computing amps on charge controler

solarvic
solarvic Solar Expert Posts: 1,071 ✭✭✭✭
I know how to figure the c charging rates now from reading some of the other post. I need some lessons on how to figure the amount of amps you put into a mmpt charge controller at higher voltage to arrive at amps charge to battery. I will have a 24 volt battery system and probably will be feeding the charge controler 2 strings of 3 stc 30.4 voc. coldest voc 109 v. @7.45 amp. How do I convert the feedin info to controler to calculate the battery amps at a c-10 of 40 amps? I have 2 unused mx-60 charge controlers I will probably use as I already have a trace 500 setup with 125 amp breakers. First I was thinking of getting a new higher midnight controler but would have to buy all new higher voltage equipment. the ps 500 I have has never been used and I don,t want to invest any more money in newer equipment when I think the mx 60 chargers will work. The limit on them for the panels I have is 2 strings of 3. The string chart that outback says I can use 2 strings of 4 down to -12 f but I don,t think the string chart is correct. :Dsolarvic:D

Comments

  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: computing amps on charge controler

    Look the max power point of the panel string.

    Easy example: single string.
    10 panels: 20V 10A at the MPT for each.
    200V @ 10A = 2000W
    If you have two strings double the power if they are identical.
    The efficiency of the controller/wiring losses: 95% delivered.

    2000W*.95=1900W to the batteries.
    Current from power with known voltage. (24)
    http://www.sengpielaudio.com/calculator-ohm.htm
    79.16A to the battery.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: computing amps on charge controler

    The output of the MX60 (or any similar MPPT charge controller) is limited to 60 Amps @ system charging Voltage; in this case approximately 28.4. If you multiply that to Watts you get 1704. That's the basic.

    For more accurate results you have to include compensation for the controller efficiency (about 92%), wire loss, averaged panel output (varies with temperature), and so forth.

    So if you want 40 Amps maximum output you convert that in to Watts: 1136
    Then you can use a "typical" derating factor of 77% over-all to calculate the array size: 1475 Watts

    Keep in mind that the venerable MX60 works best at about 75% capacity (45 Amps output - so you're good there) and a nominal array Voltage no more than 2X the nominal system Voltage - in this case that would be a 48 Volt array.

    You really only have to worry about cold temperatures causing the Voc of the array to exceed the controller's input maximum. In this case that is 150 Volts, so four panels in series with a Voc of 30.4 each would no doubt be safe down to -12F. The string calculators provided by the various manufacturers are quite accurate; you can trust them. :D
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Re: computing amps on charge controler

    I still don,t know or understand the answer to my question. What I ask was how much power the mx-60 needs put into it at 107 voc and 2 strings of 3 panels, vmp of 7.45 for 1 string. I realize I would have 14.9 vmp at 2 strings but would like to know how to figure what battery charging amps that would translate to about 45 amp charging for a 24 volt battery system. I have to run a higher voltage than what cariibookoot mark told me as my wire run from solar will be between 150 and 200 ft. So in mx chart I will probably be using the chart that converts the 96 volt down to the 24 volt.
    Vic :Dsolarvic:D
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: computing amps on charge controler

    think more on the output of the mx60. if going by nec for a 24v battery bank this is 48a x 24v = 1152w. normally that's the answer, but there are losses in cc operation and the wire resistances and you might bump it to account for that, but i don't think the nec allows you to do that. also, there's a mod you can do for the mx60 to allow it to be a 70a cc and 80% of that is allowed by nec rules making it .8 x 70 = 56a. at 24v this is 24 x 56 = 1344w. so what this means is it doesn't matter about the current input as it is about the current output and the wattage involved that this represents. if the nec holds it to xx watts output then it is held to that on input.

    if non-inspected go straight to the mx60's full current output as the gauge for wattage output. that will be the input wattage as well. the input and output wattages are looked upon as the same because the downconverting from the mx60 can be compared to that of transforming ac power and the nec does not account for losses in the transformation. of course you still need to watch other parameters like overvoltage due to temp or just too many pvs seriesed.

    does this help?
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Re: computing amps on charge controler

    Niel I understand about what the output of the mx is. What I don,t understand is how much voltage and amps you put in to the mx to figure what the output is at a lower voltage. I am just trying to figure how this is figured so I can do this on my own. I have 8 schuco 180 watt panels and 2 mitsubushi 175 watt panels. They look identical and I read on here that mitsubushi made these shuco panels. I know they have the identical specs and look like twins. The models of these panels are out of production now but I still can get some more of the schuco 180 watt panels if I don,t procrastinate too long. I only wanted 4 more of the schuco,s but the seller only sells them in quality of 8. I might get 8 more and sell the ones I don,t need. Why I pm you once. Outbacks string chart says you can run 2 strings of 4 down to -12 F which seems kind of high voltage to me. the record cold temp for here is -22 which I havn,t seen for 20 years. Actually hasn,t been more than -3F here for a few years. I have a weather station that records the coldest and hotest temp till you change batterys. Really like that. Hottest I saw here since I had it is about 98 F. At -22 the voc is 141 0 F is 139voc. For every 5 degree warmer the VOC lowers 1 degree. So at 80F the voc is 121. Owners manual spec. listed at limit 120voc imput to the mx. Manual seems to dispute what thier chart says. I can list the spec. of the schuco panels. probably will use the 2 mitsubushi 175 watt panels elsewhere. ISC 8.03A 30.4V IMP 7.45A VMP 24.2V VTOC 0.0928 VTMP 0.07744 reccomended maximum array wattage 1600 taken from MX string chart and verified with Schuco spec for model S 180 SPU-4. I wanted to know how much voltage and amps these panels would down convert from either 2 strings of 3 or 2 strings of 4 si I can decide what kind size of batterys or configuration to buy. Someone else can do it for me but I want to learn how to do it myself. Since I am not going too big as I am grid tied I just want to do this for power outages and still make some use of my gaswell as I don,t use what I am alloted and I will never get paid for what I don,t use. Thinking that probably 8 of these panels will be all I need either to get 1 set of 4 l-16 or 2 sets of smaller batterys to run my SW 4024.Then just run the gen if there is a long extended power outage or I want to let the gti system build up credits so I never have to pay the electric co any more than thier standard charge. Before I hear any speechs about the paybacks I already have most things I need except batterys. I figured I might as well spend my money as my son inlaw is in the higher income bracket where he makes 3 or 4 times more than I ever made. :Dsolarvic:D
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: computing amps on charge controler

    Convert it to Watts, as I said before.
    If you're looking to get 40 Amps @ 28.4 Volts out, that's 1136 Watts.
    So if you have an array capable of 1136 Watts after efficiency losses (about 1475 Watts) you'll get the desired output. The efficiency difference is averaged panel output + normal wire losses + controller efficiency rating. Of these you can check the wire losses for your particular install, because long lines lead to greater losses. It's best to keep them under 3%.

    But remember that neither the array output nor the charge rate is fixed. The array's output will vary with the amount of sun hitting it and how hot it gets. The charge rate will vary according to the needs of the battery bank. You will never see a steady state of in/out power.

    As far as limiting the input to the charge controller, any "over Wattage" will simply not be harvested as the charge controller's maximum output is 60 Amps @ whatever charge Voltage. The only thing to watch out for is having a Voltage Open Circuit (Voc) that exceeds the maximum input Voltage of the controller (150 Volts in this case).

    The eight Shuco panels @ 180 Watts each = 1440 Watts. That's about the limit for a desired 40 Amp output. Two strings of four each would work fine. The Imp of this array would be 14.9 and the Vmp would be 96.8. The Voc would be 121.6, going down as the panels get hotter. There is a slim possibility that the Voc may hit peak input if you should get a spell of unseasonably cold Winter (like -30).
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: computing amps on charge controler

    let me ask you firstly if you are doing this per nec/inspection standards and secondly if the mx60s you have are modified for 70a operation?
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Re: computing amps on charge controler

    Ok Mark, Think I understand it more. Now I have another question about the battery charge rate. To keep the charge rate to c-10 for a 24 volt 400 amp battery string that would need 40 amp. So if at times I get more than 40 amp would the battery accept it? I don,t expect that to happen much as I live below lake erie which causes more cloudy weather than lots of other areas. So if I was using some of the power then getting over 40 amps wouldn,t overcharge the battery. I am thinking of Crown battery l-16 395 amp or us battery at 400 amp l-16, 1 string of 4. I also checked with deca batterys and the were stock out of gc-15 6v @ 230 amp 20 hr rate so offered me a bigger battery GC25 for the same price as gc-15. GC-25 has 235 a @ 20 hr rate. 2 strings of 235 would give me 470 amp battery bank. Would I be better having that than 1 string of l-16. Do you think the smaller batterys would last as long as the l-16 batterys? I am mostly going to use this as a backup for my GTI system and plan on maybe dedicating it to pumping my water and fridge circuit so I do use it wisely to keep everything exercised and it just don,t set around like a lump. I have a grundfos sq pump that uses 1100 watt to start and 1000 w to run. !10 v. Thanks for your help mark and niel. :DSolarvic:D
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Re: computing amps on charge controler
    niel wrote: »
    let me ask you firstly if you are doing this per nec/inspection standards and secondly if the mx60s you have are modified for 70a operation?
    My mx 60,s are not modified. ! is first version of serial no. and other is 2nd version of serial no. I am not going to get it inspected. Looks like the stock amps will be ok. I am just going with that to start and see how well it works. I am going to use one with the solar and other to downconvert my 48 vdc generator to 24 vdc for charging batterys. I had a post about the generator about a year ago and there were it was interesting and different folks seemed to think this will work. I might ask about the generator on midnight forum to get some of those peoples ideas. :Dsolarvic:D
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: computing amps on charge controler

    Solarvic;

    If the system is designed for a 40 Amp potential peak load there's not a very big chance it will exceed this. Most of the time the batteries will require less, and they are probably safe with 13% input on occasion (the max recommended). As you know, the actual charge rate is what comes out of the controller less any loads being supplied. If you're really concerned about it, the Outback's can actually be programmed to limit the max current output to less than the 60 Amp full rating.

    As for choosing one battery over another, the small "golf cart" type are usually least expensive per Amp hour and can last up to eight or even ten years with care. Without care you don't drop so much money when they fail. :p The L16's have a need for higher current charging - the 10% rate is a must or else you'll be equalizing more often. They need the extra jolt to keep them stirred up; tall case stratification.

    General one string of batteries is easiest to cope with; fewer connections and less chance of imbalance of current across all batteries. However two strings in parallel is viable too. So you could go with 450 Amp hours of T105's for example (4 in series for 24 Volts * 2 strings). Whatever fits your budget best!

    No reason I can think of that an MX60 wouldn't handle a 48VDC gen output to charge a 24 VDC system, so long as the gen's output is "clean" (fully rectified - you might want to add some capacitors or even a small amount of battery to filter out any AC signal that might be present - depends on the design of the gen).
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: computing amps on charge controler

    ok so we'll figure the max pv wattage to be 60a x 24v = 1440w. this is input and output, but we arrive at it via the output specs of the controller. the input is just reconfigured by the mppt action and although there is a bit of loss input to output we will go by the same wattage. this works until the parameters of the cc can be exceeded.

    the voltage possibilities you list are possibly damaging to the mx60 under extreme conditions and, although you haven't had those conditions recently, you still have to look at that as a real possibility in the near future. i know what your area is like and it's always about 6 degrees colder there than here on average and hitting minus temps in my area happen nearly every winter even though the number of days and how far below 0 being rarer the farther you go. go by your min temp in your calculations and i recommend you not use that many of those pvs in series seeing as how the worst temp will hurt the cc.

    now i believe you are trying to size the array to arrive at a good charging current. use the total wattage to arrive at a rough charge current. for example a 1000w array you will use the typical max charge voltage for the battery and would represent a minimum charge current under full sun. i don't remember what trojan specifies off hand, but lets use 14.5v for this. now you are using it in a 24v battery system making that x2 or 29v. that 1000w divided by 29v gives a current. 1000w/29v = 34.483a. this means that when the battery voltage is lower it could put forth more current to the batteries like 12.5v per battery for a total of 25v. 1000w/25v = 40a. if the battery bank was drained even further then the current delivered to the batteries will be higher yet providing the max current spec on the cc was not exceeded, which is much higher for the mx60. as you can see the current of the pv or the voltage of the pv does not play as big of a roll as you thought when dealing with mppt as the input wattage is considered more as long as the max voltages or currents are not exceeded to the cc input. so for example 4 pvs in series if they exceed the max input voltage spec then rearrange the same pvs so the the voltage does not go over the max specs of the cc. now a 2 strings of 3 in series arrangement does work to prevent overvoltages to the cc and will still be within the wattage specs the cc can deliver as 180w x 6 = 1080w. remember at 24v it can be as high as 60a x 24v = 1440w so 1080w is within spec. now do you have the ability for buying that number pvs (6 for 2 x 3 and you can do 3 strings of 2 as well, but with much lower voltage for vdrop losses translating to a higher %) or will you need to buy different pvs to meet you goals it's hard to say. the slightly lower voltage may mean a bit larger wire to use in your run though.

    editted due to error in my calculations, but is fixed i think.
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Re: computing amps on charge controler

    Because of the length of wire I will need I probably will go with the highest voltage I can get away with. I probably could buy a new classic for the higher voc but I think these outbacks should do the job so why spend more. If the temp gets way below zero I could just shut the charge controler off. I have a 500 ft roll of #2 aluminum urd that I probably will use. I have read up on some of the drawbacks but mine is new enough that I think it should be safe. It is listed for using in gasoline and good for 90 degree in conduit. Since I think I have enough I might bury 2 sets of it in case I decide to add something more later. This won,t get done for awhile as this is a project for the summer. Now my yard is so soft that you sink to your ankles walking in it. Wish the rain would stop for awhile so Texas can have some. :Dsolarvic:D