Changing charge controllers, should I rewire panels?

cruiser guycruiser guy Solar Expert Posts: 87 ✭✭✭✭
My current charge controller at my house is currently undersized. It is a 25 amp PWM charge controller going into a 24v battery bank with 790 watts of panel. I'm totally off-grid in Sierra Leone, West Africa.

I'm considering a Morningstar Tristar 45 MPPT with the remote display.

The current panels are all 12v nominal panels, 4 are 135 watt and 2 are 125 watt. Currently they are wired in groups of two in series, (both 125 watt panels form one group and the others are of course all 135 watt panels) and all groups in parallel. When/if I go with the TriStar MPPT should I rewire to three in series, 2 @ 135 watt and one 125 watt panel in each string) to increase the voltage and let the controller do what it does to charge everything or leave it as it is or is there some other combination? The problem is the 125 watt panels have the older connectors and the 135 watt panels have the latching connectors. Also, the panels are slightly different in ratings.
Naturally I want the biggest bang for the buck.

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    we had a similar question not too long ago. you can do it either way as there will be a tad of loss either way with dissimilar pvs. what you could do is wire the 135w pvs all in series through the new controller and put the 125w pvs in series through the old controller. mismatch problem solved.:D
  • PhotowhitPhotowhit Solar Expert Posts: 5,365 ✭✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    If you were to put them all on the new controller, either do strings of 2 - no loss or put 3 - 135 watt on one string and the 2 - 125 watt and one 135 watt on the other, when in series the string voltage's add and they work at the amperage of the lowest panel. This assumes that the VMP voltages are all pretty close as when you run the strings in parralell the amperage adds and they work at the VMP of the lowest panel or string of panels.

    Clear as Mud?
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • cruiser guycruiser guy Solar Expert Posts: 87 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?
    niel wrote: »
    ...what you could do is wire the 135w pvs all in series through the new controller and put the 125w pvs in series through the old controller. mismatch problem solved.:D

    No problems with the same battery bank being fed by two different charge controllers? I like this idea.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    i see no problem.:D
  • cruiser guycruiser guy Solar Expert Posts: 87 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    Just curious. As the battery voltage gets closer to a full charge, or goes into an equalization charge, I assume the PWM controller may not be able to "keep up" with the voltage increase the way the MPPT controller would. I assume the PWM will only ever have the nominal 24v from it's two panels in series while an MPPT controller with 4 panels in series would have a nominal 48 volts that it then modulates down to the voltage needed to charge the batteries.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    under conditions that would cause the pwm cc to not output enough it will be the same for the mppt in that the pv performance is not up to snuff due to clouds or whatever. you are thinking the 48v string during under performing times that the voltage may drop to say 44v that it will still work. with currents that low from the pvs it would essentially fizzle out too.
  • cruiser guycruiser guy Solar Expert Posts: 87 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    We're going to run the old panels on the original PWM controller and switch the 4 newer panels to the new controller. Now to decide what that new controller should be!

    Where we are here in Sierra Leone we have only the dry season and the rainy season. Temperature does not vary much except obviously a panel getting rained on is going to be cooler than a dry one in full sun. The lowest recorded temperature in the capital city of Freetown is 19 Celsius so cold weather performance is not a concern.

    Here I can get the Morningstar TS45 PWM controller in country for about $350 including the meter, much more than the same thing stateside but it's here, no worries about theft in transit or potential importing hassles (solar is NOT duty free here).

    How much of a benefit will I get from an MPPT controller vs. the PWM controller? The time that we struggle is obviously the rainy season as we can have a period of several days to a week of inclement weather and low PV output. Is an MPPT controller going to give me a noticeable improvement over a PWM controller in that situation or am I in trouble either way? I'm trying to decide if I get the TriStar TS45 MPPT and have it brought or sent over or go with what is locally available which is the TriStar TS45 PWM.

    I could probably get the MPPT brought over in someones luggage but there are no guarantees and if luggage goes missing it's gone.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    low lighting conditions are just that and that directly affects the output of the pvs before it even gets to a controller.
  • cruiser guycruiser guy Solar Expert Posts: 87 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    So then since the MPPT optimizes amperage then in conditions when the PV output is poor like in cloudy overcast conditions I may not get much of a boost, especially since the ambient temperature is still up there? In the summer when my panels are out in the hot sun their output goes down and again not much of a boost from the MPPT controller.

    When the PV output goes down is it the volts that drop or the amps or both? Does it follow a somewhat linear scale? It appears so from the graphs that one sees everywhere. If both volts and amps drop and volts go below what the battery voltage is can the MPPT controller convert the voltage up so charging continues, even at the then lower amperage?

    The way I see it, the MPPT is most useful and advantageous in conditions where it is cold and clear. That's a condition that we NEVER see here in Sierra Leone.

    Am I on the right track?

    I'm trying to sort out in my mind if the premium for an MPPT controller is worth it in my circumstances especially given the greater difficulty in getting one here.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?
    The way I see it, the MPPT is most useful and advantageous in conditions where it is cold and clear.

    Yes, it is most useful in cold & clear conditions. But it is still useful in other conditions.

    Suppose you have a 180 watt panel (Imp=10, Vmp=18 ). It is hot out and your Vmp is reduced from 18 volts to 16 volts.

    When you start charging a half discharged battery its voltage is 12 volts. With a PWM you are putting 120 watts (12 volts x 10 amps) into it. With a MPPT controller you would be putting 160 watts (10 amps x 16 volts) into it.

    Later in the day the battery voltage is up to 14 volts. The PWM is now able to put 140 watts into the battery and the MPPT is still pushing 160 watts into the battery.

    Note: these examples assume no efficiency loss in the controller.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • PhotowhitPhotowhit Solar Expert Posts: 5,365 ✭✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?
    I'm trying to sort out in my mind if the premium for an MPPT controller is worth it in my circumstances especially given the greater difficulty in getting one here.

    There are many variables in deciding if a MPPT controller will be 'cost effective' for you;

    Some people are using numbers as low as 400 watts of array before MPPT charge controllers are 'cost effective' but it all comes down to 2 questions, The cost difference and how valuable it will be to you vs the value of adding panels. I argue, in hot temperatures, with balanced systems, it is more valuable to add panels until you have a much larger system.

    How much are solar panels in a 12 or 24V range costing you (delivered) vs how much the Charge controller costs for you delivered?

    How you use your array/system?
    In a balanced system, in hot temperatures/full sun, on an MPPT charger you will get a boost(recovery of the current above that needed to charge the battery) during the bulk charging, if you don't use more than 20% of your battery capacity, you don't Bulk charge fore very long, once you reach the absorb state your charge controller reduces the amount of current going to the battery. While you might get some added benefit, by the time you are charging the last 10% you are likely receiving no benefit from the MPPT type charge controller.

    So on cloudy days when your in bulk longer, and temperatures are not as hot at the panel, your benefit is greater. Weather it is greater than adding panels depends on the panels prices vs the charge controller prices. On sunny days when your panels are hot and if you've used little, your benefit is minimal or non existent.

    Part of the reason, I believe, MPPT charge controllers are not as useful as many make out, is that off grid solar systems must be over built to provide for the health of the batteries. So, much of the time you don't need the extra energy your array is producing. In the top 10% of charging your system cuts back and likely, there is no added benefit.

    Much depends on the availability of reasonably priced paanels designed for nominal battery voltages (12 or 24V, VMP of about 17.5 and 35V respectivly) Here, in the United States, grid tied systems are popular and larger than most battery charging systems so Grid Tie panels which use the best effective layout of wattage are cheaper. With a couple of exceptions, they cost 1 1/2 that of grid tie panels as these sources disappear, MPPT will be needed to get reasonable output from the grid tie, type panels. Likely battery based systems are more prevalent where you are, so the cost difference might be minimal.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • BB.BB. Super Moderators, Administrators Posts: 31,068 admin
    Re: Changing charge controllers, should I rewire panels?

    Another issue related to panels to worry about...

    Physical size of panels... The largest 12 volt (Vmp~17.5 volt) panels these days seem to be around ~125-140 watt units. These are also about the maximum physical size that can be delivered using non-truck methods (in US and Canada). Higher wattage panels can get very large--and cost of shipping small numbers of panels can go up too.

    Then there is the issue of ~175 watt panels are about the largest that can be handled by a single person. Larger panels will need two people to move around safely (this is assuming standard crystalline cells--Thin film panels are about 2x the surface area for same wattage ratings).

    In the end, try a paper design for both MPPT and PWM type controllers (wire lengths & gauge/costs) and your higher temperatures (hot panels, lower Vmp, may not fully charge/equalize batteries on very hot days--I.e., Vmp-rated=17.5 volts or lower, voltage drop through wiring, controller drop, may reduce charging voltage too much with PWM controller).

    I don't make a big deal about amount of "extra" energy MPPT controller will generate (usually in the 10-15% or so in sub freezing weather). The big deal for MPPT is the ability (with higher end controllers) of running Vmp~100 VDC (or higher) and reducing your wiring costs for the solar array back to the controller/battery shed.

    Also, higher end MPPT controllers frequency have more data/communications options and production logs--if useful for you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    i'm going to add to what photowhit said in that if you intend on adding pvs in the future then why get a pwm now and a mppt later as you can avoid the duplication? don't get me wrong as you may still want to go with the pwm 1st if initial costs prevent you from getting an mppt or if it's your intention to have a backup cc for your large array. yes, controller companies can fix a cc under warranty, but that won't give you power from your pvs to your batteries in the mean time so that is food for thought.

    do understand that although under poor conditions both types will have far less to work with as i pointed out, but an mppt does optimize what it is given. on the other hand, a pwm will give a lower operating loss than most mppt controllers so one pwm may be drawing 35ma or 50ma to operate while the mppt may be .5a or even 1a so in my opinion the pwm wins under low light conditions simply due to lower operating requirements that facilitate an earlier charge to the batteries. during that early turn on the output will be small though and you lose the extra kick you will see when fully illuminated with mppt. although the average extra kick from mppt is about 10%, the extra kick is stronger when the battery soc is smaller.

    there are good points/bad points to both under varying conditions and it may seem like i'm not helping, but i am giving you info to make your decisions with.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    I live in a cold place compared to you, -2 C* right now, so I really like MPPT during the winter for the gain it can harvest at that time.

    One benefit I see from MPPT use, is the ability to expand your array without changing any wiring, using multiple PVs per string and realizing smaller voltage losses in transmission from PV to CC. The higher the voltage the smaller the percentage lost, => more panels per string less % loss.

    However as has been mentioned, but not stressed much, is that MPPTs allow you to run PVs in series strings and thus you can still get a charge in lower light conditions where a PWM can not perform as it may not be getting more than the battery needs.. But a series string WILL as the PVs would be supplying at least 2 times, or more depending on the number of panels in the string... all be it at the Amperage of 1 panel, then the MPPT CC will convert any excess Volts to amps...

    I vote for an MPPT particularly if you are going to add more panels.

    hth
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • cruiser guycruiser guy Solar Expert Posts: 87 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    OK, here is the cost on the Kyocera 135 watt panels, 3,500,000 SLL each which sounds like a lot and it is at $800/ea. in Sierra Leone. The Morningstar TS45 PWM controller with meter is 1,500,000 ea or about $350/ea. An MPPT controller will need to come from the states either by post, courier or a visitors luggage. No chance of the luggage option till January since elections will be rocking the country next month and then comes Christmas.

    As you can see, the component costs are significantly more over here than stateside.

    As it is now, I have 4 of the Kyocera 135 watt panels and 2 Carmanah 125 watt panels for a total array of 790 watts. I am running a 24v battery bank using 6 - 12v 200aH truck batteries (deep cycle batteries are similarly priced to the panels and charge controllers while truck batteries are a little more reasonable (six truck batteries were significantly less than a pair of 200Ah deep cycle gel batteries). Wiring is 4mm from each pair of panels and run about 30-40 ft. from the roof to a terminal strip where all groups are then paralleled to the controller and battery bank, both of which are within 5 ft. of the terminal strip.

    The array is far oversized for my 25 amp PWM charge controller.

    I don't really see a need to add to the array unless it will significantly improve my inclement weather performance. In the dry season I am working in the top 20% of the batteries. We have few high draw items. The fridge is the biggest energy user at 90 watts on the nameplate. The fridge is run on a timer and runs from 10AM to about 4PM in the rainy season and I find it can run 24hrs/day in the dry season. All lights are 2 watt LED in fixtures from 1 to 4 lamps. Outside on the veranda we have a 10 watt LED "fluorescent" replacement and a 9 watt LED which is rarely used. We run two inverters, a 600 watt 110vAC 60Hz sine wave inverter for our American appliances and a 1400 watt 230vAC 50Hz sine wave inverter for the lights and local appliances. Both inverters are left on 24/7. The 230v inverter will act as a 13 amp charger if it is fed with an external 230v source like a generator. This is what we did this past rainy season when the batteries started to be drawn down too far. Occasionally we didn't catch it and the inverter dropped out on low battery:blush:

    I do have a cheap watt meter that I could hook up for a few days or a week to see the actual loads in the house if that helps. Unfortunately the watt meter is only for the 230v side of things and the fridge is on the 110v inverter.

    There is an outside chance of getting panels from the states as there is another building which may be getting some solar power so if additional panels are warranted then let me know too.

    So based on this information what is the consensus? PWM or MPPT for the charge controller to take some of the load off my poor undersized PWM controller?
    westbranch wrote: »
    I live in a cold place compared to you, -2 C* right now, so I really like MPPT during the winter for the gain it can harvest at that time.

    However as has been mentioned, but not stressed much, is that MPPTs allow you to run PVs in series strings and thus you can still get a charge in lower light conditions where a PWM can not perform as it may not be getting more than the battery needs.. But a series string WILL as the PVs would be supplying at least 2 times, or more depending on the number of panels in the string... all be it at the Amperage of 1 panel, then the MPPT CC will convert any excess Volts to amps...

    I vote for an MPPT particularly if you are going to add more panels.

    hth

    This may be the reason why I want an MPPT controller.

    I know too well the cold temperatures. I'm also from B.C., born in Vancouver and lived there till 2004 then to Guatemala and back to B.C. to the Kootenays for a year or so and now Africa! Currently in shorts and no t shirt 'cause it's too hot!
  • cruiser guycruiser guy Solar Expert Posts: 87 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?

    Just how "smart" are the MPPT charge controllers?

    I understand they will take the higher voltage from a string of panels and factor it down to the voltage needed to charge the batteries, upping the amps in the process. What I have not found out is if, in inclement weather when the voltage from the panels is less than needed to charge the batteries (battery voltage exceeds array voltage), will the charge controller boost the voltage at the expense of the amps and still be able to put something into the batteries?

    If it all works strictly on watts=volts*amps then both the following scenarios should work:

    We'll assume a series string of 100 watt nominal 12v panels and a 24v battery bank. Forgive me if my voltage and amp assumptions are off in the following, it's the principle I'm trying to understand.

    full sun we assume the series string of nominal 12v 100 watt panels makes 17*4=68 volts and about 6 amps. The MPPT takes that and for a 24v battery bank converts it to approximately 28volts and 14 amps charging things up.

    in an inclement weather day lets assume our solar array is making only 25 volts and we'll assume 3 amps and our battery bank is at rest around 25.4 volts but we know our battery needs 28+ volts to charge so nothing flows into the battery.

    Does the MPPT controller take the 25 volts and 3 amps produced in inclement weather and convert it to 28 volts and 2.6 amps still giving us a charge (albeit a small one) or is the MPPT algorithms or charge controller electronics not capable of that sort of conversion where voltages are boosted and amps reduced?
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Changing charge controllers, should I rewire panels?
    in an inclement weather day lets assume our solar array is making only 25 volts and we'll assume 3 amps and our battery bank is at rest around 25.4 volts but we know our battery needs 28+ volts to charge so nothing flows into the battery.

    Does the MPPT controller take the 25 volts and 3 amps produced in inclement weather and convert it to 28 volts and 2.6 amps still giving us a charge (albeit a small one) or is the MPPT algorithms or charge controller electronics not capable of that sort of conversion where voltages are boosted and amps reduced?

    The problem is with your assumptions. In almost any light at all the panels will put out their full voltage, but with very little current.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Changing charge controllers, should I rewire panels?
    Just how "smart" are the MPPT charge controllers?

    I understand they will take the higher voltage from a string of panels and factor it down to the voltage needed to charge the batteries, upping the amps in the process. What I have not found out is if, in inclement weather when the voltage from the panels is less than needed to charge the batteries (battery voltage exceeds array voltage), will the charge controller boost the voltage at the expense of the amps and still be able to put something into the batteries?

    No. An MPPT controller can not make up for having less than charge Voltage available from the panels.

    What you can expect for maximum charging current is approximately: array Wattage * efficiency (typically 77%) / system Voltage = current potential

    When conditions are cloudy you will not see any great advantage in an MPPT controller either, as the panels are a current source; low light = low current. When the power is not there, it's not there. The charging advantage in MPPT is being able to use any Voltage available above the battery Voltage to increase current. On a small, low Voltage system there is not much advantage (as in 17.5 Volt panels on a 12 Volt system where there is very little Voltage difference between panel output after losses and battery Voltage).

    A panel will try to put out its Imp 'first' and allow the Voltage to go to whatever level. So a panel with Imp of 7 may put that out at 17.5 Volts (Vmp) or at 1 Volt.
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