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

[JRBiz]

I've figured out and purchased all the components that I can afford at this point for a new system -- but one array configuration question keeps nagging at me. I have 9 new identical panels with these STC-rated specs:

Pmax = 136w
Vmp = 33.0v
Current@Pmax(Imp)= 4.1a
Open-circuit Voltage (Voc) = 46.2v

I already have a 24v AGM Battery Bank (currently 210ah) and do not want to go to 48v.

And I already have a Morningstar TS-MPPT-45 charge controller which takes PV output up to 150v and 1200 watts @ 24v and deliver a maximum of 45a charging current (this is the most controller I can afford... a bigger controller just isn't in the budget).

I'm lucky -- there is only 20 feet of distance from my panels to my charge controller in the house and the batteries are in the same closet. I'm in North Florida with quite a few 5-to-6 hour peak sun days during the year. Really cold panels generating over-voltage is not an issue for me either.

THE QUESTION: What's the best approach to connecting the panels together as a single array for input into this controller and charging this battery bank -- go all parallel or use a series/parallel combination? With the short run from the panels to the controller, wire cost is not really an issue. I know that in general the more amps you can throw at a battery bank the better -- but is it better to hit my controller with a high amperage current by connecting all 9 panels in parallel or to connect the panels in a series/parallel combination and input a lot of excess voltage at lower amperage and let the MPPT covert all that excess voltage to charging range current? Unless I'm missing something, I have two options:

Connecting all 9 panels in parallel would hit the controller with 41 amps at 33v at peak sun -- the controller would still have 4 amps of excess charging capability and a small amount of excess voltage coming into do its MPPT thing with.

Connecting 3 panels in series to create 3 groups which would then be connected in parallel. That would hit the controller with only 12.3 amps but a kickin' 99 volts -- tons of excess voltage for the MPPT function to work on, but maybe more than it can reasonably be expected to convert to the 25-28v charging range for my 24v battery bank?

Which of the above approaches is going to work best/most efficiently with the least amount of wear and tear on the controller -- or does it really make any difference? If some other series/parallel combination would strike a more suitable balance I could get one more identical panel for a total of 10, although that would put me over my controller's stated maximum of 1200 watts of PV input at peak.

Any thoughts or experience with the Amps vs. Volts choice where MPPT is concerned would be most helpful and appreciated.

-- J.R.

[Mike90250]

price the cost of wire and conduit size to carry 41 amps - all 9 in parallel is not the way to go.

VMP of 33V is cutting it close, for controller losses, and trying to EQ the battery. (I know, AGM you don't EQ, but you may change in 5 years) And 33V could use PWM controller, you won't get much MPPT action, if any, just the lower losses.

The best approach would have been 2 panels in series, in 5 banks, but you only have 9 panels. Can you snag and mount 1 more ?

3 panels in series, on a frosty morning, (I've seen photos of frozen Orange Groves) @ Voc 46.2v hits the controller w about 140v at 70F, and more as it goes colder. Do you feel lucky ? What does the calculator say for your panels at 20F ? Only takes a second to fry the input with too high a voltage. Oh, the thunder storms and clouds keep your area warm ? Be sure you have a way to totally disconnect the PV from the CC when sparks are flying, or the induced voltage in the wires will fry everything. Use a pair of Surge Protectors, I like the Midnight and Delta working together.

I would not worry about over paneling the power spec of the controller, it has a robust heat sink, and throttles back if heat or amps becomes a concern.

[Sunking]

I already have a 24v AGM Battery Bank (currently 210ah) and do not want to go to 48v.

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.

With 9 panels you have 3 options of all in parallel, 3 x 3, and all in series. Since you did not buy a 600 volt input controller we can eliminate 9 panels in series, so that leaves you with either all in parallel or 3 x 3 .

Lets start with all in parallel with a 20 foot ling run. First you will need a minimum 10 port combiner, 9 inputs with breakers for each panel, and one output with a breaker to the controller. That is expensive. With 37 amps of current the feeder from the combiner over a distance of 20 feet will need to be #6 AWG copper. FWIW 6 AWG copper cost roughly $1/foot and you will need a 50 foot reel.

3 x 3 is quite a bit less expensive because of the higher voltages and more efficient. You would need a 4 port combiner with 3 input ports with breakers, and one output with breaker. That lowers the current down to 12.1 amps which means a much smaller less expensive 12 AWG copper conductor (about 25 cents per foot).

FWIW all 9 in series requires no combiner or breakers, just a #14 AWG conductor going to the CC, but that option is not available to you with your controller.

So you might want to rethink that 24/48 volt thing again. The higher the operating voltage, the less expensive things are. But I guess expense is not an issue with you right?

Couple of last comments: Operating at 24 volts with a 45 amp controller you are maxed out at 1200 watts. At 48 volts you can go up to 2400 watts. At 1200 watts operating at 24 volt battery minimum battery capacity is 320 AH, you only have 210. Since it is AGM you can get away with it but you will not be able to utilize the potential capacity of the 1200 watt solar panels. A 24 volt 210 AH battery can only give you 1 Kwh of usable power per day. A 1200 watt system in your area with a properly sized 24 volt 500 AH battery would give you about 3 Kwh of usable energy per day or 300% more than you will be able to get. You have really limited yourself. YOu batteries will be fully recharged around noon, then the system will just shut down and collect dust the rest of the day instead of generating power. That is extremely expensive electricity you are making. Battery cost alone will exceed $1/Kwh. You can buy power in FL for roughly 10-cents per Kwh.

[Mike90250]

... 48 volts is 400% more efficient than 24 volts......

That copper losses only, in the wiring, and some reduction of Peukert loses in the battery, not the overall panels & inverters.

[Sunking]

That copper losses only, in the wiring, and some reduction of Peukert loses in the battery, not the overall panels & inverters.

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

[JRBiz]

OK... but what about that Question?

Thanks for the replies so far. I understand and agree with what's been said and know about the need for disconnects/breakers/fuses throughout the system and proper array-to-battery sizing -- 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? Is there more wear and tear on the controller with one approach compared to the other? Perhaps more important: Is there a limit on how much of the excess voltage my controller can convert to usable charging current into my 24v batteries once I up the storage capacity to 400ah - 500ah? I haven't been able to find a solid answer about this, even in the controller manuals.

If the series/combo is best balance (for the controller) I can get one more identical panel so I could do 5 banks of 2 panels in series. I could barely squeeze in 10 panels on my roof if the series/parallel combination is going to put less wear and tear on my controller and/or be more efficient in charging.

Naturally, I am going to at least double my battery storage capacity as soon as I can. I waited to buy my four 105ah AGM batteries until recently, after I had purchased everything else and hope to be able to get more before actually putting the system into operation so they are all identical and in matching condition.

As for 'cold starts'... I do get a few mornings per year where it breaks 32 degrees F and on rare occasions goes as low as 20 degrees F for a few hours before warming up to at least 50 degrees by 11 am. In the 20 years I've been living here the most that has happened was for 12 mornings in a row. Usually it is a only day or two here and there and since I have a sizable garden and orchard I have to monitor those conditions anyway -- it would be a simple matter for me to disconnect my array from my controller for the few mornings per year when there would be any danger of over-voltage conditions. But that would only be necessary if I did 3 banks of 3 panels in series which doesn't seem to be the best option anyway.

Since committing to this project I have become much more knowledgeable on wiring, battery capacity, and the advantage of 48v over 24v. If I had known all that when I first started purchasing I might have decided to take 2 years instead of 1 year to get my components, given the extra out-front costs a 48v system requires. But maybe not... because my budget was and still is very tight. I purchased components as I went along over the past year -- when I had scrapped together some 'extra' money and found a really good sale price on one of the components I had identified as the best quality/value I could afford. So I'm stuck with them and a 24v system for now (and probably until I win the lottery). A quality 48v inverter is very pricey compared to my 24v Samulex.

-- J.R.

[Sunking]

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? Is there more wear and tear on the controller with one approach compared to the other? Perhaps more important: Is there a limit on how much of the excess voltage my controller can convert to usable charging current into my 24v batteries once I up the storage capacity to 400ah - 500ah? I haven't been able to find a solid answer about this, even in the controller manuals.

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.

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.

[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

[Shockah]

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

[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

[paulcheung]

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

[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

[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]

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

[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.