2,499 ✭✭✭✭✭
    jonr said:
    With a MPPT charge controller, array current and load current should not be compared.  But as long as battery terminal voltage is at or above float voltage, the array/charge controller is supplying the entire inverter load.  Anything not used by batteries or loads ends up as heat in the panels.

    " Anything not used by batteries or loads ends up as heat in the panels."

    What is not used is not used, there is no heat ending up in the panels, I'm sorry your statement confuses me, the potential energy wasted can be used to power a load, I do it all the time with a MPPT controller, when in float, opportunity loads are used, @jonr could you elaborate on this statement?
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • jonrjonr Solar Expert Posts: 1,187 ✭✭✭✭
    edited May 2017 #7
    The energy from the sun that hits the panel always goes somewhere.   If it isn't turned into electricity and removed by the electrical wire, where else would it go?    But we agree that a MPPT controller can (and often does) provide energy to a load (instead of to a battery).
  • PhotowhitPhotowhit Solar Expert Posts: 4,781 ✭✭✭✭
    Will an MPPT charge controller supply power to the battery based inverter when the batteries are fully charge and there is plenty sun.

    IN effect a solar panel doesn't have to produce power, unlike wind and spinning hydro (hydro can sometime be diverted). Once the batteries are full, the charge controller cuts back the amount of energy produced and allows just enough energy to hold the battery at a fully charged level called 'float', usually around 13.4-13.6 volts per '12 volt' nominal Flooded Lead Acid battery (FLA) battery.

    The math is that the 12 volt battery has an actual voltage of about 12.6 volts and requires a voltage of 15 - 20% above it's voltage to effectively charge. When there is a load the charge controller will increase the current from the array to maintain this voltage level. If required for large loads the battery bank will supply some of the energy and you will see the voltage drop. You can setup some charge controllers to compensate for the time the voltage drops below the 'float' level.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • HorseflyHorsefly Registered Users Posts: 321 ✭✭✭✭
    jonr said:
    The energy from the sun that hits the panel always goes somewhere.   If it isn't turned into electricity and removed by the electrical wire, where else would it go?    But we agree that a MPPT controller can (and typically does) provide energy to a load (instead of to a battery).
    Two things here:

    First, if no current is passing through the panels (i.e., the charge controller isn't consuming any of the power to charge batteries), the panels only have a Potential. That is what the open circuit voltage Voc is. There is no current, so electrically, there is nothing that is converted to heat.

    Second, sunlight hitting any surface does convert to a varying degree into thermal energy, and moreso with darker objects (like a solar panel). But this is independent of electrical potential that is generated by the photons knocking loose electrons in the cells.

    All in all, the amount of heat generated when the sun hits a solar panel is not affected much by whether or not current is flowing through the panels.

    At least, that's how I have digested the physics.   :)

    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
  • bill von novakbill von novak Solar Expert Posts: 822 ✭✭✭✭
    Horsefly said:

    First, if no current is passing through the panels (i.e., the charge controller isn't consuming any of the power to charge batteries), the panels only have a Potential. That is what the open circuit voltage Voc is. There is no current, so electrically, there is nothing that is converted to heat.

    Second, sunlight hitting any surface does convert to a varying degree into thermal energy, and moreso with darker objects (like a solar panel). But this is independent of electrical potential that is generated by the photons knocking loose electrons in the cells.

    All in all, the amount of heat generated when the sun hits a solar panel is not affected much by whether or not current is flowing through the panels.

    At least, that's how I have digested the physics.   :)

    There are two possibilities.

    One is that there is no change in albedo (amount of reflection) when the load is changed.  In that case, the panel always absorbs the same amount of solar radiation.  When you draw power from the panel, some of that solar radiation is converted to useful energy and dissipated somewhere else.  When you do not draw power from the panel, that power still must be dissipated somewhere; thermodynamics and all that.  Thus it is dissipated in the panel.

    Two is that the panel (effectively) gets lighter when it is lightly loaded.  (Most panels do NOT do this.)  In this case, it is possible for the panel to reflect more light when it is lightly loaded, thus absorbing less solar energy.  In that case, the panel need not dissipate more heat when unloaded.

    Let's take an extreme case - a 50% efficient 1 meter square solar panel.  The panel absorbs 90% of the energy falling on it.  Thus, an unloaded panel absorbs (and dissipates) 900 watts.  If you put a load on it, you are now using 450 of those watts for something useful, and dissipating those 450 watts somewhere else.  At that point the panel cannot be dissipating 900 watts; that would mean that the system (panel+useful load) was dissipating 1350 watts, or 350 watts more energy than it received.  That, again, is impossible thermodynamically.
  • jonrjonr Solar Expert Posts: 1,187 ✭✭✭✭
    edited May 2017 #11
    I'm with Bill on option 1 - but if someone sees their panels getting brighter (visible spectrum, not IR) at no load, let me know.    

    You don't need current flow through wires to dissipate energy.
  • Raj174Raj174 Solar Expert Posts: 650 ✭✭✭✭
    This is from a MPPT wiki -
    When the batteries in an off-grid system are fully charged and PV production exceeds local loads, an MPPT can no longer operate the panel at its maximum power point as the excess power has no load to absorb it. The MPPT must then shift the PV panel operating point away from the peak power point until production exactly matches demand.
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
  • BB.BB. Super Moderators, Administrators Posts: 28,314 admin
    Sorry, missed panel is not labeled.

    So you know the wattage of the old panel?

    - Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    Will an MPPT charge controller supply power to the battery based inverter when the batteries are fully charge and there is plenty sun.
    Yes.
  • SchnipperSchnipper Registered Users Posts: 1
    jonr said
    But as long as battery terminal voltage is at or above float voltage, the array/charge controller is supplying the entire inverter load.

    So if I have a large enough solar array can I just have an MPPT controller and inverter and run loads directly without batteries at all?
  • BB.BB. Super Moderators, Administrators Posts: 28,314 admin
    In practice no... It is the battery bank that sets the system voltage range (i.e., from 10.5 to ~15.0 volts for a 12 volt battery bank).

    If the voltage drops below ~12.7 volts, the battery supplies current to keep the voltage in range. If it is above ~12.7 volts, the battery absorbs the extra current instead.

    Most MPPT charge controllers are "relatively" slow (cannot respond instantly to changing loads). To respond quickly to changing loads, you need a "buffer"--Either a bank of capacitors or a battery bank.

    And, in practice, most MPPT controllers get the electricity to power their onboard computer+electronics from the Vbattery connection. And many will set their battery bank voltage based on battery bus voltage (i.e., 9.5 volts to ~18 volts = 12 volt battery bank; 18 volts to 36 volts will be a 24 volt battery, etc.).

    What some folks have done is to connect their solar array directly to the input of a standard off grid AC inverter. And if there is enough sun (and not to large of starting surge for the AC loads), they can run the AC inverter without a battery bank (there are some other issues that do need to be addressed--Not a plug and play solution usually).

    And you could get a buck mode DC switch power supply which could take the Varray voltage and down convert to the Vbatt voltage needed to run an AC inverter (it can work in theory--I have not seen anybody try it). Again, some other issues with the AC inverter need to be addressed too (for example, many AC inverter "lock out" if the battery voltage fall below ~10.5 volts--And you need to cut/restart DC voltage to reset the inverter so it will run again).

    There are also Grid Tied AC inverters. These will take a bunch of solar panels and connect directly to your AC mains and reduce your electric bill. And one mfg (at least), SMA, has a version that will output 120 VAC to a power outlet if the AC mains fail (GT inverters shut down for safety if the AC utility fails). Kind of nice (has limitations).

    In general, placing 120 VAC loads directly on solar panels is kind of not great... Most AC loads do not like being cycled on/off during the day (clouds overhead, sun sets, birds fly overhead, heavy motor surge current shuts down inverter, etc.). And those loads that can be AC Cycled--Many times the users are not happy (TV, Radio, going off when watching).

    There is one class of loads that does work very well with solar panels and a form of AC inverter--Those are three phase water pumps and VFDs (variable frequency drives--Basically a DC to variable frequency 3 phase AC inverter). The VFD has "soft start" (works well with solar panels and battery systems), and will vary the AC frequency depending on available solar energy (i.e., start the pump running at slow speed in the morning, run pump at full speed middle of the day, and ramp down again to slow speed in the afternoon).

    Becoming a popular method to pump water and replace fossil fuel motor driven pumps.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stmoloudstmoloud Registered Users Posts: 72 ✭✭
    This thread is about MPPT & fair enough that's what the OP asked. Just curious if the above applies to PWM.
    Two  hours of good sun and my batteries are at float (14.6 v).
    So are the amps thereafter coming directly from the CC, or are they routed via the battery bank? 
    I take Bill's point about AC not being comfortable with direct connection, but I guess it could be possible to connect your DC directly via the CC in a dire get-you-through situation, if, for instance, your batteries were stolen?

    760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter. 
  • BB.BB. Super Moderators, Administrators Posts: 28,314 admin
    PWM controllers are "even worse" (than MPPT controllers) at regulating voltage...

    PWM is Pulse Width Modulation... Think of a switch that turns on every 1 second, and stays "on" from near zero seconds (0.001) to 0.99 of second. It is a "square wave" that simply connects the Solar array output directly to the battery bank (or DC loads in your example).

    If the controller decides that more current is needed, the switch simply stays on closer to 0.99th of a second. Turns off, then turns on again for another part of a cycle. The pulse width is adjusted by the controller's CPU to hold the average voltage.

    And this is where the battery comes in into use... It is averaging the input current/voltage and the CPU is adjusting the pulse width to keep the battery bus voltage at an average of 13.6 to 14.8 volts (or whatever the float and absorb setpoints are, and what the controller thinks the battery needs--float or absorb charging).

    Obviously, is there is no storage element (Battery) then the loads will see zero voltages and Vpanel (something like 15 to 22 volt) voltage as a square wave. Basically, your loads are being switched on and off constantly for PWM.

    Now... In some cases, the cycle is slow (something on the order second or so +/- for Morning Star in "electrically quiet mode"), or faster in the hundreds to thousands of cycles per seconds.

    If you wanted to run without a battery, you would need a relatively large set of Capacitors to "average" the voltage (other things used include inductors and other components too better average/store/filter the output voltage).

    In general the answer is no--There is no easy/good method to run your off grid system without a battery bank.

    Is it possible to do this... Folks have done it--But it takes some more knowledge and experimentation to make something like this to work.

    My suggested starting point would be to put a DC to DC step down power supply/converter between the solar array and your DC loads.

    My guess is that the solar array will need to be >> than your steady state/surge loads and it will only support those larger loads in the middle of the day.

    Start with something "cheap" like this and see what happens?

    https://www.amazon.com/DZS-Elec-Adjustable-Synchronous-Rectification/dp/B07L8BTZW5

    I don't know. Chances are that you could overvoltage your loads--So I would pick "cheap loads" (or 12 volt automotive lights of some sort as loads) to start.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stmoloudstmoloud Registered Users Posts: 72 ✭✭
    Ok Bill thanks it was just a what-if question.
    As always, your expertise and thorough answers are very much appreciated.   
    760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter. 
  • BB.BB. Super Moderators, Administrators Posts: 28,314 admin
    My pleasure.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 4,781 ✭✭✭✭
    stmoloud said:
    This thread is about MPPT & fair enough that's what the OP asked. Just curious if the above applies to PWM.
    Two  hours of good sun and my batteries are at float (14.6 v). 
    14.6 is NOT a normal float voltage. usually much lower around 13.5
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • NANOcontrolNANOcontrol Registered Users Posts: 96 ✭✭
    I heat water at my small camp system.  The add on controller looks at the panel voltage and when it exceeds the power point voltage the excess power goes to a water heater. Whether that be as little as 5W or 500W. Typically I harvest more than 2800W a day. Typical PV systems really do waste a lot of potential power.
  • softdownsoftdown Solar Expert Posts: 2,006 ✭✭✭✭
    edited January 18 #23
    A little baby photon named Sonny was born one fine summer day. Its family was extremely warm and soon it was filled with tales of high adventure traveling the near by solar system. It quickly learned that Earth was a veritable paradise with opportunities ranging from swimming in vast oceanic expanses to helping Himalayan mountaineers work on their sun tans. The pinnacle of existence was to provide power for the little earthlings and their existence over 90,000,000 miles away. Sonny was informed that if his navigational skills were truly extraordinary, he could provide power for the earthlings and be remembered kindly for generations.

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    Sonny watched in horror as most of his greatly diminished gang simply bounced off the EMF and were lost into oblivion. Sonny was a true survivor and dauntlessly forged ahead. His informal education led him to believe that the ripest targets were in Europe or N. America. Sonny chose N. America with California as his ultimate destination. All looked very good as Sonny sped towards the vast state filled with solar panels yearning for the likes of Sonny to help fulfill their mission in life. That mission being to provide energy for the earthling and his partnership of solar panels.

    Alas, Sonny's courageous life would come to an abrupt and unfulfilling finish. Sonny had arrived too late. It was 2:25 PST and the solar cells were no longer able to feed their earthling with the protons they harvested. Sonny slammed into a panel and was shed like water off a ducks back. Not the smallest quotient of energy was used. In despair, Sonny simply vanished into thin air. Never to be seen again. Never to be thought of again. Not even a memorial service. It is my sincere hope, that in writing this eulogy for Sonny, we can at least remember the rigors and ultimate failure of his doomed life. Let us strive to help future Sonny's fulfill their ambitions. Let us learn to eventually realize proper utilization of Sonny's progeny. Save a proton!
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
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