MPPT Charge Controller or PWM?

utsvt_billutsvt_bill Registered Users Posts: 6
I am in charge of re-designing the electrical system for the University of Texas Solar Vehicle Team trailer.

I currently have planned out to use 4 125Ah deep cycle batteries in a 24V bank feeding a 1500W inverter. This is going to power a quad-core computer, a monitor, 4 small lightbulbs, and a fan for ventilation.

Conservative calculations tell me that the power draw of these devices will be ~1110W. So my calculations tell me that my design will provide roughly 5 hours of power. I do realize that a battery rated at 125Ah discharged in 5 hours will probably only provide ~100Ah, but my estimated power draw is also slightly exaggerated just to be sure I get it right.

I had found an affordable MPPT that also had 3-stage charge capability, for $120, ( http://s4wsbox.com/?q=node/33 ) but when I ordered it, I was informed he was out of stock and probably not going to be making any more.

So now I have everything designed, ordered the inverter, and am about to build the box for the batteries, but lost my charger.

I've done some searching and put together a list of potential MPPT charge controllers, the most attractive currently looking like the Morningstar or the Steca Solarix, based on cost. We are a student organization, so funds are limited. I also liked the looks of the Rogue MPPT, but it is a little pricey and I'm not sure if the professor would go for it.

So to get down to it...

We are in Central Texas... It is hot. Will I get that much better performance out of a cheap MPPT controller than I will from a $100 PWM charger? Can I buy a PWM charger and then put a MPPT between the charger and the PV array?

Oh, and currently we have an 18V array putting out about 5-6A, but we are expanding this to 24V, and have 30 ft of trailer roof to put arrays in parallel to increase current output.

Thanks in advance for helping out a newb.

Bill

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: MPPT Charge Controller or PWM?

    hi bill and welcome,
    i am unclear on some aspects of your setup. are those 4 12v batteries at 125ah or 4 6v batteries at 125ah and what type are they, fla, agm, or gel? how did you determine those loads as they are high? what and how many pvs do you have now as opposed to what you will be getting, for 18v pvs and 24v pvs do not say enough about them? this specific info makes it easier for us to comment on what you have and propose to have.

    in general an mppt controller may typically recover about 10% of the power that would've been lost in a standard pwm arrangement and does vary per setup. be aware of the controller's output capabilities so that your future pvs will not exceed the capability of the controller.

    do know that you need at least a 5% charge rate from the pvs to charge the batteries. that is a 20hr rate of which solar may have 5-6 full sun hours per day making it many days to reach a full charge. also note that you should not drain the batteries to 100% and in fact should not be drained more than half way (50%) or the batteries will not last long.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: MPPT Charge Controller or PWM?

    In addition to what Niel said,

    A MPPT controller gains advantage with lower battery states of charge (generally) or a bigger spread between PV voltage and battery voltage, AND/OR as PV panel temperatures drop. Low panel temps and low battery voltage CAN see MPPT advantage ~30% best case. Higher temps, fuller batteries and the advantage nears zero. More like 10% on average.

    So to some great extent it depends on how far you are going to routinely draw down the batteries.

    Tony
  • utsvt_billutsvt_bill Registered Users Posts: 6
    Re: MPPT Charge Controller or PWM?

    It is 4 12V flooded lead-acid batteries. I will configure the bank so that there are 2 parallel pairs of series 12V batteries.

    To determine the load, I looked at the power supply of the computer, and used that as a maximum power draw for that, and then found specs on the fan and monitor to determine their power draw, and added the wattage of the CFLs in the trailer.

    The quad-core has a 610W power supply, so I assumed that was the maximum power it would require, the fan will draw ~400W on startup, and around 200W continuous, and the lights and monitor draw 40W(lights) + 34W(monitor) = 74W, so I rounded up to 100W. It is my understanding that the inverter will not produce a constant 1500W, but is capable of producing 1500W continuous, delivering the amount of power necessary. Is this assumption incorrect?

    I am not sure about specifics of the PV array, as I have not quite gotten to that point. My project lead told me to leave those details until the end, and to just design a system that will deliver the necessary power for the necessary amount of time, ~5 hrs. All that I have gotten out of the guy that designed the previous system is that the array produces 18V nominal and 5-6A. If I had to guess, and based on the only time I have looked at the roof of the trailer, I think we currently have three 6V nominal arrays wired in series. Unless I am mistaken, my EE background tells me that it is absolutely essential to expand the array to atleast 24V to charge the batteries at all.

    I was planning (if time permits) to add another 6V nominal panel in series, and then hopefully a few more sets of 4 series 6V panels in parallel.

    I do realize that discharging a battery fully is not the best plan, but this system will not be used very often, maybe for 2-3 races per year.

    Basically, I really just want the charger to be able to increase the time that the system will be capable of producing the required power, and maintain a float charge on the batteries while the trailer is being driven to destinations or sitting un-used. It would be ideal if I could design the system such that I could completely recharge the batteries in one day, or produce almost the necessary power from the PV array alone, but given time constraints, this is not feasible. I have about 4 weeks left to finish this project.

    We also have a generator that produces a 60Hz 120V signal that we can plug into a 24V battery charger, and we can use grid power to charge the batteries if necessary.

    I am open to any suggestions beyond the charge controller as well... PV array suggestions, expanding the battery bank may be possible (but another 24V costs another $150)
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: MPPT Charge Controller or PWM?

    why don't you buy this meter so that you can take the guesswork out of the draws.
    http://store.solar-electric.com/kiacpomome.html
  • utsvt_billutsvt_bill Registered Users Posts: 6
    Re: MPPT Charge Controller or PWM?

    We do have a kilowatt meter, I just haven't had the opportunity to use it yet. My senior design project is due Tuesday, so that is my main focus right now.

    I really was trying to design the system with a little extra capacity, so I'm not super concerned with the exact draws, I figured if I calculated for a power draw greater than the true draw, it would provide extra capacity.

    I think what I need to do, and correct me if I'm wrong, is to expand the PV array, and then a PWM charge controller will be sufficient for now.

    Is my assumption about the inverter correct? Will it deliver the necessary power, up to 1500W, or will it continuously produce 1500W, regardless of the load?
  • mikeomikeo Solar Expert Posts: 386 ✭✭✭
    Re: MPPT Charge Controller or PWM?
    Is my assumption about the inverter correct? Will it deliver the necessary power, up to 1500W, or will it continuously produce 1500W, regardless of the load?
    Your inverter draw will be based on what ever load you have up to 1500 watts plus the extra current consumed by the inverter, see the efficiency chart for the converter that you have if available. Usually an inverter if from 80% to better than 95% depending on load and design.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: MPPT Charge Controller or PWM?

    sorry as i forgot to address that point and 1 other.
    yes, it will basically draw the load you present to it plus the power needed to operate the inverter itself. some types of loads will draw even more power if this is an msw inverter with the big load culprits having inductive loading such as fans.

    you can use a pwm cc and later on replace it with an mppt just as long as the mppt cc you chose meets the specs your setup demands. you don't want an mppt controller that handles less current or voltage than the pwm cc would be handling. an example would be a blue sky sb2000 mppt cc that only handles 12v and will not work to a 24v battery bank or from the higher voltage pvs.
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: MPPT Charge Controller or PWM?

    MPPT or PWM ? Find one with both. Blue Sky has one that's interesting: http://www.infinigi.com/blue-sky-energy-sb3024dil-40a12v-30a24v-mppt-charge-controller-p-2432.html . Might be pricey but efficient.
    Nature's Design & Green Energy on FaceBook : Stop by and "Like" us anytime.. Many up-to-date articles about Renewables every day.
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  • Solar GuppySolar Guppy Solar Expert Posts: 1,973 ✭✭✭
    Re: MPPT Charge Controller or PWM?
    MPPT or PWM ? Find one with both. Blue Sky has one that's interesting: http://www.infinigi.com/blue-sky-energy-sb3024dil-40a12v-30a24v-mppt-charge-controller-p-2432.html . Might be pricey but efficient.

    I would disagree with the efficient statement for the Blue Sky unit. I tested this unit last year and due to the fact its not a synchronous design its the lower efficiency of the pack of mppt units on the market. It also completely fails to track when there is 120hz inverter ripple on the battery bank, this happens when there are AC loads on the battery bank.
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: MPPT Charge Controller or PWM?

    Didn't say it was any good. Said it looks "Interesting"
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  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: MPPT Charge Controller or PWM?

    GPM--You probably are not going to win this one. ;)

    Bill,

    If I had to choose--is this a "One Time" class project--or is it planned to be around for 5-20 years?

    If the system is going to be used for many years, and by many people, I would go with a good MPPT controller (Xantrex XW 60 or possibly the new MorningStar--due out soon). Lots of educational opportunities with the "high tech controller" and you can mount panels remotely when needed (trailer inside at show, under trees, etc.). However, for the most part, the controller will give you reasonable gains in cool weather/remote mounting of panels/arrays >~400 watts. Large MPPT controllers with small arrays typically waste power running the internal electronics (poor efficiencies at lower power levels).

    If cost is important, and/or will not be used for many years/in temperature extremes/smaller arrays--a good PWM controller will probably be a better choice vs purchasing an unknown and/or poor performing MPPT controller.

    -bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • utsvt_billutsvt_bill Registered Users Posts: 6
    Re: MPPT Charge Controller or PWM?

    This will probably be the system they use for a couple years max. The array is quite small, and we will be dealing with very high temperatures. I do plan on expanding the array but I don't know how much I will be able to do in a month... I am trying to graduate and my Power Quality and Harmonics class is trying to interfere with that LOL.

    I saw someone mention MSW above, my inverter is pure sine wave w/ efficiency around 90-95%.

    Can y'all recommend a good PWM controller for ~$100?
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: MPPT Charge Controller or PWM?

    Morningstar: http://store.solar-electric.com/mochco.html

    Tony
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: MPPT Charge Controller or PWM?

    Xantrex "C" series PWM is also a solid oldie-but-goodie.

    Also, if you are going to be using this system among many different groups/people/situations... A hydrometer + thermometer (for flooded cell batteries) and a Battery Monitor is pretty much required for good battery life (or other cumulative Amp*Hour meter--This company sells smaller units for RC work--may be a cheaper alternative/hackable alternative)...

    You really need to keep the battery >50% state of charge most of the time; and >75% state of charge for storage (below 75% SOC, the battery sulfate and begin to lose capacity after a day or so--and will die in months of storage <75% SOC).

    The major factors for early battery death--Over discharging/under charging, overcharging/boiling dry, and not keeping the battery water up with distilled water.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • utsvt_billutsvt_bill Registered Users Posts: 6
    Re: MPPT Charge Controller or PWM?

    Thanks for the recommendations.

    I do realize the harm that completely discharging the batteries can do, but I'm assuming the system will only be used a few times a year, and a float charge will be maintained on the batteries between uses (provided by the pv array). I may propose adding 2 more batteries to the bank to get another 125 or so Ah.

    Hopefully I will have the time to add another 100-200W nominal to the array, that should help minimize the battery discharge.
  • utsvt_billutsvt_bill Registered Users Posts: 6
    Re: MPPT Charge Controller or PWM?

    Oh, and also I'm designing a track to mount the battery box on so that it can slide out from underneath the shelves it will be located under. That way people will have easy access to maintain the batteries.

    So another dumb question... Since I'm wiring pairs of series 12V in parallel, can I just run the inputs to the inverter and the outputs from the charger to the positive and negative terminals of one 24V pair, or should I do positive of one and negative of the other? The positive and negative terminals of each pair should be essentially a node at +24V and a node at 0V, but I have heard of people connecting their charger output to the pos of one battery and the negative of the other when running two 12V in parallel.
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: MPPT Charge Controller or PWM?

    All Loads and Chargers, should connect a the SAME place

    Additionally, check this site for wire connection layouts to properly/equally share currents between batteries : http://www.smartgauge.co.uk/batt_con.html
    Powerfab top of pole PV mount | 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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: MPPT Charge Controller or PWM?

    "or should I do positive of one and negative of the other?"

    correct as each string of batteries shall appear as the individual batteries do in mike's link. in other words the battery symbol shall represent a string of your 2 seriesed batteries.

    as to the reference i made to the blue sky controller it was only to illustrate that it only worked at 12v so a blanket recommendation of going with just any mppt controller would be wrong to say. the mppt controller i had in mind was the sb2000e as you could not use this cc. on the subject of blue sky controllers, one would have to be certain their pv's open circuit voltage ratings will not be equal to or higher than the controller's input voltage rating and in my opinion not to even being within 10% of it.
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