What happens to excess power when batteries are full?

geoffreykailageoffreykaila Registered Users Posts: 1
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: 635 ✭✭✭✭
    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 Registered Users Posts: 1,190 ✭✭✭✭
    edited May 2 #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 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • Raj174Raj174 Solar Expert Posts: 405 ✭✭✭✭
    The simple answer is if you don't find a way to use it you lose it. The potential power production is wasted.
    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
  • jonrjonr Solar Expert Posts: 844 ✭✭✭✭
    edited May 4 #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 Registered Users Posts: 1,190 ✭✭✭✭
    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 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • jonrjonr Solar Expert Posts: 844 ✭✭✭✭
    edited May 4 #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: 4,195 ✭✭✭✭
    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, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • HorseflyHorsefly Registered Users Posts: 250 ✭✭✭
    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: 694 ✭✭✭✭
    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: 844 ✭✭✭✭
    edited May 4 #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: 405 ✭✭✭✭
    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.
    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
  • BB.BB. Super Moderators Posts: 26,758 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.
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