PV Module Voltage vs Battery Voltage

[Grouch]

I started delving into sizing batteries this past week, and have these questions (based on the diagrams in Figure 690.1(b) in the 2017 NEC, showing all the main components of solar PV and battery / energy storage systems):

1. For Stand-alone systems, where you just have the solar pv array and the batteries (and the charge controller), the pv array voltage should exceed the battery bank voltage only by about 20% - 30%, correct? For example, if your battery bank is 24 volts DC, you cannot have a solar pv array at 400V-600VDC as you would in a regular interactive system with no batteries, for a residence?

2. For AC Coupled Multimode systems, here you CAN have the solar pv array voltage at 400-600VDC for a residence, correct? It seems like this would work, since the output of the interactive inverter is at 120/208 or 120/240 volts, which then goes into a multimode inverter and brings the voltage down to 24 volts DC, to charge the battery. (or whatever the battery bank voltage is).

Thanks again.

 

[Grouch]

EDIT: I probably should've titled this thread as 'PV Array Voltage' instead of 'Module'.

Mod: FIFY

 

[Carultch]

1. For Stand-alone systems, where you just have the solar pv array and the batteries (and the charge controller), the pv array voltage should exceed the battery bank voltage only by about 20% - 30%, correct? For example, if your battery bank is 24 volts DC, you cannot have a solar pv array at 400V-600VDC as you would in a regular interactive system with no batteries, for a residence?

For DC coupled systems built with a charge controller, the charge controller specs govern the allowable range of PV voltage. There isn't a rule that works for all makes / all models, as every manufacturer has their own technology for converting between DC voltages. A typical charge controller might be built for ~200V for the PV voltage when used with a 48V battery bank, though I have seen models built with a wide range input that enable the use of 600V circuits. Some also have HyperVOC technology, that give the charge controller an extended range of withstanding higher voltages at cold temperatures, even if it is beyond the usable range.

2. For AC Coupled Multimode systems, here you CAN have the solar pv array voltage at 400-600VDC for a residence, correct? It seems like this would work, since the output of the interactive inverter is at 120/208 or 120/240 volts, which then goes into a multimode inverter and brings the voltage down to 24 volts DC, to charge the battery. (or whatever the battery bank voltage is).

For AC coupled systems, the battery voltage is independent of the PV voltage. They are on separate inverters, and the power goes thru AC first, before charging the batteries.

 

[electrofelon]

To expand a bit on what Carultch said:

Many stand alone charge controllers have wide range mppt now and can take in the neighborhood of 150 volts DC in. Generally only the real small and simple charge controllers need close to battery voltage ( a little over). Last I knew there were two charge controllers on the market, Schneider and Morningstar, that can take 600 volts in.

 

[Grouch]

Thanks guys, tremendous help. Googling for these answers is near impossible, you have to go through so many links and articles... you find a lot of stuff, but have to be lucky to find the specific answer you're looking for.

Just to go off on a slight tangent... when you design a solar PV array with the battery storage and connection to the utility grid, do you have to follow the architecture / system setup shown in the 'AC Coupled Multimode System' diagram in figure 690.1(b), in order to be code compliant? I know there's a note at the bottom of the figure that states "custom designs occur in each configuration, and some components are optional". For example, if you get rid of the multimode inverter (thereby changing the whole setup to something else), is this considered against code, since you're not complying with one of those 5 diagrams in the figure?

 

[GoldDigger]

You can support both off grid and grid interactive in a DC coupled system as well as long as there is a protocol for the charge controller to throttle the inverter as the batteries near full charge.

 

[Carultch]

When you design a solar PV array with the battery storage and connection to the utility grid, do you have to follow the architecture / system setup shown in the 'AC Coupled Multimode System' diagram in figure 690.1(b), in order to be code compliant?

The figure is simply a representative schematic that is a visual aid to show the meanings of the basic terms in context. The technology of system architecture is continuing to evolve, and it is not possible to capture every possible type of system architecture in that figure.

Specifics of system architecture will come from reading the documentation of the equipment manufacturers, and cross-referencing the requirements and data for each component for inter-compatibility. It will also come from knowing what NEC requirements are built-in to the inverter as well, and what you have to separately install.

 

[Grouch]

Understood. Thanks everyone.

 

[rainwater01]

On off grid lead acid systems they want you to have a minimum of 20-30% over your battery voltage because you need enough voltage to fully charge and equalize the batteries which can range from 54 to 65 volts for a 48 volt system.


Sent from my iPad using Tapatalk

 

[Grouch]

The figure is simply a representative schematic that is a visual aid to show the meanings of the basic terms in context. The technology of system architecture is continuing to evolve, and it is not possible to capture every possible type of system architecture in that figure.

Specifics of system architecture will come from reading the documentation of the equipment manufacturers, and cross-referencing the requirements and data for each component for inter-compatibility. It will also come from knowing what NEC requirements are built-in to the inverter as well, and what you have to separately install.

I guess another to ask my question is: if I design a solar PV system with battery storage, and connect it to the grid, am I forced to have to use 2 inverters as shown in the NEC diagram? An interactive inverter AND a multi-mode inverter? and do I NEED to have a backup loads panel? I attached the diagram for an AC Coupled Multimode system for reference...

 

[wwhitney]

For AC coupled, yes there would have to be two inverters, one for the PV and one for the batteries, with an AC connection between them, and both of them grid interactive.

For DC coupled, you can get a single grid-interactive inverter that connects to both the PV and the batteries. What the internal topology is, and whether it is effectively two inverters inside, I have no idea.

Cheers, Wayne

 

[Grouch]

So if I decide to have solar PV, with battery storage, and connect to the grid... there's no other way to do it other than an AC coupled or DC coupled system, is that correct? So if i don't follow one of these 2 topologies, is it a violation?

 

[wwhitney]

So if i don't follow one of these 2 topologies, is it a violation?

Those 2 topologies are the only possibilities when the loads are all AC.

I mean, you have a system with PV, and with batteries, and connected to the grid. There is going to be some energy path from the PV to the batteries. Either that energy path involves only DC current, so it's DC-coupled, or that energy path involves AC current, so it's AC-coupled. In the latter case, there's got to be a PV inverter in the pathway, and assuming you want to actually use the energy in the batteries for your AC loads, there has to be a battery inverter.

If you want to have a DC microgrid with DC loads, then there may be other possibilities. I've not thought about that much.


Cheers, Wayne

 

[Carultch]

For DC coupled, you can get a single grid-interactive inverter that connects to both the PV and the batteries. What the internal topology is, and whether it is effectively two inverters inside, I have no idea.

A DC coupled inverter has a DC-to-DC converter built-in, that converts the PV to the voltage of the batteries, or vice-versa, or multiple converters to convert both sources to the input voltage of the inverter's power electronics. There are also ways of externalizing this, with a charge controller used along side the inverter and battery bank. The charge controller steps the PV voltage down to the battery bank voltage, so that it can either charge the batteries or deliver its power through the battery inverter to the grid.

I guess another to ask my question is: if I design a solar PV system with battery storage, and connect it to the grid, am I forced to have to use 2 inverters as shown in the NEC diagram? An interactive inverter AND a multi-mode inverter? and do I NEED to have a backup loads panel? I attached the diagram for an AC Coupled Multimode system for reference...

An inverter could exist that has both technologies built-in, but this is usually the case with DC coupled systems, rather than AC coupled systems.

The reason why you would most likely need to have a backup loads panel, is that the capacity of the multimode inverter(s) to supply power to the backed up loads, becomes a chokepoint for how much capacity of backup loads you can power. Even when the grid is available, there still is a maximum capacity that the inverter(s) can pass through from grid-tied side to backup side. Some backup inverters have ways of aggregating multiple units to increase this capacity, others have strict limits for how many backup inverters you can operate together as a system. This means you most likely need to make decisions about what loads you want as critical loads, and what loads you want as general loads that you tolerate as unavailable during an outage.

In concept you can back-up an entire service, but you usually like to avoid doing this in the interest of economizing on your system size.

So if I decide to have solar PV, with battery storage, and connect to the grid... there's no other way to do it other than an AC coupled or DC coupled system, is that correct? So if i don't follow one of these 2 topologies, is it a violation?

If there is a technology that does it in another topology, and that product passes the product listing requirements, you would most likely be able to use it. Generally speaking, you'll usually have either a DC coupled topology or an AC coupled topology. You might have DC loads which can run directly off the battery system, but this is a lot less common.

Is there a specific topology you have in mind?

 

[jaggedben]

and do I NEED to have a backup loads panel?

No.

 

[Grouch]

ok... I'm getting it now. It takes awhile for things to sink in sometimes.

In the latter case, there's got to be a PV inverter in the pathway, and assuming you want to actually use the energy in the batteries for your AC loads, there has to be a battery inverter.

So in the case of an AC coupled system, i see that the 2nd inverter (multimode) is in fact needed in order to change the AC back to DC so you can charge the batteries... so the topology pretty much has to stay as is, with 2 inverters. Understood.

The reason why you would most likely need to have a backup loads panel, is that the capacity of the multimode inverter(s) to supply power to the backed up loads, becomes a chokepoint for how much capacity of backup loads you can power. Even when the grid is available, there still is a maximum capacity that the inverter(s) can pass through from grid-tied side to backup side.

This part I don't follow completely... I get that the inverter has a max capacity that it can deliver to the backup loads panel... where I don't follow is that what makes the backup loads panel a necessity?

If there is a technology that does it in another topology, and that product passes the product listing requirements, you would most likely be able to use it. Generally speaking, you'll usually have either a DC coupled topology or an AC coupled topology. You might have DC loads which can run directly off the battery system, but this is a lot less common.

Is there a specific topology you have in mind?

Nope, no other topology in mind actually... was just curious.

 

[wwhitney]

This part I don't follow completely... I get that the inverter has a max capacity that it can deliver to the backup loads panel... where I don't follow is that what makes the backup loads panel a necessity?

OK, say you have a grid interactive PV inverter and a separate grid interactive battery inverter, so they are AC coupled. You don't need to have off-grid capability; if you're happy to have no power when the grid goes down, that's fine. Loads can be connected anywhere behind the meter, and there's nothing in the system that would be considered a "backed up loads" panel, as there are no backed up loads.

But now say you want to be to run some or all loads during a grid outage. You can't do that while connected to the grid, so you need a device that disconnects the premises wiring from the grid. That's called a Microgrid Interconnect Device (MID).

Now you have two choices of where to put loads, either on the grid side of the MID, or on the premises side of the MID (behind the MID). The former loads will not get power during an outage, and the latter loads will.

If you have sufficient inverter capacity (PV plus battery) to run all your loads during an outage, then you can put all your loads behind the MID. Then there's no particular backed up loads panel, they all are backed up.

If you don't have sufficient inverter capacity to run all your loads, then you may want to pick a subset of loads that can be run by the inverters during an outage. Those loads go behind the MID, and the other loads go on the grid side of the MID. The loads behind the MID could be in a "backed up loads" panel.

Cheers, Wayne

 

[Grouch]

OK, say you have a grid interactive PV inverter and a separate grid interactive battery inverter, so they are AC coupled. You don't need to have off-grid capability; if you're happy to have no power when the grid goes down, that's fine. Loads can be connected anywhere behind the meter, and there's nothing in the system that would be considered a "backed up loads" panel, as there are no backed up loads.

But now say you want to be to run some or all loads during a grid outage. You can't do that while connected to the grid, so you need a device that disconnects the premises wiring from the grid. That's called a Microgrid Interconnect Device (MID).

That I follow... ok so the backup loads panel is optional. Now that I reread Carultch's post, I see what he meant.

Where is the MID located, inside the multimode inverter? or is it a separate device?

 

[Carultch]

Where is the MID located, inside the multimode inverter? or is it a separate device?

It is most commonly the case that the MID is inside the multimode inverter. The multimode inverters have two AC connection sections, one to connect to the utility and general loads side, and the other to connect to the "island grid" of backed-up loads and the PV inverter(s). Different brands will have different terminology, and I recommend checking the manual and labeling your drawings consistent with how the labeling appears inside the inverter. One example is "AC1" for the grid side and "AC2" for the critical loads side. Another example is "Grid" and "Load Out" for the critical loads side.

The alternative to having this inside the multimode inverter, is that it would be a transfer switch, or group of interlocked breakers, as it is in the case of generators, in order to electrically isolate all customer-owned sources from the utility grid during an outage.

This is a device that stops either inverter from feeding the utility and general loads side of the system, during an outage, and enables them to continue to operate and feed the backed-up loads. The term I prefer for the backed-up loads panel, is "critical loads panel".

 

[jaggedben]

It is most commonly the case that the MID is inside the multimode inverter. ...

Not anymore. The most common case now is a separate 200A MID piece of equipment that is designed to work with the particular brand of inverter. We can thank Tesla for the idea. Enphase, SolarEdge, and SMA all do it now, too, plus probably others.

This design has huge advantages in installation flexibility, as well as the ability to back up the whole home if money is no obstacle.