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PWM vs MPPT charge controller for new old-parts installation

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  • PWM vs MPPT charge controller for new old-parts installation

    I have the opportunity to install a PV system using parts that are about 20 years old but were never installed. I want some opinions on the charge controller, specifically whether to use the old PWM one I have or buy a new MPPT.

    The system is nominal 48v and has a nicad battery of 40 cells. The float charge voltage is 56v to 58v. Despite their age, they seem to work well when charged and discharged with the Trace SW5548 inverter.

    The panels are BP590UL with rated peak power of 90w, rated voltage of 18.5v, rated current of 4.86v, open-circuit voltage of 22.3v and short circuit current of 5.2a.
    The charge controller is a Trace C40 which has a max input voltage of 125v DC.

    My question is whether to use the Trace controller and, if so, what would be the best array voltage. The C40 controllers are PWM so my understanding is that I would use 32 panels in 8 parallel groups of 4 in series to get 74v and 38.88a of power to the controller.

    My concern is the energy loss of using a PWM controller vs a MPPT controller. My understanding is that a PWM controller drops the voltage and keeps the current, so if the battery was charging at say 58v, it would get 38a at 58v or about 2.2kW of energy whereas about 2.8kW is available at the panel. If I used a MPPT it would take that power and convert it to 48a at 58v.
    What would the real-world gain be between a PWM and MPPT? I live in the Caribbean with an ambient temperature of 25c to 33c, few clouds and approximately 10hrs to 13hrs of daily sunlight depending on the season. The biggest MPPT advantages seem to be low temperatures or overcast skies which wouldn’t apply to my location.

    The other advantage of MPPT, higher volts for longer distances, doesn’t matter as the panels will be installed on the roof right above the inverter and controller.
    My other concern is that having a lower-voltage array for the C40 will hinder future upgrades as most new controllers seem to be MPPT. I’m only hoping to get a few years out of this old inverter and battery until lifepo4 batteries are cheaper but I want to use the panels for a long time.

    I don’t have the budget to replace a lot of parts but if using a new MPPT controller would give me significant gains I would buy one instead of using the C40.

    Thanks for your input.

  • #2
    Your panels are a pretty close match to the controller. A MPPT controller $$$ might get you a couple more amps at the initial stages of charging, but I don't think it would be dramatic enough to make that investment yet. The Trace has a setting specific for NiCad batteries too. So, I'd run what you have, and see how it goes. And keep us updated
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A



    • #3
      Thanks for this. I'll stick with the C40.