Setting aside wiring efficiency, what are the advantages/disadvantages of a higher voltage PV input with an MPPT charge controller

Set aside wiring cost and efficiency, what are the advantages and disadvantages of a high differential between PV array and battery voltage. In what specific situations or contexts is a higher differential good/bad?

I believe DC-DC converters are slightly less efficient with higher voltage differentials, is this correct?

I also believe MPPT charge controllers benefit from a higher array voltage in high temperatures and possibly in partial shade (not positive about this last point).

Higher voltage means less current on the PV side inside the controller, so, unless the manufacturer cuts corners, less heat to deal with from resistance in tracks, current through switching components, windings in inductors, shunts etc. Pretty much all MPPT controllers will be using FETs to do the switching, and FETs act like resistors, milliohms of resistance when full on (higher current devices usually have higher on resistance), so the more current flowing the hotter they get.

On the flip side you need to withstand the higher voltage so increased creepage on the board layout makes for a slightly bigger board, higher voltage specs in capacitors and transistors etc.

When talking efficiency it really boils down to the design of the power supply. Most of the buck/boost supplies people will come across use FETs. FETs act like capacitors on their gate pin, and in other ways too. The faster we have to switch them or the more often, the more current we have to push in and pull out to get them to snap on and off quickly (to avoid them being in not fully on, not fully off state and really start to get hot). High efficiency buck/boost supplies don't just turn on and off at the same frequency but varying pulse widths, they also do tricks like varying the switching frequency, drop switching cycles completely as they start to get towards their extremes of operation. For a buck this can mean the lower difference between in and out will result in less switching of the FET and presto you have more efficiency based on voltage difference. There's also losses in diodes for the same reason, but high efficiency buck/boost circuits will replace the diode with another FET, synchronous rectification.

There are so many many weird and wonderful switching power supply topologies that you could write (and there are) 1000 page books on the subject. Even things you may not think of such as using the resonance of the output inductor to increase or decrease the voltage depending on the frequency of the switching action. Panasonic (I think it was, time fades memory) loved this type of switching regulator in their VCRs.

The only advantage is less copper in the cable between PV and charge controller. The disadvantage is that the greater the voltage differential (Vmp to Vbat) the lower the efficiency. It could be a few percent. Once you get above 50V then you need to take shock hazards into account. 12V arrays have some disadvantage at extreme temperature. Once panels are above 12V then there's no temperature advantage of a 48V over a 24V panel.

For shading, the advantage goes to series 12V panels as opposed to 12V parallel panels. If you have a 12V battery with 24V panels then there shouldn't be any advantage. Bypass diodes will work equally well in both configurations.

Panel voltage falls as temperature rises. If you sail too close with voltage you could end up at the point where panel volts is the same or lower than battery volts, and then charging is no longer possible. Extreme case sure, but conceivable. Keeping Vmp a good distance over peak battery voltage ensures this never happens and that you'll be able to hold Vmp, which is the whole point.

I would start with the question about the application. I presume this is not grid tie. Is it hybrid or off grid?

Ampster said:

I would start with the question about the application. I presume this is not grid tie. Is it hybrid or off grid?

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Simply seeking to learn, but yes, I'm mostly interested in off-grid, and its really the only application I have any familiarity with.

Cal said:

The disadvantage is that the greater the voltage differential (Vmp to Vbat) the lower the efficiency. It could be a few percent. Once you get above 50V then you need to take shock hazards into account.

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You are referring to the efficiency of the DC-DC conversion here, right?

12V arrays have some disadvantage at extreme temperature. Once panels are above 12V then there's no temperature advantage of a 48V over a 24V panel.

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This has more to do with relative voltage, right? Differential between Vmpp and Vbat.

For shading, the advantage goes to series 12V panels as opposed to 12V parallel panels. If you have a 12V battery with 24V panels then there shouldn't be any advantage. Bypass diodes will work equally well in both configurations.

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I don't understand why 12v panels in series would be the best option for shade. This seems to go against conventional wisdom for shade tolerance unless I'm misunderstanding something. Can you clarify why this makes more sense than parallel connections?

@gnubie gave an informative tome on the electronic pros and cons.
From my perspective with a hybrid inverter it was a non decison because it has a string voltage of up to 600 volts. My guess is hybrid inverters are going to resemble string inverters because they are inverting the DC to 240 volt AC. There function is primarily feeding the 240 volt critical loads panel. Any need for battery charging can be efficiently accomplished by charging circuitry. My hybrid does not have a separate charge controller which is probably one thing that distinguishes hybrids from off grid inverters. I think most hybrids operate at 48 v DC and there are plenty of lower voltage off grid inverters.

Dzl said:

You are referring to the efficiency of the DC-DC conversion here, right?



This has more to do with relative voltage, right? Differential between Vmp and Vbat.



I don't understand why 12v panels in series would be the best option for shade. This seems to go against conventional wisdom for shade tolerance unless I'm misunderstanding something. Can you clarify why this makes more sense than parallel connections?

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The higher the pv voltage above bank voltage, the SOONER and LONGER through the day charging will occur.
Partial shading of a series string will cut the output of that string down in watts, but it will still produce watts into the battery.
Parallel shading of a panel close to bank V will STOP that shaded panel from producing watts into the bank.

Supervstech said:

The higher the pv voltage above bank voltage, the SOONER and LONGER through the day charging will occur.

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Have you seen actual test results or analysis showing this "advantage" is more than a pimple on an elephant? Twilight hours produce extremely little current. Any time differential during that period produces (extremely)^2 little energy.

Supervstech said:

The higher the pv voltage above bank voltage, the SOONER and LONGER through the day charging will occur.
Partial shading of a series string will cut the output of that string down in watts, but it will still produce watts into the battery.
Parallel shading of a panel close to bank V will STOP that shaded panel from producing watts into the bank.

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Very high PV-battery voltage differentials bring about another problem. The DC-DC conversion process gets less efficient, esp. at high currents.
There's another disadvantage of most installations with MPPT controllers that is not well recognized, esp. in the boondocking context. Since the panels are series wired, any break in the wiring can completely shut power generation down. Isolating & fixing it can be a challenge. Not at all that uncommon in rigs that go off paved roads. We have converted several series wired installations to parallel for this reason.

Dzl said:

I also believe MPPT charge controllers benefit from a higher array voltage in high temperatures and possibly in partial shade (not positive about this last point).

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Higher temperatures reduce a panel's Voc & Vmp and therefore, the power. There's no way around this fact. In parallel wired systems, if the panels' voltage drops too low, it may not have the 'oompf' to drive a current thru the batteries.

I forgot to add another advantage of parallel wiring of battery voltage matched panels ("12V" panels to 12 V battery or "24V" panels to 24V battery). If the controller were to fail, you can simply connect the incoming PV wires to the battery wires and bulk charge the batteries; essentially what the PWM controller does during bulk. This is an important freedom when boondocking. If the panels are series wired, you can do the same, but keep in mind you're now handling wires that are at 75V DC (or more); not something you'd want to do when you're by yourself.

Cal said:

For shading, the advantage goes to series 12V panels as opposed to 12V parallel panels. If you have a 12V battery with 24V panels then there shouldn't be any advantage. Bypass diodes will work equally well in both configurations.

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@Cal still hoping you will clarify what you mean by this

I don't understand why 12v panels in series would be the best option for shade. This seems to go against conventional wisdom for shade tolerance unless I'm misunderstanding something. Can you clarify why this makes more sense than parallel connections?

Lets start over. Given a 12V battery and three 12V panels then the series option is better for shade.

You understand the concept of bypass diodes? If one section of a panel is shaded then those cells may produce just 20% of the current the other cells produce. Since the 12V panel contain 36 cells in the series string, the unshaded cells force the higher current through the shaded cells. The shaded cells act like a resistor and will heat up and possibly burn the cell open. The 36 cell panel may contain 2 bypass diodes, each one across 18 cells. The diode associated with the shaded cells will conduct. There's no force feeding the shaded cells with high current. The panel voltage is now cut in half. Vmp decreases from 17V to 8V. However 8V isn't going to cut it charging a 12V battery. Two of the three panels are producing 66% of unshaded power.

If you got the 3 panels in series then 2.5 panels are producing 83% of unshaded power. Vmp drops from (3 * 17V = 51V) to (2 * 17V + 8V = 42V) at full current.

All clear?

You understand the concept of bypass diodes?

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I thought I did, now I'm realizing I understood how they worked in concept, but may have misunderstood the context in which they are beneficial.

Cal said:

Lets start over. Given a 12V battery and three 12V panels then the series option is better for shade.

You understand the concept of bypass diodes? If one section of a panel is shaded then those cells may produce just 20% of the current the other cells produce. Since the 12V panel contain 36 cells in the series string, the unshaded cells force the higher current through the shaded cells. The shaded cells act like a resistor and will heat up and possibly burn the cell open. The 36 cell panel may contain 2 bypass diodes, each one across 18 cells. The diode associated with the shaded cells will conduct. There's no force feeding the shaded cells with high current. The panel voltage is now cut in half. Vmp decreases from 17V to 8V. However 8V isn't going to cut it charging a 12V battery. Two of the three panels are producing 66% of unshaded power.

If you got the 3 panels in series then 2.5 panels are producing 83% of unshaded power. Vmp drops from (3 * 17V = 51V) to (2 * 17V + 8V = 42V) at full current.

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All clear?

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Well your explanation is quite clear and I can see the logic in the example you gave. What I am struggling to understand is how to reconcile your example with the conventional wisdom that I've heard many times over of wiring in parallel when shade is an issue, and the little bit of real world testing I've seen where shading reduces current and voltages remains relatively constant, and parallel wired panels exhibit less power loss when partially shaded (one example here).

Can you explain the discrepancy, is the conventional wisdom outdated wisdom that doesn't apply to panels with bypass diodes built in? Or is it possible that your explanation is outdated or overlooking some factor? I feel I am still misunderstanding something as I'm struggling to reconcile what you are saying (which makes logical sense) with what I've heard and observed elsewhere.

Was that the solarqueen in the video? Just had a discussion with her regarding circuit breakers not long ago. That was a poor choice of panels as there were 72 cells per panel (two times 36 series cell in parallel). So you got a series / parallel scenario in each panel. Not ideal when showing parallel vs series shading. The viewer can easily jump to false conclusions. Results would be different with a pure 36 cell panel.

I was a firm believer parallel is superior prior to the use of bypass diodes. The greater the number of bypass diodes on a panel the more efficient it becomes when there's shade. In the old days bypass diodes were not that prevalent in 12V panels. There's a good chance those panels didn't contain bypass diodes.

A while ago I read a short technical manual concerning a cheap MPPT controller made in China. It states that to ensure the maximum efficiency of the controller. There should be a 2.0-2.5 to 1 ratio from of the PV Volts to the Battery volts.

DASH said:

A while ago I read a short technical manual concerning a cheap MPPT controller made in China. It states that to ensure the maximum efficiency of the controller. There should be a 2.0-2.5 to 1 ratio from of the PV Volts to the Battery volts.

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I recently heard something similar.

I'm thinking that this ratio is a balance between (1) minimizing difference between array voltage and battery voltage, which would be theoretically most efficient for the DC-DC conversion part of the process, and (2) ensuring the difference between array voltage and battery voltage is high enough that power will still be supplied to the battery in conditions where the array voltage is reduced (like high temperature or maybe partial shade).

Just a guess though, I don't really know how that ratio is derived or what contexts it applies to.

*IF* you are making high volts/low amps keeping the volts as close as possible (or higher) means efficiency.
As Gnubie said, less losses to heat, induction issues, etc.

Since I switched to AC Coupled micro-grid, and I started with smaller panel strings/voltage and moved up,
The higher panel string voltage is more efficient in the inverter.
While it's DC voltage that has to be switched to produce AC, it doesn't have to be 'Stepped Up' or 'Stepped Down' through Inductors, so the less it has to be 'Worked' the less losses you have.

What I'm currently doing is looking for where my panel voltage gets through inverters with the less losses.
I add panels until losses start to rise, then reduce the panel string voltage until efficiency starts to rise again.
'Clipping' is wasted power, heat is wasted power, so I simply look for the point where I don't have heat or clipping losses.

NOT running the inverter at it's absolutely maximum, around 80-85%, seems to be where I get my best efficiency,
The thermocouple says the heat sink is happy (but I haven't got into summer heat with the new system yet), and heat is lost power...

According to the manufacturer, a cooler running unit is a longer lived unit, it's supposed to be built to take maximum input and 'Clip', but according to the factory engineer maxed out isn't good for anything...
He said what I found, about 80% is where the unit will be happiest, and I want longevity.

Since this ISN'T the typical small solar system, different rules apply,
The 'Grid Tie' inverters produce directly in 240Vac, are parallel connecting on the AC lines, and communicate through those AC lines.
It's really stupid simple to connect these together, but it's infrastructure intensive...
I run the Grid Tied inverters to produce AC power directly from panels and put it on common AC lines,
BUT, and it's a BIG BUTT (like HUGE), I need a hybrid (battery) inverter to produce the sine wave the grid tied inverters need to syncronize with.

That inverter MUST be at least 10% higher capacity than my total grid tie inverters (plural) combined,
And it MUST support frequancy shifting, this is how the inverters communicate via AC lines.

Having the big (and costly) hybrid inverter increases inverter cost,
But it reduces battery cost, the battery only having to support your system while there is no sun situation,
You would be surprised what a 96% efficient panel string & grid tie inverter will produce on even the most overcast days...
(No 35% losses to battery & battery support)

I don't know if this will work for anyone else, but I suspect anyone that needs to power home, small business, basic homestead (not big farm) it would work for them like it is for me.
I'm going slow, trying to figure it out as I go, squeezing as much information out of the manufacturers as possible to maximize this,
I'm by no means the 'Last Word' authority, this is all new to me, but I do have spare inverters if something cooks, so I'm experimenting a little...

A few extra inverters,



Literally a truck load of inverters...