Feed constant voltage DC into the input of a MPPT controller?

[Red Squirrel]

Is it safe to do this? Stay below the max voltage of course. Basically I would run mains through a variac, drop voltage to something sensible, rectify and filter it then feed the controller. This is a temp setup, I just want to use one to charge a battery, and to also test it out indoors before I put it in service. The controller is still going to limit how much current it takes based on it's rating right? Ex: if it's 20a it will not pull more than what it needs in order to output 20a, correct?

 

[mikefitz]

Sounds like fun, my guess is that components in the MPPT or your crude power supply will fail.
Mike

 

[Warpspeed]

It depends if the particular MPPT circuit has a maximum current limit.
It may not.
If the specifications say something like 50 Amps max, you would not connect it up to a bank of solar panels capable of more than that.

Solar controllers generally do not have inbuilt current limit as such, but its possible.
No real need, because the solar panels as current sources are fundamentally current limited in themselves.

There may be other inbuilt protective systems such as thermal limiting, but I would not count on that.
It may be just that a high heat sink temperature starts up a fan, or something really crude like that.

My guess is your MPPT software will quickly discover that the more the duty cycle is increased the higher the measured power. It will just keep increasing the power flow to destruction.
Are you feeling lucky ?

 

[Red Squirrel]

Oh ok, yeah that's what I was worried about, whether or not they have current limiting. Guess it's best not to try it! So that also means one should ensure to oversize the controller to ensure that solar panels don't produce too much current at any time such as winter when it's cold and sunny.

 

[Warpspeed]

Always a good idea to stay well within the published specifications for a maximum anything.

It may actually work, but you are at the absolute mercy of what the manufacturer provided, and they don't usually incorporate protection against what might happen if the equipment is used for other than its designed purpose.

 

[Kemills1]

Normally you can "over panel" a MPPT SCC and it will limit it's output current to it's rated value or to a user defined limit so in theory, what you're proposing should work. Power up could be a problem though. What SCC were you planning on using?

 

[Red Squirrel]

It's a Renogy Rover 20a. It's going into a portable solar setup that will only use 60w of panel, but I also have 2 320w panels so I was thinking I could set those up too in a tree so when I'm at the main camp area I would plug into those. When I need to bring it somewhere else I'd bring the small folding panel.

I just was not sure if the "max" spec was the max it can use, or the max it can actually take.

 

[rickst29]

Is it safe to do this? Stay below the max voltage of course. Basically I would run mains through a variac, drop voltage to something sensible, rectify and filter it then feed the controller. This is a temp setup, I just want to use one to charge a battery, and to also test it out indoors before I put it in service. The controller is still going to limit how much current it takes based on it's rating right? Ex: if it's 20a it will not pull more than what it needs in order to output 20a, correct?

A variac is an AC->AC transformer. I use a well-regulated DC->DC "boost" controller from more-or-less "unlimited" source (the car alternator and battery) to run into my MMPT controller, and the total power consumed by the MMPT is limited by its maximum output current parameter.

But you seem to be describing a home-built rectifier bridge, with maybe a couple of capacitors, as a sufficient high voltage DC power supply. Your result is almost certain to have tons of AC harmonic distortion within the supposedly DC "constant voltage" result, and you're probably going to blow up your MMPT if you try. The quality of your filtering must be super high, you must basically build a competent AC->DC power supply.

You can't build that for the price of buying one.

 

[Red Squirrel]

Yeah this was just for a quick test for something else, I guess it's not worth the hassle then! I was thinking if the MPPT is smart enough to limit current then it does not matter if the input is voltage source or current source, but guess that is not the case.

But it does also answer another question I always wondered, and it's that I probably should not over panel the control either. I was going to hook it up to 2 320w panels but that is exceeding it's rating by a lot, especially when it will hit -40.

 

[Bud Martin]

"Basically I would run mains through a variac, drop voltage to something sensible, rectify and filter it then feed the controller. "
BTW, VARIAC does not provide isolation form the Line Voltage, so it will be deadly.

 

[Red Squirrel]

Yeah this would not be a permanent setup anyway, just a bench test thing. I've done much worse when it comes to safety with temporary setups lol. The cat stays out of the room for those questionable experiments so that nothing that should not be licked gets licked.

If I needed a DC power supply permanently I would just build a proper one. I do want to play around with power electronics eventually.

 

[pollenface]

I've had mixed results.

A 1.5a 18v psu plugged into my vehicles MPPT input on the dc-dc controller (Ctek d250s) worked perfectly at keeping both batteries 100% while I was away from home.

A 5a 24v psu running from off grid inverter and plugged into a 10-30v grid tie MPPT inverter (from amazon) pulled the voltage down to 0v

In the future I will be using an industrial 400w 36v psu connected to an Epever VS3024AU PWM controller for my shore power needs.

 

[GXMnow]

Is this just to do a system test with a DC power source?

If that is the case, you could add a series resistor to make the DC power source look more like a solar panel. Let's say you use a 24 volt power supply, and put an 8 ohm resistor in series with it to the solar panel input.

At 0 current you get 24 volts, but no watts. That is your VOC of your "fake" solar panel.
Into a dead short, you would get 24 / 8 = 3 amps. That would be your ISC current.
At 1.5 amps, the resistor will drop 12 volts, and you get 12 volts into the charge controller. This should give you 18 watts going into your system, and that 8 ohm resistor will also be throwing away 18 watts of heat. This is why I only suggest this as a test.
As the MPPT searches, it should find this maximum power point. If it climbs to 1.6 amps, the voltage getting into the system drops to 11.2 volts, x 1.6 amps = 17.92 amps, so the power is falling. And if it drops to 1.4 amps, the voltage only climbs to 12.8 volts, so the power is once again 17.92 watts. So a good MPPT controller, operating in it's tracking range should find this maximum power point where your series resistor drops 1/2 of the DC power supply voltage. The lower the resistance, the more current it will pull to still get the voltage to 1/2 of the supply before the resistor.

If a power supply has a very low output impedance, the MPPT will keep jacking up the current until the voltage drops to half. Oops. On a lot of modern power supplies, hey will do their best to hold the voltage up until the current reaches a "trip point" and then the voltage drops fast, or a fuse pops.

 

[robby]

Is it safe to do this? Stay below the max voltage of course. Basically I would run mains through a variac, drop voltage to something sensible, rectify and filter it then feed the controller. This is a temp setup, I just want to use one to charge a battery, and to also test it out indoors before I put it in service. The controller is still going to limit how much current it takes based on it's rating right? Ex: if it's 20a it will not pull more than what it needs in order to output 20a, correct?

Your better off not running this test unless you have a bench power supply with current limiting built in.
If you just want to charge the batteries buy a charger.

 

[rickst29]

In my earlier post, I attacked the idea of a simple variac + diode bridge + a couple of filter capacitors being equivalent to an AC -> DC regulated power supply. However, the other half of the idea works well (at least with my own EpEver MPPT "Tracer-3215BN" controller as the current limiter), with a regulated DC Boost Converter as the source (11-14 VDC input to 36.0 VDC output).

My Boost Converter has no internal power limiter. I have been told that it will simply burn out if too high a "load" (in Amps) is placed across its output leads. Though several years of use, it has never had a problem. The Tracer-BN, while running in MPPT mode, refuses to draw more than the current required to create 30A of battery output current (at 13.6 - 14.4 volts, depending on the LFP SOC and also limited by a fairly short "maximum boost time" parameter. After 60 minutes into a hungry "12v" battery array, output voltage drops from 14.4V down 14.2V.

If the Tracer-BN drops into PWM mode (I don't know whether it even does that), the proportion of connect time is so low that the Boost Converter remains unaffected. The "30A" MPPT is my only power limiter. This has been running for years, charging Travel Trailer LFP batteries from an under-the-hood Tow Vehicle Battery connection. My details are within This Thread, which contains wiring diagrams and quite a bit of Q-and-A. I pull a maximum around 500 watts from the Boost Converter, with up to about 450 watts actually reaching the batteries.

 

[Red Squirrel]

Yeah going to just forget this test. I thought maybe MPPT controllers basically could double as a power supply if you feed it an arbitrary voltage below it's max, but I guess it really wants a current source as it will just try to pull as much current as it can otherwise.

The first test was to simply test the controller itself (easier to do on bench then outside in the cold) and the 2nd test was because I'm having an oddball issue with a brand new AGM battery making oddball sounds during the 14v phase so I've been discharging it and charging it to see if this clears up or not so I can do warranty claim, and the current chargers I have keep overheating as they're not meant for continuous use. One of them is a larger metal car booster pack, so maybe I will just add a fan to it as there should be room in the chassis.