Configure Array for High Amps or High Volts for MPPT Controller?

[Mike90250]

There are many factors that go into a properly designed system. the distance from PV to batteries is one. Battery voltage is another. Existing appliances is another too.
Designs that factor in all the components, Loads and usage patterns, will yield a much more robust system than getting some parts at a store and hoping they work.

I prefer to work with mfg's data, not random unnamed charts off google. Generally, a PV voltage configured to be 1.5x battery voltage will work for most MPPT charge controllers. When you have narrowed your choices, then start looking at the specs for each piece of gear. This is the time to dig deep, inspect efficiency curves, performance specs and even expected solar hours for the site. The more attention to details at this stage, is directly related to performance and happiness when the system is in use.

[travissand]

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Please accept you are not always right, you don't know everything and your opinion is just that. Any good scientist will tell you, the more they learn the more humbly they realise how very little they know. Upgrade the arrogance to respect for others and their views, most people on this forum are well meaning and just trying to nut out the truth.
You will get more respect, and the credibility of your knowledge will approach its maximum power point.

Thank thank you for your well put together post. I was one of those people sunking was arguing with. The moderator put an end to the pissing match and closed the thread. But basically yes you are 100% correct in your statements. I for one have tried high voltage versus lower voltage in a side-by-side comparison just to put an end to my curiosity. And yes you are correct.

[webdude]

I'm surprised no one has accepted Sunkings challenge to "Show Me".

Google: dc dc conversion efficiency

Or, here is a succinct explanation:

https://www.homepower.com/input-voltage-controller-efficiency

And something more technical:

https://www.researchgate.net/publication/257891029_PERFORMANCE_OF_MPPT_CHARGE_CONTROLLERS_A_STATE_OF_THE_ART_ANALYSIS

It has always been my understanding that it is harder/heavier/hotter to convert DC to DC where the ratio of Voltage differences are large than when they are more similar.
Indeed this was one of the many selling points to go to a 48v battery system, as it seems 48v DC to 240v AC inverters (ratio 5:1) are slightly more efficient than 12v DC to 240v AC (ratio 20:1). But technology is always finding new ways to do things and I came to this thread hoping for respectful discussion on whether this has changed yet.

JR that is just plain ole silly. You want to operate both battery and panel voltages as high as you can to keep installation cost as low as possible, and efficiency as high as possible. 48 volts is 400% more efficient than 24 volts.

I appreciate I am new to this forum but I hope you can take some constructive criticism from a reader who has read many of its threads in the quest for solar know-how.
Maybe someone else has already addressed this with you, I have not read all of your 23067 posts to date but I have read enough to see a tone of berating those who you rightly or wrongly feel have inferior information. This thread is one of the milder examples, but given the amount of participation you have in the discussions and your overall authoritative disrespectful tone of others, this reflects on solarpaneltalk as a friendly place to gain credible knowledge.

While I sense you understand a lot of the subject matter, I find more and more that I jump past your replies where you start reminding people about how stupid they are and how little they know. In some threads after seeing you ragging others, further down I found you were wrong and I starting loosing any respect for the potentially valuable knowledge you do have.

Forums are places for people to help each other. Your knowledge will be respected when you show respect for others and their opinions, especially when you know (or think you know) they are wrong. You can be and have been wrong too, these are the times to be humble, accept the mistake and thank those that have helped to further the accuracy of your understanding.

True Dat, but copper are the greatest losses that are controllable by design and the most important factor.

Not always true. Amongst other benefits, I appreciate the value of sending power over distance at higher voltages, but it is not always the best way to go, cable size & cost is not always the greatest consideration.

The question has already been answered. Always run panel and battery voltages at the highest possible voltage to minimize installation cost and highest efficiency. That is why electric utilities run high voltages period. Low Voltage = high current = fire.

Yeah that's great for sending massive power over long distances, but why do they transform the voltages down to 415v / 240v / 120v. Why doesn't my toaster accept 33,000 volts, perhaps for different purposes / circumstances there could be other factors to consider ?

The controller wil run much cooler at higher input voltages with less stress. It is a NO BRAINER decision. What is not to like? Less expense, higher efficiency, less stress, and safer.

You regularly accuse others of dishing out bad and/or dangerous information, you are also human.

Incorrect response. With MPPT you ran as high of a voltage as you can.

Incorrect response.

Google Images: mppt conversion efficiency curves

very 1st picture in the results:

https://image.dhgate.com/0x0/f2/albu/g2/M00/94/41/rBVaGlaCQfGACKLGAADQMznFR74757.jpg

A 24v battery, at 400w charging power seems 33v in is 98.2% efficient but 115v in is only 95.7% = 2.5% loss, and look at what happens down near 100w, more than 7% difference.
Maybe cable size is not always the most important factor.

I have a beefy 48v setup at home but have inherited a 12v system in another off-grid home we have purchased. Many appliances including an expensive DC fridge, lights, water pump etc and the MPPT itself will run 12v/24v. The cost of replacing and upgrading all these perfectly good items to 48v or 240v AC with another inverter, will far exceed the savings of some 48v system efficiencies so the plan is to reconfigure the batteries and panels for 24v for a cheap improvement. Other (not silly) people also sometimes have their own reasons for not jumping straight into the higher voltages you preach as the only way to go.

The distance from our panels to MPPT is reasonable and I have plenty of spare cable up to 25mm2. The roof real estate is limited so more panels is not simple. Generally efficiency losses end up as heat and we are in a very hot climate. As everyone knows, heat is the biggest killer of silicon chips. Preventing a fire (how much inefficiency would that produce?) or an early MPPT failure and purchase of an expensive replacement is a big consideration in this overall design. And what if the MPPT does have an over-voltage de-rating (on colder days) and/or over-temperature de-rating (hot days), shouldn't that occasional power loss also be factored in?

I am not interested in sending the max juice into the MPPT to max it's allowable input voltage. So long as it always has enough extra volts to always poke its best attempt into the batteries, then I want the input voltage that will do the most efficient conversion and to run at its coolest.

Please accept you are not always right, you don't know everything and your opinion is just that. Any good scientist will tell you, the more they learn the more humbly they realise how very little they know. Upgrade the arrogance to respect for others and their views, most people on this forum are well meaning and just trying to nut out the truth.
You will get more respect, and the credibility of your knowledge will approach its maximum power point.

[Mike90250]

Show me.

I can tell you. My Classic 200, is fed with a 2KW array at about 165VDC, and charges my NiFe bank at 66VDC. It's well within it's rating, and makes a great little heater. Even with it's fans screaming, it's much hotter than the morningstar.

Someday, I'm going to get some ckt breakers to use as switches, and switch my 110V 3Kw array from the morningstar to the classic, and see how it does with lower voltages.

[Sunking]

That Midnite chart is an OUTPUT POWER chart, not an EFFICIENCY chart. 90 volts input is the lowest voltage they tested on a 12 volt battery. If the input voltage were to go lower than 90 volts, the output power of the Classic coluld not increase (ampacity limitations), but the efficiency would increase and the unit will run cooler.

According to the Midnite engineers (on their user forum), the Classic 150 is most efficient with an input voltage about 12-15 volts above the equalization voltage (which is the highest voltage that the Classic must produce).

--mapmaker

Show me.

[mapmaker]

Every MPPT charge controller will tell you the most efficient voltage to run at a specific battery voltage. Example the Midnight Solar Classic 150 operating into a 12 volt battery is a Vmp of 90 volts. Look for yourself. Now as you go up in battery voltage Vmp and battery voltage get closer, but for Midnite Solar 90 volts is optimum voltage for all battery voltages up to 48 volts.

That Midnite chart is an OUTPUT POWER chart, not an EFFICIENCY chart. 90 volts input is the lowest voltage they tested on a 12 volt battery. If the input voltage were to go lower than 90 volts, the output power of the Classic coluld not increase (ampacity limitations), but the efficiency would increase and the unit will run cooler.

According to the Midnite engineers (on their user forum), the Classic 150 is most efficient with an input voltage about 12-15 volts above the equalization voltage (which is the highest voltage that the Classic must produce).

--mapmaker

[rithym]

Incorrect response. With MPPT you ran as high of a voltage as you can. Every MPPT charge controller will tell you the most efficient voltage to run at a specific battery voltage. Example the Midnight Solar Classic 150 operating into a 12 volt battery is a Vmp of 90 volts. Look for yourself. Now as you go up in battery voltage Vmp and battery voltage get closer, but for Midnite Solar 90 volts is optimum voltage for all battery voltages up to 48 volts.

The correct answer is look at the SPECS and weigh it against the distance you have to run the wiring.

Okay, thank you for giving your perspective on your understanding of voltages in to MPPT controllers. I have been searching on the MStar website for clarification but have not found anything except to say the controller I have derates at 140V so on a 24V battery and with 12 panels according to the Mstar string calculator I have the choice of 6 x 2, 4 x 3 or 3 x 4... question is which configuration is best?? Perhaps they should extend the calcualtor software to include cable run lengths from panels which obviously makes a huge difference to the possible configuration.

[Sunking]

You are correct in your thinking... higher input voltages make the controller less efficient and therefore warmer.

If you have a great distance between the array and the controller, it MAY make sense to configure the panels to a higher string voltage in order to save on cable costs.

--mapmaker

Incorrect response. With MPPT you ran as high of a voltage as you can. Every MPPT charge controller will tell you the most efficient voltage to run at a specific battery voltage. Example the Midnight Solar Classic 150 operating into a 12 volt battery is a Vmp of 90 volts. Look for yourself. Now as you go up in battery voltage Vmp and battery voltage get closer, but for Midnite Solar 90 volts is optimum voltage for all battery voltages up to 48 volts.

The correct answer is look at the SPECS and weigh it against the distance you have to run the wiring.

[rithym]

You are correct in your thinking... higher input voltages make the controller less efficient and therefore warmer.

If you have a great distance between the array and the controller, it MAY make sense to configure the panels to a higher string voltage in order to save on cable costs.

--mapmaker

Okay, thank you for clarifying my cable run is not long so keeping the strings the way they are is probably better for the controller long term

[mapmaker]

So by this logic I would be better feeding my 60A controller with 3 strings of 4 in series than what I configured currently which is 6 strings of 2? This would be more efficient and better for the controller?? I really don't get that...

You are correct in your thinking... higher input voltages make the controller less efficient and therefore warmer.

If you have a great distance between the array and the controller, it MAY make sense to configure the panels to a higher string voltage in order to save on cable costs.

--mapmaker

[rithym]

The question has already been answered. Always run panel and battery voltages at the highest possible voltage to minimize installation cost and highest efficiency.

The controller wil run much cooler at higher input voltages with less stress. It is a NO BRAINER decision. What is not to like? Less expense, higher efficiency, less stress, and safer.

So by this logic I would be better feeding my 60A controller with 3 strings of 4 in series than what I configured currently which is 6 strings of 2? This would be more efficient and better for the controller?? I really don't get that... please elaborate I thought keeping the panel voltage closer to the max Vb required by the battery (32V) would mean LESS work for the controller, LESS heat and BETTER efficiency??

200W Hyundai panels
Mstar MPPT 60A
4 x Trojan LR16 6V
Panels located on roof 3 metres above controller

[paulcheung]

Or he can return or sell the charge controller and get a classic 200, 250 or the 600 volts.

[mapmaker]

but I can't help noticing no one has answered my burning Question...

Since #6 AWG twisted copper wire would only cost me about $50 for my short run, I'd like to ignore that issue for the purpose of answering this:

Is it better (for my Morningstar MPPT 45 controller) to have my array feed it a high amperage current (41 amps) at 33v -- or to feed it a high voltage (66v) much-lower amperage current (20.5 amp) and let the MPPT covert all that excess voltage to my 24v battery banks' charging voltage?

Happy to answer: the less down conversion of voltage that MPPT must do, the happier it will be. Therefore, 9 panels in parallel is the answer. BUT there is a problem... (Mike mentioned it earlier)

The problem for you is that 33 volt Vmp is not high enough to charge a 24 volt battery bank. The voltage of a hot panel is reduced. (and you're in a hot climate) Your batteries could need ALL of that reduced voltage, but the MPPT circuitry needs some headroom.

The engineers at Midnite solar have pointed out that some folks have seen a decline in their production after switching from PWM to MPPT. That only occurs when the headroom issue comes up... most PWM can get by with a little less headroom.

NEXT issue... Three strings of three is not the answer for you either. You will exceed Vmax of the controller. You think you are warm enough to avoid exceeding 150 volts... you are not. Learn about radiative cooling. On a clear, still night the reason the air gets cold is because surfaces (like your panels) radiate their heat into space. The cold surfaces (by conduction) cool the air. The surfaces are colder than the air. You need to plan for several degrees BELOW the record low temperature for your area.

Really, the only solution for you (once again Mike mentioned this earlier) is to get another panel (or lose one) and make strings of two.

--mapmaker

[Shockah]

I have nothing to add,
except a big THANKS to Mike90250 & Sunking for their PRICELESS input on this topic!!

[Mike90250]

Go to the morningstar site, and enter your panel specs and weather records into the array sizer. Great if you can do 3x3 panels, but you do know that the big freeze will happen the day you sleep in late.

String Selection Tool: http://www.morningstarcorp.com/en/strings/calc.php