hi. i'm amy beaudet from the alte store. thankyou for watching the 4th of our video series on designing an off-grid solar system. thisvideo will explain how to size the charge controller for your off-grid solar system.we'll discuss the 2 major types of charge controllers, and figure out how to selectthe right size for your system. our previous videos helped us determine how much powerwe needed to make, what size battery bank
Solar Panel Output Calculator, was needed to store it, and what solar panelswere needed to generate that power. now we'll figure out what charge controller is neededto manage that power. we're getting closer to completing the system, we've figured outthe house loads, done the batteries and panels, now let's do the charge controller. we'vegot another video available that goes into
great detail of the difference between a pwmand mppt charge controller, so i'm not going to repeat much of that. the short versionis, charge controllers are available with 2 different technologies, pwm and mppt. howthey perform in a system are very different from each other. an mppt charge controlleris more expensive than a pwm, and it is often worth it to pay the extra money. pwm chargecontrollers operate by making a connection directly from the solar array to the batterybank. during bulk charging, when there is a continuous connection from the array tothe battery bank, the array output voltage is "pulled down" to the battery voltage. asthe battery charges, the voltage of the battery rises, so the voltage output of the solarpanel rises as well, using more of the solar
power as it charges. as a result, you needto make sure you match the nominal voltage of the solar array with the voltage of thebattery bank. a 12v panel can charge a 12v battery. a 24v solar array is needed for a24v battery bank, and 48v array is needed for 48v bank. mppt charge controllers measuresthe vmp voltage of the panel, and down-converts the pv voltage to the battery voltage. becausepower in = power out, when the voltage is dropped to match the battery bank, the currentis raised, so you are using more of the available power from the panel. so you can use a highervoltage solar array than battery, like the nominal 20v grid-tied panels that are morereadily available. with a 20v panel, you can charge a 12v battery bank, or 2 in seriescan charge a 24v, and 3 in series can charge
a 48v. this opens up a whole wide range ofpanels than now can be used for your off-grid system. to determine what the specs are fora solar panel, you can look at its datasheet or the label on the back of the panel. whensizing a charge controller, the 2 numbers you are most interested in are open circuitvoltage, or voc, and short circuit current, or isc. these are the volts and amps the paneloutputs when it is not connected to anything pulling it down. you keep hearing me talkabout nominal voltage. nominal voltage is a way to categorize battery based solar equipment.because a higher voltage is required to charge a battery, nominal voltages are used to helpsee what equipment goes with what. so a nominal 12v panel, which actually has a voc voltageof around, 22v , plus or minus a volt or 2,
and a vmp of around 17v. and if you countthe number of cells, or silicon squares on the front, it will likely have 36 cells. likewise,a panel that was designed to charge a 24v battery bank will have a voc of around 44vand a vmp of around 36v. counting the cells will come up with 72, twice as many as a 12vpanel. if you wire 2 24v panels in series, or 4 12v panels in series, you can chargea 48v battery bank. this was all well and good for battery based systems, but then alongcame grid-tied systems, and 12, 24, and 48v became meaningless. so the industry sort ofstandardized on 60 cell, 20v nominal panels. alone, they are too big to charge a 12v battery,and too small to charge a 24v battery. an mppt charge controller solved that, by reducingthe voltage down to the required range, and
in doing so, increasing the current output,so you are not losing power. so let's start with sizing a charge controller with matchingpanels and batteries. this can be done with a pwm charge controller. our first exampleis going to be one string of panels. we check the label or datasheet and confirm with thevoc of 22.1v that it is a nominal 12v panel, and the isc is 8.68a . we then multiply theisc by the number of parallel strings, 1, and multiply it by nec's safety factor of1.25, to get 10.85a minimum charge controller amperage requirement. great, i'll round upto a nice 15a morningstar prostar 15m with a meter. now let's try it with 4 parallelstrings of the same 140w panel. notice i'm not talking about how many panels are in eachstring, because i'm using a pwm charge controller,
i know that i'm using the right number tomatch the voltage of my battery bank, so i've got 1 for a 12v, 2 in series for a 24v, and4 in series for a 48v battery bank. so here we go with 4 parallel strings. 4 strings xshort circuit current x 1.25 nec = 43.4a, equals 43.5a, so i'll round up to a nice morningstartristar 45 charge controller. now let's get into the mppt charge controller. if you areusing the same nominal voltage panels as battery bank, the math is the exact same as with thepwm charge controller. an mppt charge controller can manage the same or higher nominal voltagepanels as batteries. there are some "boost" charge controllers that can take a lower voltagepanel, and boost it up to the correct voltage to charge a higher voltage battery bank. theseare generally designed to take a single panel
to charge a 36v or 48v golf cart. however,most mppt charge controllers need the nominal voltage to be equal to or higher than thebattery bank. because watts equals volts x amps, and the watts are constant, if the voltagedrops, the current goes up. for example, if you have a 200w panel with a voltage of 40vand a current of 5a, if the voltage drops in half to 20v, the current doubles to 10a.200w remains the same. when the voltage of the array is higher than the battery, thecharge controller will reduce the output voltage to charge the batteries, and increase thecurrent output. a lot of mppt charge controllers can handle a higher input voltage that nominal48v. this allows you to use several solar panels in series to increase the voltage inthe line going from the panels outside to
the charge controller and batteries inside.when you see the max voltage rating of a charge controller, that is the highest voc voltageof the string of panels, plus, a temperature compensation for the coldest temperature thepanels will be exposed to during sunlight hours. so if it ever gets to -20 degrees fahrenheitduring a cold winter morning, like it does here in new england, you need to multiplythe voc of the panel by 1.21 to determine the highest voltage the panels may potentiallyput into the charge controller. so to figure out how many panels you can do in series fora charge controller that is rated for 150v, you multiply the voc of the panel by the numberof panels in series, by the temperature correction factor. if we try putting 4 nominal 20v panelswith a voc of 39.4v in series, when we multiply
x 4, we are already over the 150v limit, evenwithout the temperature correction. so if we drop down to 3, times 39.4v x 1.21 temperaturecorrection, we are just under at 143v. within the acceptable voltage range. so let's lookat what happens when we put a string of 3 240w panels in series and send it into a 12vbattery bank through our mppt charge controller. we have 240w panels times 3 of them givesus 720w. next, we take the 720w divided by a 12v battery bank equals 60a out of the chargecontroller. ok, so i know i need a charge controller that's rated to handle at least60a. outback makes a fabulous flexmax 60, i can see if that will work for me. the nextstep is to confirm with the specs of the charge controller you want to use that it can handlethe full wattage of the array at that battery
voltage i see in its specs that for a 12vsystem, it can handle up to 750w. 720w is less than 750w, so we are good to go! so,let's use the off-grid example we've been using throughout this video series to figureout what equipment we need. looking back through our list of steps, we have everything we needto calculate the charge controller needed from our previous videos. we had decided todo a 48v battery bank, and need an array at least 1168w. first let's do a pwm charge controller.we know we need at least 1168w of solar. we also know we need to have the nominal voltageof the array equal the 48v battery bank. so we will use 2 24v panels in series to equal48v. if we use the solarworld 315w panels, 1168 divided by 315w = 3.7, which means weneed 4 of the panels. so we'll have 2 parallel
strings of 2 in series. to figure out whatsize charge controller we need, we take the 2 strings times 9.19a short circuit currenttime safety factor of 1.25 equals 22.9a. so we can use a 25a or 30a charge controller,depending on which model we like, for instance the bluesky energy sun charger. now let'sdo an mppt charge controller. we know we need at least 1168w of solar. we also know we needto have the nominal voltage of the array equal to or greater than the 48v battery bank. so,if we wanted to use nominal 20v panels, traditionally used in grid-tied, we would have to use 3in series to get at least 48v nominal. if we use the kyocera 260w panels, 1168 dividedby 260w = 4.5, but we know we have to do strings of 3, so we have to use a number divisibleby 3, so we round up to 6 panels. so we'll
have 2 parallel strings of 3 in series. solet's look at what happens when we take our 2 parallel strings of 3 kyocera 260w panelsin series and send it into a 48v battery bank through our mppt charge controller. the wattageof each panel is 260w, and we have 6 of them, for a total of 1560w. next we take that 1560wdivided by a 48v battery bank equals 32.5a out. ok, i can probably use morningstar'strisar 45. let's double check with their specs so this is interesting, even though our mathsays we need at least 32.5a output, the morningstar specs are saying that their 30a charge controllercan handle up to 1600w with a 48v battery bank. that is because mppt charge controllersare designed to current limit their output. even if i am putting enough power in to generate32a out, the charge controller will limit
it to 30a. so i have a choice, i can slightlyundersize the charge controller and get the tristar 30, knowing that on occasion it mayclip the output, or i can move up to the tristar 45a charge controller, which can output upto 45a. either one is fine, i just need to decide if i want to pay a little more fora bigger charge controller that will not limit the output, or if the occasional loss of 2ais adequate. with the 45a controller, i also have room for additional panels in the futureif i want to expand. being able to handle up to 2400w, i could add a third string of3 panels in the future, bringing me up to 9 panels and 2340w. you may be adding additionalloads in the future, so it may benefit you to have some wiggle room for growth. that'sit for the this. you can go to our web site
to check out our selection of pwm and mpptcharge controllers. watch the final video in this series for how to select the inverter,using the numbers we came up with from our loads list. also watch more of our variousvideo series on our web site. we've got a team of highly trained technical sales repsavailable to help you plan your system, give us a call.
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