DC-DC Boost converter = MPPT?

KC-watts · January 2013

I have a >90% efficiency boost converter (minimum 10.5 V with a variable output of 24-27.5 V). Am I essentially creating a MPPT if my 16.9 V panel uses this device to feed a 24 V PWM charge controller for series twin lead acid batteries?

PS. I'm trying to justify keeping this DC-DC converter. It's a new Analytic Systems VTC305-12-24; if someone is dying to make an offer on a new item that retails for $700, please do and I'll happily buy a MPPT from NAWS. Happy to pass it on because I'd like to stay 12 V (boater).

BB. · January 2013

Re: DC-DC Boost converter = MPPT?

No, it is not a MPPT type charge controller... From looking at the specs., it is difficult to figure out what it would do when charging a battery bank.

Also, it has a "Output Crowbar Programmed Output Volts x 1.2" -- You would not want a crowbar over voltage protection on a battery charger. It the battery overvoltaged, the power supply would short out its output to "protect the loads". In this case, it would attempt to short out the battery bank. Not a good thing.

-Bill

KC-watts · January 2013

Re: DC-DC Boost converter = MPPT?

Bill, Thanks. I've worked with other DC-DC converters that simply pass through overvoltage and didn't realize this one clamps down using a crowbar circuit at only 1.5 V over the setting; scary event indeed for a charger, but nice for it's intended use as a voltage regulator. If one were to have a non-clamping DC-DC converter, would that turn a 12 V solar panel and 24 V charge controller into a 24 V MPPT?

inetdog · January 2013

Re: DC-DC Boost converter = MPPT?

KC-watts wrote: »

Bill, Thanks. I've worked with other DC-DC converters that simply pass through overvoltage and didn't realize this one clamps down using a crowbar circuit at only 1.5 V over the setting; scary event indeed for a charger, but nice for it's intended use as a voltage regulator. If one were to have a non-clamping DC-DC converter, would that turn a 12 V solar panel and 24 V charge controller into a 24 V MPPT?

No, that would still not do it. You might have an MPP but not an MPPTracking system.
The problem is that finding the maximum power point for a PV array involves presenting a different current load to it under various conditions to maximize the product of V and I.
Your DC to DC convertor would allow using the maximum possible panel power output at exactly one voltage (which would not allow for temperature or shading effects) and will crash the panel voltage if the load needs more current than the panels are currently able to produce.
If the light is low (clouds, time of day, etc.) your DC output will still try to deliver all of the current the batteries will take at the fixed output voltage. An MPPT system would only deliver as much current to the batteries as the panels could produce and would let the voltage drop if necessary to limit the current to the batteries.

KC-watts · February 2013

Re: DC-DC Boost converter = MPPT?

I need some more explanation. If I set a DC-DC converter to output 27 V to a 24 V PWM solar controller, wouldn't that create a condition where maximum panel current is used for charging when the panel is within 10.5 - 16.9 V? Does the PWM get burned out if it can't lower input voltage? What if I set my C40 PWM solar control to Load Control mode?

Just continuing the idea as my project became uneconomical since I can't use my voltage regulating converter and DC-DC converters are pricey by themselves.

inetdog · February 2013

Re: DC-DC Boost converter = MPPT?

KC-watts wrote: »

I need some more explanation. If I set a DC-DC converter to output 27 V to a 24 V PWM solar controller, wouldn't that create a condition where maximum panel current is used for charging when the panel is within 10.5 - 16.9 V?

Let's do a concrete example: Your 24 volt battery is at an SOC where it will take 20 amps at the preset output voltage of your supply (set to Vabsorb)
That is roughly 500 watts. (20 x 25)
At noon your 13 volt are producing 600 watts, in the form of 30 amps at 20 volts. (A little high in voltage for a nominal 12 volt panel, but it makes the numbers nice.)

The DC to DC boost convertor will take only 25 amps of the potential output of the panels so that it can deliver 500 watts to the battery. (I am assuming 100% efficiency. :-))

Now look at what happens later in the afternoon:
The panels now have a maximum output of 300 watts because the sun is lower in the sky.
That would be 15 amps at 20 volts, since Vmp is not going to change much.
The batteries still want to draw 500 watts, since they have not charged up much.
The Boost Convertor will try to draw 25 amps from the panels so that it can maintain 20 amps on the output.
That will not happen, and as the convertor keeps trying to draw more current, it pulls the panel voltage down below 20 volts without actually getting any more current from them. If the BC turns itself off at 10 volts input, then no charging. (The worst case). If the BC lets the voltage drop to 10 and keeps trying, it will find that it cannot hold 25 volts on the output anymore, so it will finally let the output voltage droop. The panels are now producing only 150 watts (15 amps times 10 volts) instead of the 300 watts they are capable of.

A true MPPT controller in the same situation will realize that the absolute best it can do is get 300 watts from the panels, so it will do that: 15 amps at 20 volts. And it will immediately drop the output voltage to the point where it is delivering only 300 watts to the batteries.

I have left out the PWM controller, since it is not really playing any role at all here and will just be trying to deliver full output. All it will do is introduce some additional power loss.

Did that clarify the difference?

boB · February 2013

Re: DC-DC Boost converter = MPPT?

A boost type of charge controller would certainly be interesting ! Instead of worrying about PV input voltage drop from array to charge controller, the opposite
might be the case if the batteries are a long ways away from the batteries... Instead of putting the CC near the batteries and optimizing the PV input
wiring, you'd want to place the CC near the array and optimize the cable run to the batteries in which case the wire size would be larger and cost more
than it would with a typical buck type charge controller.

Doesn't sound like the most efficient method but I suppose it has its uses.

boB

Cariboocoot · February 2013

Re: DC-DC Boost converter = MPPT?

There actually are "power optimizers" sold to be put on panels to boost their output Voltage in the even of shading so that the panel is not 'dropped' from the array. The formula remains the same, though, and the higher Voltage is got at the expense of current.

Any such device is not in itself a charge controller or power point tracker.

Although it would be interesting to incorporate such circuitry in a charge controller for those days when the sun doesn't shine and thus > battery Voltage isn't available, usually you just run some panels in series to be sure it is (since it would be an MPPT controller anyway). Merely upping the Voltage from the panel, even with a PWM controller, would not be much help most of the time.

Manipulating the I/V comes with a cost of some W no matter which way you do it. If the power to charge isn't there it isn't there.

DC-DC Boost converter = MPPT?

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