backchannel - mppt [dis]advantage
…in which I take issue with tangential comments in St. Sternwake’s posts*.
From this post:
The loud touting of MPPT increased harvest, over PWM controllers elsewhere on the net when 12v nominal rated panels are employed, can be considered grossly overexaggerated by those who do not really understand what is going on.
Even though I am not a touter, loud or otherwise, I’ll comment on this. Full disclosure: my own charge controllers are mppt, pwm, and shunt. There is a place for each, IMO.
How this for a reasonable claim: “MPPT will have substantial increased harvest over PWM in Bulk. It will have a noticeable increase during Float when loads are applied. Results during Absorption are mixed.”
For our comparison we will use a normal setup:
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a PWM and an MPPT controller at the same setpoints one might see in a t105bank. Vabs = 14.8v, Vfloat = 13.2v/13.4v/13.8v depending on preference. Other voltages may be considered if time permits.
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95% efficiency assumed for MPPT, 100% for PWM
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200w of Renogy mono/poly panels, Vmp 18.9 and 17.8 respectively.
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75F ambient temperatures
We will look at this in order of increasing voltage, and therefore decreasing MPPT advantage.
Parallel panels - Bulk - MPPT clear advantage
In bulk stage from the controller’s standpoint there is no difference if there is a load; the controller is maxxed out, shoving as much current as possible into the battery.
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Scenario: morning after 50% DoD cycle, bank discharged to 12.2v. MPPT makes 26% more power on mono, and 19% more on poly.
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Scenario: morning after 20% DoD cycle, bank discharged to 12.5v. MPPT makes 23% more power on mono, and 16% more on poly.
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Scenario: morning after emergency discharge to 80% DoD, bank drained to 11.5v (!). MPPT makes 33% more power on mono, and 26% more on poly. No, we shouldn’t do this to our batteries. Yes MPPT will help them recover faster.
Parallel panels - Float - MPPT some advantage with loads
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Low Vfloat, as when in storage, 13.2v. MPPT has 16% more power for loads on mono, and 10% on poly.
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Medium Vfloat, as when in storage, 13.4v. MPPT has 15% more power for loads on mono, and 8% on poly. <– less than 10% advantage on poly
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High Vfloat, as when deep cycing each night, 13.8v. MPPT has 11% more power for loads on mono, and 5% on poly.
Parallel panels - Absorption - minimal advantage, possible _dis_advantage
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Low Vabs, as with some AGM, 14.2v. MPPT has 8% more power for loads on mono, and 2% on poly.
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Medium Vabs, 14.4v. MPPT has 7% more power for loads on mono, and 0% (!) on poly.
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HighVabs, as with T105, 14.8v. MPPT has 4% more power for loads on mono, and -2% (!!) on poly.
Hitting the Vabs wall
The combination of high temps and low Vmp panels is a bad combination, particularly for MPPT with their 5% hit on efficiency. In order to hold 14.8v at the battery a PWM only has to have ~14.8v coming in, but MPPT will require 15.54v to overcome the inefficiency (assuming it doesn’t have the smarts to skip the DC-DC conversion - Tracer models seem to turn tracking/conversion on when power output hits ~10W).
This means the MPPT will hit the wall (unable to reach high Vabs == 14.8v) at about 68F (!) with the poly panel and about 86F with mono. PWM will chug up to 85F for poly and 100F for mono.
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68F ambient: MPPT + poly hits the 14.8v wall
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70F ambient: MPPT + poly hits the 14.6v wall
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78F ambient: MPPT + poly hits the 14.4v wall < – human comfort limit
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82F ambient: MPPT + poly hits the 14.2v wall
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85F ambient: PWM + poly hits the 14.8v wall
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86F ambient: MPPT + mono hits the 14.8v wall
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88F ambient: PWM + poly hits the 14.6v wall
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90F ambient: MPPT + mono hits the 14.6v wall
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92F ambient: PWM + poly hits the 14.4v wall
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93F ambient: PWM + poly hits the 14.2v wall
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95F ambient: MPPT + mono hits the 14.4v wall
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100F ambient: PWM + mono hits the 14.8v wall
- 100F ambient: MPPT + mono hits the 14.2v wall
Will hitting the wall kill your battery? No, but it’s less-than-optimal. At 100F MPPT + poly would only be able to hold 13.15v, which is barely Vfloat. On mono it would be 13.96v. Of course, with MPPT you could run the panels in serial to double panel voltage, and make “the wall” a non-issue at temps other than lava.
Conclusions
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In Bulk stage MPPT has a clear advantage, particularly with mono panels.
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In Float stage (no load) both types perform the same. The system is idling.
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In Float stage MPPT will provide ~10% more power for loads. More with mono, less with poly.
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At ambient temps comfortable to humans ( <= 78F) , all controller/panel combinations can hold a midrange Vabs == 14.4v.
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At ambient temperatures > 68F, MPPT + poly will not be able to hold aggressive Vabs == 14.8v. With mono panels it can hold 14.8v up into the mid-80s. So don’t run MPPT controllers with single or paralleled poly panels.
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In temps in the 80s and higher, holding a high Vabs == 14.8v would require:
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a pwm controller; or
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mppt with mono panels; or
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mppt with panels connected serially
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If going to 24v Nominal residential panels then MPPT is necessary.
Not necessary in the normal sense. It won’t damage anything**, it just won’t generate much power.