MPPT/PWM

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adam1984
adam1984 Solar Expert Posts: 72 ✭✭✭✭
in Solar Beginners Corner #1
I just want to know if I understand this correctly. With MPPT you can charge a 12V battery with a 24 volt panel, but with PWM you must use a 12V panel for a 12V battery? So if you don't have an MPPT, you couldn't use a 48 volt panel to charge a 12v battery at all? Would it just be a waste of 36 volts or would there be damage? Confused and thanks. By battery i mean the whole bank.
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  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    #2
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    Re: MPPT/PWM

    Also, using an MPPT, if you use a 48nominalV panel with a 12V bank are you losing are the extra volts being wasted, does it charge the battery faster? Does the tracking optimize efficiency? I know a ton of questions but thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
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    Re: MPPT/PWM

    With PWM type controllers there is no "down converting": the nominal voltages of battery and panel array must be the same. If you try to run 48 Volts through a PWM to charge a 12 Volt battery the current will be very high and something will burn out. Hopefully it will be the fuse you put in to protect the circuit. But it is likely to damage the controller even so.

    An MPPT controller will 'convert' the higher panel Voltage into greater available charge current, and adjust as needed. The disadvantage is, of course, greater cost.

    For choosing, think of PWM as being best suited for "smaller" or "occasional use" systems and MPPT for "whole house" set ups. This isn't a 100% rule, of course.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
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    Re: MPPT/PWM

    basically anything (edited to add i meant voltage) over what the battery is is technically wasted. some will be needed to press the charge into a battery as 0v with any supposed current is 0w of power and as such means no current will flow without a higher voltage. a much higher voltage will still put a charge into the battery. the extra voltage that is fed the pwm controller can strain some designs such as the shunt design and i don't know to what degree of excess most series designs will handle. example- 24v in with a 12v pwm output setting. outputting 24v in that scenario takes any strain off of the controller, but the higher voltage will cause the battery to continue its charging far after it has reached its full charge as the controller wants to start to regulate at a higher voltage.
    as such if the controller handles it you can do it, but this makes as much sense as walking 100 miles instead of taking your car. pv watts are expensive to make and buy so pressing higher voltages that are not going to be used when a lower one will work is dumb 99.99% of the time. it won't necessarily charge faster as the current input is what determines that and tracking has no bearing on the higher pv voltage issue. pun was intentional.:p
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
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    Re: MPPT/PWM

    Remember, solar panels are really current sources. No matter the voltage (between 0 volts and ~Vmp) the output current of the panel will remain nearly 10 amps (there is no damage if you short circuit a single solar panel), ... If you have Vmp=18 volts and 10 amps -- that equals a 180 watt panel.

    However, the output energy depends on the load (for PWM). The current is almost the same if Vpanel=18 volts or Vpanel=0 volts:
    • P = I*V
    • P = 10 amps * 18 volts = 180 watts
    • P = 10 amps * 14.5 volts = 145 watts
    • P = 10 amps * 6 volts = 60 watts
    • P = 10 amps * 0 volts = 0 watts
    So--within reason, matching the Vmp of the panel to that of the Vbatt-charging+Voltage-Drops will give you optimum PWM controller power transfer.

    With an MPPT charge controller--they typically use a Buck Mode switching power supply to take "high voltage/low current" from the solar panel and down converter it to "low voltage/high current" required by the battery bank.

    The rough equation is:
    • Ppanel - Controller-losses = Pbatt
    • Vmp*Imp - ~5-10% controller-losses = Vbatt-charging*Ibatt-charging
    As always, the details of what is going on in the MPPT switching controller is more complex (the control loops inside the MPPT controller are pretty amazing).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    #6
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    Re: MPPT/PWM

    So you couldn't use a voltage converter to charge a 12v battery with a 48 volt panel? Like the converter couldn't step down the panel (48v) to 12 volts?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
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    Re: MPPT/PWM
    adam1984 wrote: »
    So you couldn't use a voltage converter to charge a 12v battery with a 48 volt panel?

    Kind of depends on what you mean by "voltage converter". Technically, any type of charge controller or DC-to-DC converter takes an input Voltage and regulates an output Voltage based on the device's design. I mean, that's what an MPPT controller does.

    What you're going to find in the practicality of the real world is that there's no "48 Volt" panel and 'ordinary' voltage regulators aren't designed to drop 48V down to 12V. Most solar panels are either 12 or 24 Volts nominal, and their actual output in terms of VOC or VMP is somewhat higher (like a 12V panel having a VMP of 18V or so and a VOC of 20V or more).

    And be advised that a Voltage converter doesn't function the same as a charge controller. The latter will actually output different levels of current/Voltage according to the charge function. You may have read about "four stage" or "three stage" charge controllers: when charging a battery (as in RE application) it is necessary to bring the Voltage above nominal for a while, hold it there, and then let it drop back to a lower yet slightly elevated level. This is the Bulk, Absorb, Float type of charging suitable for lead-acid batteries. Beyond that there is the "fourth stage" of Equalization, used as needed to keep cells' SG close to one another.

    We have on this forum explored the possibility of running an MPPT controller from a higher Voltage battery bank to charge a lower Voltage bank and it is a "yes", but with the usual provisos.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #8
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    Re: MPPT/PWM

    What do you mean by Voltage Converter?

    In general, a solar panel is a current source. Slap on a "voltage converter" to the solar panel--what happens.

    A converter follows P=I*V:
    • Iin * Vin = Iout * Vout
    Say you convert 100% of the 80 watt panel's output, Vin=10 volts and Iin=8 amps. Then a bit of cloud/bird/shadow crosses the panel. Iin available falls to 4 amps--but the converter is trying to output 80 watts:
    1. Iin*Vin=8a*10v=80 watts output to battery charging
    2. Iin*Vin=4a*10v=40 watts < 80 watt demand
    3. Iin*Vin tries 16 amps * 5 volts = 80 watts
    4. But source is current mode, draw more than 4 amps @ 10 volts, output will collapse to 4 amps @ 0 volts = 0 watts
    When you look at how devices work--they are more complex than originally thought.

    For example a Voltage Converter requires the input voltage to fall in a defined range and assumes that if the converter's output requirements go up, that it can take more current to support the output demand (and if the voltage falls a little bit--the total P=I*V -- the increased current part -- is still larger in the end).

    And the output of a voltage converter. Place a 14.5 volt output converter on a "dead" 12 volt battery--it could take hundreds of amps to "pull" the battery to 14.5 volts--well beyond the capacity of a the voltage converter.

    So, the Voltage Converter has one of three options:
    1. Burn up because of excessive output current.
    2. Pop an internal fuse.
    3. Collapse the input power sources' ability to provide current/voltage.
    4. Got into current limit mode and become a current mode supply until battery voltage equals 14.5 volts--where it then becomes a voltage converter again.
    So, on the output of a battery charger, they all have some sort of current limit and/or power limited output feed back control. You cannot "charge" a voltage source with a voltage source safely.

    And back to the input. With a current mode source--Power transfer is variable:
    • P=I*V
    • P=10 volts * 8 amps = 80 watts
    • P=5 volts * 8 amps = 40 watts
    • P=0 volts * 8 amps = 0 watts
    • P=11 volts * 0 amps = 0 watts (exceeded Vout capability of current source)
    So--to successfully run load on a solar panel, there are only a few options:
    1. Run load less than Imp so that panel acts like a voltage source
    2. Run loads that are resistive or fall back on power usage as I available falls (water pump, fan, run slower with less I and V available).
    3. Run loads that do not "care" about current (charging a battery at 14.5 volts work if there is 1 amp or 8 amps available).
    4. Design a converter that searches for the maximum power available from the solar panel using the equation Pmax=Imp*Vmp and re-evaluates the Imp/Vmp points every so often (from 120x per second to once per 15 minutes). That variable power is then transferred to a load that can use that variable power.
    To common uses of that power are charging a battery bank (variable power transfer during charging/use is standard operating procedure for battery type voltage source).

    Another is a Linear Current Booster--This is a type of MPPT converter that is used with pumps (for example). Turns out that motors work better with "low input power" if they have high current and low voltage... (High voltage and low current for a motor really just turns into low voltage and low current and therefore P=Imotor*Vmotor is a small number). Instead, the LCB converts the high voltage/low current of the solar panel into low voltage high current for the motor. A solar pump will start earlier in the day and, overall, pump more water per day than the same setup without a LCB (talking about a batteryless system here).

    So--there are ways to make the best of a solar panel's current mode output--but they are not usually not both simple and cheap at the same time (use more panels and run in constant voltage region--which is a waste of money--or use a charge controller and battery bank or a linear current booster to maximize the available energy from the solar array which adds costs and complexity to the basic solar power system; plus batteries age and need replacement every few years).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    #9
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    Re: MPPT/PWM

    I meant a charge controller. So now im really confused. I had it correct. You cant use PWM to charge a 12v battery with a 48 volt panel, MPPT you can. Just talked to a woman from Xantrex and she said they offer one PWM charge controller that can do that. The c-40 she said has a max Voc of 125 volts, meaning the 48 volt panel (Voc=82 volts) would charge the battery bank of 12v, but would be really innefficient, but it wouldn't damage the controller. SO for that model, PWM is ok to charge a 12v battery with a 48v panel? She said it could handle the current as well. Now im way more confused, thought PWMs couldn't do that. Sorry guys please dont get frustrated with me.
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  • lorelec
    lorelec Solar Expert Posts: 200 ✭✭
    #10
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    Re: MPPT/PWM

    You can charge a lower voltage battery from a higher voltage array with a "PWM" controller, so long as the controller is rated to handle the voltages and currents involved. You'll waste a lot of PV this way, but it's doable. You can also charge a lower voltage battery from a higher voltage array without any charge controller (and within reasonable limits) -- the battery will simply drag the PV voltage down. Again, lots of potential power wasted....and with this method, you have to do the regulating, meaning that you'll need to disconnect the battery from the array at the appropriate time to prevent overcharging. The MPPT controller is the ideal, because it (theoretically) doesn't waste anything and it also does the regulating. As with other controllers, though, you'll need to make sure that it's rated for the job.

    Marc
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #11
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    Re: MPPT/PWM
    adam1984 wrote: »
    I meant a charge controller. So now im really confused. I had it correct...

    Terms matter--you had me type up a page about voltage converters and you you wanted PWM charge controller???? ;)

    I am not one to complain about misusing term--I am terrible at that (and anything in English--my only language :cry:)...

    What Marc (lorelec) and Marc (Cariboocoot) said.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lorelec
    lorelec Solar Expert Posts: 200 ✭✭
    #12
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    Re: MPPT/PWM

    Also, I think some confusion lies in the terms used. Almost every solar controller made today that's based on a switching power topology is PWM (pulse width modulated) - based. The difference is that the "MPPT" controllers make use of an "LCD" (inductor-capacitor-diode) cell such as you'd find in a buck or boost converter, which works to conserve energy. What's sold as a "PWM" controller simply chops up the output to the battery as a form of regulation when the battery reaches full (or near full) charge.

    Marc
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  • adam1984
    adam1984 Solar Expert Posts: 72 ✭✭✭✭
    #13
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    Re: MPPT/PWM

    Last question and THANK YOU ALL SO MUCH! For example, the C40 has a rating: Charging/Load Current =40 amps DC
    Max. Peak Current= 85 amps
    Max Voc= 125Volts DC

    How do I know if it can handle the current? Lets say a 60W 48V panel with a
    Voc=90volts
    The currents for the panel:
    I short circuit= 1.19amps
    I max power= .9 amps

    I have read FAQ's and cant find it. SO obviously the controller can handle the panels voltage. Controller can handle 125VDC, and the panel is 90V. Clear on that. I assume the charging load current is referring to the battery. Imp for the panel is .9 but 85 for the controller. Super confused. For sake of example say i have a 12volt 225AH battery.
    How do I figure out if the charge controller can handle the current to charge this battery from this panel. THANKS SO MUCH REALLY!
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  • lorelec
    lorelec Solar Expert Posts: 200 ✭✭
    #14
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    Re: MPPT/PWM

    You're well under the 40A limit of the C40 with your 60W panel. The Isc is 1.19A.

    Marc
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #15
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    Re: MPPT/PWM

    That controller has a maximum charging current of 40 amps (plus a safety factor). (remember, PWM is basically just a switch/transistor that turns on and off electronically--pretty much the same if you put a light switch in to control charging... Turn it on to charge, turn it off to stop charging).

    And, remember that solar panels are really current mode power supplies... You could use Imp (current maximum power) or even Isc (current short circuit) for maximum current...

    So, in your case, the maximum current Isc is 1.19 amps:
    • 40 amps / 1.19 amps = 33 panels maximum in parallel for C-40 controller
    Now, if you remember, the output of the solar panels is really based on sunlight. And there are times when you can get more than 100% rated sunlight on a panel...

    If you wanted to be really safe, you could derate the current by a factor of 0.8 -- So that you either assume the panels can output 1.19a/0.8=1.5 amps maximum, or that the controller can only manage 0.8*40a=36 amps maximum.

    Always read the instruction manual... However, the Xantrex stuff is "lost in transition" on the web right now and I am not going to bother searching for the manual myself at this moment.

    When reading the manual, always check that you are not double adding safety factors. The vendor should have their own safety factors when they rate the product. And adding another 0.8 just ends up costing you more money (more charge controllers, fewer panel supported, etc.)--For example if they have the 0.8 and you add another 0.8:
    • 0.8*0.8=0.64
    You are well below the "real" safe operating limit of the controller if you do too much derating.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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