Forum Discussion

BFL13's avatar
BFL13
Explorer II
Jun 22, 2013

Test Method PWM vs MPPT ?

I might get a chance to compare MPPT with my PWM on the set of two 100w panels. I get 12.6 (Isc) now with PWM so that is the "standard"

I was thinking of a quick connect set-up to swap controllers in place, and also to run the two 100s as 12v like now and as a 24v in series.

I gather MPPT does better on lower battery voltage. Does having a loaded voltage on the battery work for that same as if the SOC were lower? If so I could do two voltages, one low one higher on each test. Or?

Is it ok to swap controllers in place? I might have voltage drop that has no effect on the PWM but might bother the MPPT, no idea.

Any suggestions on how to do a quickie test (of course with no clouds that day, and do it mid-day, etc etc.)?

I have no idea how to test for the (bogus?) claim that MPPT will have a greater percentage gain in the shoulder hours. If there is an easy test for that I could do that I suppose (have Trimetric)

Not clear yet whether I will get the chance but hints of it. (borrowing the MPPT) Want to be ready ahead of time so can prepare a test set-up that is fair. (Otherwise, those MPPT guys will moan and whine :) )
  • pianotuna's avatar
    pianotuna
    Nomad III
    Jun 22, 2013
    Hi BFL13,

    Applying a load to the battery bank causes my MPPT to max out.

    What would be best is to have twin battery banks, with twin loads. That way moment to moment differences would not occur.
  • BFL13's avatar
    BFL13
    Explorer II
    Jun 22, 2013
    I am not worried very much about moment to moment as long as there is blue sky, no clouds. July has some of those :) Get steady amps with PWM, have swivel, can keep aiming at sun and keep getting 12.6a PWM for quite a while. So swapping out delay in time should not cause a change in test conditions as feared.

    Don't have two identical banks to test with, I would just compare amps to the battery bank as shown on Trimetric when going from PWM to MPPT. Same with the two MPPT tests at 12 and 24.
  • hmknightnc's avatar
    hmknightnc
    Explorer
    Jun 23, 2013
    Keep it simple to test the theory. My suggestion and assumes starting with a battery bank discharged to approximately 60% SOC so that amps aren't limited by absortion any more than necessary

    Start with PWM and put a load on the battery approximately equal to the expected solar amps (so that Solar is putting out maximum available between load and charging). Take your data for 30 minutes or so

    Turn off load and turn off/disconnect solar.

    Swap controllers onto same battery bank and same panels. Turn load back on. Now take same measures as before. In theory if the MPPT is good one you should see maybe 10-15% increase in amps supplied by solar to battery.

    Be interesting to see your results
  • BFL13's avatar
    BFL13
    Explorer II
    Jun 23, 2013
    Why run each one for 30 min? I thought the idea was to get a quick reading of one and then the other in amps before "conditions" changed.

    If we are to test for accumulated AH in a given time, such as to test the maybe bogus claims for MPPT in the shoulder hours, then that might be different,

    If I get hold of this thing, I want to do it to advantage in the time allowed so comparisons would be valid. Those MPPT guys have so many excuses!

    I expect right off they will say that the MPPT brand/model I borrowed is not typical. :)
  • pianotuna's avatar
    pianotuna
    Nomad III
    Jun 23, 2013
    Hi BFL13,

    So long as it is not one of the ones with a "pot" to be set and forgotten the comparison will be of interest.
  • HiTech's avatar
    HiTech
    Explorer
    Jun 23, 2013
    I would run both against the same battery bank under load simultaneously, if you have a way to measure the amp contribution of each comtroller. That is what I plan to do running my manual tracking MPPT setup against my PWM setup

    You will want enough load and/or low enough state of charge for the voltage under charge to be 12.xx volts I would think.

    Jim
  • BFL13's avatar
    BFL13
    Explorer II
    Jun 23, 2013
    So is it confirmed that loaded voltage is the same thing as unloaded but low SOC voltage as far as the MPPT controller is concerned?
  • HiTech's avatar
    HiTech
    Explorer
    Jun 23, 2013
    As long as the controller is in bulk, then I believe the voltage differential (when actually under charge) is all it needs. If you want to give MPPT max benefit of the doubt, pull the voltage down to or near 10.5v with a large inverter load and see how it does.

    Jim
  • BFL13's avatar
    BFL13
    Explorer II
    Jun 23, 2013
    I would keep loaded voltage over 11 if you don't mind, since the racket of the alarm from 11 down to 10.5 would be "too much."

    Please clarify regarding testing at 12v vs 13.6v (say) for how much if any MPPT does better. I lose a tiny bit of amps into the 13s on PWM due to the IV curve for a 12v panel
  • HiTech's avatar
    HiTech
    Explorer
    Jun 23, 2013
    11 will still work, but the lower the voltage that the MPPT controller is driving, the higher its advantage will be. That's the big benefit of MPPT. It counts on PWM losing those watts that it does not as voltage drops. It only makes extra power where the voltage of the load is well below the real, this minute MPPT voltage of the panel, at temperature. At higer voltage MPPT drops its advantage over PWM even though PWM looses a little on AMPs. MPPT drops a lot more on watts up high than PWM. PWM is actually gaining a lot of watts at higher battery voltage, even though it is losing a bit in terms of amps. So the gap between technologies closes.

    Running it against a single panel is also best case for MPPT. Multiple or especially mixed multiple panels cannot yield the exact max power from each panel simultaneously (due to variations even in the same model of PV), so the controller has to pick a compromise max for the system.

    Jim

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