Mppt vs pwm real world testing

mountainmanmountainman Posts: 124Registered Users ✭✭
I've been wondering about the benefits of Mppt over pwm. So I did a side by side test. I put 2 100 watt panels in series 38 volts 50' 1.40% volt drop on 10 awg on a 30 amp mppt cc. Then on a pwm  30 amp cc. 2 100 watt panels in parallel 65' 19 volts 4.57% volt drop on 8 awg.  On a cloudy  88f day starting at 10 am both systems charging the same roughly 65% soc 12 volt bank. Connected to rc watt meters. After 2.5 hrs pwm 210 whrs 15 amp hrs. mppt  203 whrs 14.52 amp hrs.  Conclusion on a small system and in summer pwm wins.
Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.

Comments

  • BB.BB. Posts: 27,785Super Moderators admin
    Yea... The are about the same "efficiency" for a properly designed MPPT and PWM systems. That is because the Vmp~35-38 volt array on a warm day is just about the "perfect voltage" for charging a 24 volt lead acid battery bank in nominal/warm conditions.

    Also, many MPPT controllers really don't do MPPT "tracking" if the input voltage is too low... More or less, a minimum of 1.3x Vbatt-charging voltage is needed:
    • 29 volt battery charging * 1.3x = ~38 volts
    And remember that warm/hot solar panels, Vmp-std of 38 volts does drop and can be much close to 35-31 volts (depending on actual temperature of cells).

    With very low Varray voltage, a MPPT controller does not do much different than a PWM controller. A nice working Vmp-array voltage is ~2x battery charging voltage. If your MPPT array was Vmp~72 volts (roughly), then you know the MPPT controller is functioning as designed and "Tracking" Vmp/Imp of the array.

    Also note that for off grid DC power systems... The loads+battery state of charge control the actual harvested energy--Once the battery bank is full and no loads, there is no more harvest.

    And when FLA batteries are over ~80-90% state of charge, the controllers go into "absorb" mode and are supplying only the charging current need by the battery bank and MPPT/PWM does not make any difference (i.e., both controllers are outputting less than maximum current that Bulk Mode charging is capable of).

    So unless you controlled for loads and battery state of charge--Your test may actually not reflect the actual panel+controller harvest capabilities. Day to day weather/temperature/etc. condition variations can easily ad 10% or more error/change in harvest between otherwise "identical" weather days.

    There are just a bunch of variable test conditions that have to be controlled for to get useful results here.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mountainmanmountainman Posts: 124Registered Users ✭✭
    Bill this was on a 12 volt battery with no loads. So the mppt was 2x battery charging voltage. The mppt was consistently showing higher watts amps etc. Then it would drop to 100 watts for 20 seconds or so and then back to 140. At the same time pwm was showing constant 138. Even during clouds mppt would show slightly more. But over all pwm even with more voltage drop from cc to panels had the advantage. I'm gonna try it again on a clear cool day just to see the out come.
    Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.
  • NANOcontrolNANOcontrol Posts: 76Registered Users ✭✭
    I think this is a very hard thing for the general public to measure and your results do not mean a lot. A battery is an indeterminate load and the pulsed nature of the power is hard to measure. In this situation, I wouldn't see much gain because the difference in voltage is so small. I think of MPPT as solar for dummies. You hook things up with no understanding and the controller figures it out.   Someday, they might be called panel killers. But, going to something like grid tie panels, smaller wire and long runs has distinct advantages. I would never run MPPT in my systen, I run panels at power point voltages (MPPC) which gives you better than 90% of the advantages of MPPT.  This allows me to set priorities as to where the power goes so nearly 100% of the panels capabilities at the moment are used.  My charging system is based on cheap $5 buck converters and because I choose to store energy elsewhere I only need a very small battery. It's solar upside down.
  • mcgivormcgivor Posts: 2,164Solar Expert ✭✭✭✭✭
    Bill this was on a 12 volt battery with no loads. So the mppt was 2x battery charging voltage. The mppt was consistently showing higher watts amps etc. Then it would drop to 100 watts for 20 seconds or so and then back to 140. At the same time pwm was showing constant 138. Even during clouds mppt would show slightly more. But over all pwm even with more voltage drop from cc to panels had the advantage. I'm gonna try it again on a clear cool day just to see the out come.


    In order to achieve more accurate results, if both controllers were connected to the same battery with a resistive load greater than both arrays combined, applied to a common output this  would divert all production of both controllers to the load, the only purpose of the battery would be to power the controllers, this may be more accurate to compare outputs. Truly I appreciate you experimenting, just trying to provide information to obtain a more accurate result.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • myocardiamyocardia Posts: 118Solar Expert ✭✭✭
    I think this is a very hard thing for the general public to measure and your results do not mean a lot. A battery is an indeterminate load and the pulsed nature of the power is hard to measure. In this situation, I wouldn't see much gain because the difference in voltage is so small. I think of MPPT as solar for dummies. You hook things up with no understanding and the controller figures it out.   Someday, they might be called panel killers. But, going to something like grid tie panels, smaller wire and long runs has distinct advantages. I would never run MPPT in my systen, I run panels at power point voltages (MPPC) which gives you better than 90% of the advantages of MPPT.  This allows me to set priorities as to where the power goes so nearly 100% of the panels capabilities at the moment are used.  My charging system is based on cheap $5 buck converters and because I choose to store energy elsewhere I only need a very small battery. It's solar upside down.
    So, where exactly are you storing your power?
    DoD= depth of discharge= amount removed from that battery   SoC= state of charge= amount remaining in that battery
    So, 0% DoD= 100% SoC, 25% DoD= 75% SoC, 50% DoD= 50% SoC, 75% DoD= 25% SoC, 100% DoD= 0% SoC
    A/C= air conditioning AC= alternating current (what comes from the outlets in your home) DC= direct current (what batteries & solar panels use)
  • jonrjonr Posts: 1,066Solar Expert ✭✭✭✭
    > Then it would drop to 100 watts for 20 seconds

    Sounds like a poorly designed/configured MPPT controller.   The only conclusion I would draw from the test is that PWM sometimes slightly beats MPPT.
  • mountainmanmountainman Posts: 124Registered Users ✭✭
    I don't doubt that turning on loads of more amps than the panels can produce would make different results. But who does that. My test was about which mppt or pwm would charge my battery the best. I unhooked my array on tue afternoon. So Tuesday nite. wednesday and wednesday nite. My usage was 900 watt hrs or roughly 65% soc on my battery. I ran the test for 2.5 hrs during bulk with , both cc on 2 hrs absorb both cc were still showing 11 amps when i stopped the test. My thoughts are my mppt cc gets lost tracking ( 100 watts for 20 secs) a better one would give better results for mppt.
    Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.
  • NANOcontrolNANOcontrol Posts: 76Registered Users ✭✭
    At the heart of every MPPT system is a buck step down converter.  There are various designs that have different conversion efficiencies from 70 to 95%.  Much depends on the nominal voltage of the battery system and what the MPPT cost. Indeed there can be little difference between PWM with a 12V system with low array voltage.  Your results are worth noting, but not a surprise.  You would be surprised at how much power the average system wastes beyond what it can store.  As I said before, I store cold and hot water.
Sign In or Register to comment.