Intro to Lithium Batteries


In 2021, I began working for lithium lithium-ion phosphate battery manufacturer headquartered in Southampton, PA, just north of Philadelphia.  On our office roof, we have multiple solar arrays directly wired into our testing lab, where we test various inverters on our battery systems.

The job responsibility includes testing, research and development, and technical support. Education is a big part of this industry and Fortress allows me to continue to support the classroom during the work day. Now I get experience with solar battery projects across the globe.

This class is also listed by Fortress Power for NABCEP recertification credit, so if you already have a completion certificate for this class, feel free to skip ahead. Although we do continuously maintain the course content, so don’t forget to check back as the content will change over time. It is a fast moving industry.

For this first hour, we will start with a brief industry overview, talk a little bit about lithium chemistry, and then get into design principles that installers in the field should be aware of to ease installation woes and have better solar and battery designs all around.  Lastly, we will talk about battery management systems.

Speaking of which, the next class in this Fortress Power series will focus on installation pain points based on the technical support issues received in our corporate headquarters, so stay tuned for that webinar.

This Fortress Power class is available for NABCEP credit so if you are a NABCEP solar installer registered for this single class, at the end of the program please fill out the survey form and we will send you a completion certificate within 24 hours. More on that at the end of the hour. 

 So with that, let’s dive into our class material.

 This is an exciting time in the rapidly changing battery industry. Solar installers 15 years ago were only installing battery projects, but for the last decade, the grid-tied solar market largely went without batteries. Our webinar audience indicates 35% of their solar projects now have batteries and its definitely on an upward trend.

My battery experience started in the off-grid market with lead acid, but it’s clear to me that lithium batteries should be the default battery option. Lead acid still has a part to play in battery banks that are used infrequently or off grid projects requiring hundreds of kilowatt hours of storage, but that really isn’t where most battery applications are today. For less than two days of storage, or for daily use batteries, lithium is the clear leader so that will be the focus of this introduction. 

This recent shift from off-grid batteries to grid-tied batteries, coinciding with shorter duration battery banks and daily use, is actually fun because designers have to improve their knowledge and learn a little bit more about the nuts and bolts behind the systems being installed. 

Growth rates in the battery industry are very exciting. Even with the coronavirus, the US residential battery market has grown! There is  also robust growth in the utility and commercial sector. If you want to increase your business and growth, adding batteries to your solar projects is absolutely worth considering. 

Lithium Battery Chemistry

This is a nail test demonstrating the difference between a lithium cobalt pouch battery and a lithium iron phosphate pouch battery, so this is an apples-to-apples comparison of different lithium chemistries and the footage is quite dramatic here, reminiscent of airplane fire disasters in recent years.

Here is the lithium iron phosphate battery and the results are quite different. It’s the exact same test and they drill straight through without a fire starting.

Let’s watch this test one more time, and note the cell voltage for the cobalt battery is higher than the iron phosphate battery, and again fire safety is something we have to address in the energy industry and as you can see, definitely one of the values of iron phosphate compared to cobalt is iron phosphate takes a higher temperature to combust, and that’s what this nail test is demonstrating.

Lithium batteries have a longer lifespan than lead acid. In overall cycle cost, it is cheaper. It is also more efficient, and lightweight compared to lead acid. The lithium batteries are still heavy, so perhaps it is better to state that you get better storage density than lead acid.  

These are sealed batteries. The ventilation requirements of unsealed batteries are not applicable to sealed batteries. There’s a zero maintenance philosophy behind lithium that does not always apply to lead acid, and there is more technology built into professional lithium battery packs. So, if you want a battery where you can set it and forget it, using a full storage capacity of the battery, which performs at a low depth of discharge as well as when full, with fast charge and discharge rates friendly towards solar charging, then you should seriously consider lithium over lead.

Note that lithium cobalt, compared to lithium iron phosphate, has a higher native voltage. If you consult your copy of the National Electric Code, it’ll state that lithium batteries have a particular cell voltage without distinguishing between the two chemistries, but NEC is not a precise document that captures exact numbers. The code’s primarily concerned with establishing a minimum standard for safe electrical installations. This enables electricians to do some design work without electrical engineers having to check every aspect of the system along the way. 

So lithium cobalt cell voltage is higher than lithium-ion phosphate and that has a lot of different implications. For example, the activation energy required to ignite the battery is related to the cell chemistry, which also drives this higher lithium cobalt voltage. 

One NEC quirk is that systems operating under 60 DC volts are subject to fewer requirements than systems over 60 volts. The battery industry and NEC will still call these batteries 48 volts but in reality a 48V nominal lithium bank has an operating voltage that is higher than 48V.  In other words, the 48V classification is a product class, similarly descriptive to the vehicle industry’s usage of the term, “mid-sized sedan”. It is a broad definition. 

Looking at the cell voltage of lithium cobalt versus lithium iron phosphate, the true nominal voltage is lower for lithium iron phosphate, which means a fully charged 48 volt lithium iron phosphate battery will still operate at fewer than 60 volts, whereas lithium cobalt will be at just over that 60V benchmark, triggering more robust NEC requirements.

Here is the warranty of one of the more popular lithium cobalt systems

on the market. This is a 14 kilowatt hour lithium cobalt battery

bank, and we’re going to do an apples to apples comparison.  A lot of these warranties are based on total throughput.  So, this particular battery has a 37,800 kilowatt hour, 10-year warranty. 

If you were cycling that battery at a 80 depth of discharge, we can see it has a warranty of the equivalent of 3375 cycles, and when you look at that cycle number and if you subject this battery to a daily cycle at its full rating, the life of the battery, the throughput of the battery, will expire before you get to that 10-year warranty mark. It’s whichever comes first. 

Here is our Fortress eVault, which is 18.5 kilowatt hour battery, and we’re cycling our battery at 80 depth of discharge and you’ll get 5891 cycles out of it.  Our warranty is 10 years as well.

Which is better?  Which is more cost effective? How do you choose?  Cycle ratings are a little bit of a marketing gimmick; you really want to focus on longevity. Look at the total throughput warranty and how the battery is actually used in the field. In a perfect world, being able to have one cycle a day on the battery through a perfectly inspect charge and a perfectly inspect discharge would be great.  But we all know how we charge our cell phone batteries: intermittently.

An intermittent power supply like solar, as well as variable loads, means the battery is going to be charged and discharged and charged and discharged.  You’ll have days where it’s partly cloudy, so you’re not just going to have one charge and discharge cycle. With the higher cycle rating and the higher throughput rating of the lithium-ion phosphate battery, its actual life will expire after the warranty rather than before the warranty of ten years.

 This is a simulation we do a lot of for battery right-sizing and what is green is the solar production and what is in yellow is the battery. We can see the solar production coming up and then it’s a partly cloudy day and it’s going back down and then going back up again and in turn you see the battery  discharging in the morning and then it charges and then the sun goes away and it discharges and then it charges again. As with most things, the theory is different than the practice.  The real takeaway here is that apples to apples, lithium iron phosphate is going to last much longer than lithium cobalt.

Before we leave this slide, what we see modeled here is the commercial facility demand being perfectly level. This might be a “time of use” scenario where daytime electricity is billed at a higher rate.  It could also be a demand management scenario, where the peak of that commercial facilities electrical load is their most expensive electricity.  There’s a lot of different ways you can use these batteries.

 In continuing to answer the question, “Why would you use lithium iron phosphate over lithium cobalt the takeaway is that lithium iron phosphate compared to lithium cobalt has increased fire safety and a longer life.  When you combine the two, that’s really what customers want. But, in addition to that longer life, it also translates into a lower total cycle cost.  Where maybe the higher voltages that lend to slightly faster charge and discharge might be better for an electric vehicle application, when it comes to integrating batteries into buildings you want the battery to last as long as possible.

You want it to be a daily use battery where you’re competing with the cost per kilowatt hour not necessarily the cost of gasoline!  You need that economic competition and advantage of the longer cycles, better throughput, total longer life of the battery is the advantage of lithium iron phosphate over lithium cobalt. 

Market Applications

Let’s talk about some market applications of batteries.  This is a slide for Georgia Power’s rate structures and I would always encourage solar installers to get on the websites of their local utilities and look for variable rate structures.  They might come in the form of a free nights and weekends plan,  or a “time of use” plan that will adjust your electric rate

based on time of day.  

Georgia Power has a smart usage plan that basically takes a flat energy rate and converts it into a demand charge for them. Georgia has a very limited net metering program right now, but historically, they have not had net metering.  How do you install a solar array on a power grid that doesn’t have a net metering policy in place?  Start to look at the variable rate structures and design an entire system around that. 

Here at Fortress Power, we can assure you, a solar array without a battery is an incomplete system!  All joking aside, it is!  When you combine an array with a battery, more risk is mitigated and you can use variable rate structures to save even more money. For example, here is a $30,000 solar project in the southeast US.  Say it’s a larger home with an electric vehicle on site.  They’re using 2,500 kilowatt hours of electricity per month at $0.12/kilowatt hour and a variable rate structure.

A variable rate structure is something you’re probably already familiar with, but if you aren’t I will explain.  Often described as, “doing your laundry at night”, this structure charges more at peak times and less at other times like nights and weekends. Normally you would have a flat fixed rate for your electricity, whereas demand charges are more common to commercial businesses.  The demand charge availability for residential in this example is $0.10 per kilowatt then they give you an off peak electric rate of $0.01/kwh. The flat rate is $0.12/kwh.  If you work non-traditional hours as so many do now, this can be a real savings. You aren’t in the shower at peak rate hours, etc The household must evaluate usage and decide.

Disclaimer: This is a sample budget. Obviously, this is an educational example. We’re not talking about the actual price; instead we’re talking more loosely, so the actual price and availability of the actual product is subject to change.  Now, back to the show.

Let’s say we add this 8 kilowatt battery inverter. We can reduce the power draw of the home.  Let’s say their old power draw was 15 kilowatts and now it is 7 kilowatts. They didn’t have the demand charge on their old bill, but now they have the demand charge. We’re assuming the solar array provides enough energy to charge the battery.  These are conservative assumptions. Run the PVWatts model for more detail, but we’re assuming the battery and the solar array can fully offset the customer’s peak energy and then what they have left is off-peak energy.

So you can see that even though the demand charge is $70/month the access to that cheap electricity means they have quite the savings on their electric bill so we took at that this 30 project budget.  We apply the federal tax credit and looking at $200 of monthly savings.  Your $24,000 budget after tax credit divided by $20,000 a year in cost savings means you get a 10-year payback.

While a 10-year payback may not initially dazzle you, think about it: First, all of your products involved have a 10-year warranty. Add the energy security of having that solar array and battery on site.  Now, when a hurricane takes out the power grid, you know your family still has power.  The neighbors won’t.   They are throwing out food, but you aren’t, and that is a massive savings itself. Medicines are the proper temperature.  Your kids can still distance learn. You can still work remotely.

You can add an electric vehicle, something that is going to be more and more prevalent over time. But batteries aren’t just for electric cars. 

With the price of lithium iron phosphate coming down, new markets emerge for solar as well.  Now, thanks to affordable batteries, you can have an array in an area where there’s no net metering policy at all. Prior to the battery, solar payback would be non-existent.  All of a sudden, you add a battery and a variable rate structure, and any consumer can get an array if they wish. Maybe the utility does not buy back the solar at a good rate; just add a battery!

Another very popular application is commercial demand management. This is where even just a small solar array that doesn’t do 100% of the building’s electric bill and a small battery that doesn’t do 100% of the building’s demand,  target exactly where you have enough solar to charge the battery and enough battery capacity to dispatch the battery exactly when it’s needed on a demand charge. 

The demand component is your instantaneous power draw.  That’s the most expensive component of the bill, so even a small solar array and a small battery on a commercial project can result in a rather large savings, especially if your business has a load profile where you have a energy intense process for a very short period of time, and the rest of the day you’re operating at far below that demand, you can use your commercial demand management application and have it be quite cost effective in that scenario.

Now, we have batteries applications for South Eastern PA Transit Authority, or SEPTA, here in Philadelphia is using our batteries for their applications. Telecom; there’s a lot of small applications for this technology as well and so when you’re up against a variable rate structure where it’s worthwhile to use batteries well

Here’s some battery budget estimates. These aren’t necessarily what we’re selling the batteries for but they’re ballpark figures and when you take the throughput warranty of the batteries and you divide by their capacity you’re seeing prices of $0.11-0.14/kilowatt hour, but don’t forget that the batteries do apply for the federal tax credit so when you add a solar project to it that drops the price to $0.08- 0.10/kilowatt hour.

Now, explaining some of these differences, here we basically have a larger battery and a smaller battery, and it shouldn’t surprise anyone that the larger battery has a little bit lower cost at the end of the useful life than the smaller battery. So, just some ballpark figures, what I would say is if you look at your time of use rate structures and there’s a differential of  $0.08-0.10/kilowatt hour, that’s an application where you can start putting in a battery on site and then particularly when the time of use rate structures are even larger than that, then you start to really make some money.  

Here’s an electrical cooperative in Mississippi. Are you shocked that Mississippi does not have good solar policy? They have very bad solar policy, at least from the homeowner perspective, but even when you have bad solar policy you can still have a great variable rate structure. And that’s all a solar battery really needs.  Here we see Coast Electric’s variable rate structure.  Normally they’re paying around $0.09/kilowatt hour for their electricity for three hours a day they increase it by four times to $0.36 and the rest of the year they cut it in half down to $0.05 and so you see this differential that’s greater than $0.08- 0.10. That means this is a great time of use rate structure for solar + battery.

Here’s a study by National Renewable Energy Labs of demand charges throughout the country. What I like about this graph is we can see there are high demand charges in California, sure, but there are also high demand charges in the middle of the United States, which is less obvious.  You can find, especially in rural areas, the demand charges start to uptick and so you might find yourself putting a battery and maybe a small solar array, just enough to charge the battery, just enough to lower the building’s peak demand.

That’s another very robust, viable application of solar power and it’s really the next step in what we need for solar to power 100% of the grid or solar and batteries, you know, to really transform our country into more than just for the luxury energy market, but for the entire United States. Our next step in growth is adding batteries back into solar projects. So here are some project photos. It’s all about having that backup power.

Design Principles

Now we’re going to cover some design principles. These are things that you really need to know before you begin your solar project with a battery bank.  These are our sanity checks, to make sure you’re on the right track.  

Lithium works best at a two hour discharge rate.  One of our competitors in the lithium cobalt industry packages an inverter along with the battery and if you look at the inverter size and the battery bank size you’ll realize it’s impossible to discharge that battery faster than two hours with that very popular package.

The same thing applies with lithium iron phosphate.  You know, both technologies, lithium cobalt and lithium-ion phosphate, have a longer life at a slower discharge rate and you’ll see that reflected into warranty issues. So, faster rates of discharge are possible but they’re typically only reserved for commercial application like demand management. The fact of the matter is if you discharge your battery over one hour time frame that’s not going to give you a lot of backup power capability and so that means that faster rates of discharge are really reserved for larger commercial applications and not something that is particularly useful to the residential market.

So what does that mean? If I say lithium is a two hour battery, this is kind of a call back to lithium versus lead acid, and so what will happen when you discharge a lithium battery at a two hour rate is that it’s its voltage will remain relatively flat until the battery is mostly drained. If you go and apply that same load to an equivalent size lead acid battery. It’s curve is going to be way further down.  It might only have half of its useful life or even less.

The voltage might drop so much that the load might even not turn on, and you can see that reflected in lead acid spec sheets. So, this is a solar lead acid spec sheet. They don’t give you a two hour discharge rate on a lead acid battery. The spec sheet says they’re rated for a 10-hour discharge rate.

If you fully discharge the battery at anything less than that, the battery is not going to work!  You’re going to get into reliability issues. The customer may turn on heavy loads and the voltages will drop so much that those loads will turn off and now all of a sudden you’re doing the opposite of what you wanted the battery bank to do: it’s turning off your electricity rather than keeping it on! So, particularly when we’re looking at smaller battery banks, lithium-ion is a clear winner.  Lead acid is just not rated for rapid discharge, just buying a few kilowatt hours, but not running a relatively high load off of it.

Now all that said, there still is a limit to lithium technology. We’re not giving it a 30 minute discharge rate, we’re giving it a two-hour discharge rate, and maybe a one-hour discharge rate with fewer warranted cycles in a commercial specific design scenario. Lithium-ion is still a two-hour battery and so for a quick sanity check to see if the design has an undersized battery, take the battery capacity and divide by the AC inverter output, and if you’re over a two hours discharge, you’re in the clear on your design.  You’ve cleared the minimum battery bank size that’s necessary in order to pass this this test.

The mistake that we see in the field from our battery designers is they’ll just buy the smallest 48-volt battery and the largest 48-volt battery inverter.

That results in a 5 kilowatt battery on an eight kilowatt inverter. If I ran this inverter at full capacity it would only take me theoretically 40 minutes to drain that battery. That’s not within the specifications of the battery. So, you’re going to run just like you would with lead acid at less than a 10-hour rate or less than a 6-hour rate. You’re going to start running into issues with lithium at less than a two hour rate unless it’s a special circumstance.

Another design mistake we see is designers sometimes do not respect the maximum inverter output on a battery inverter! If you put more load than the battery inverter can handle, it’s going to turn off.  It’s going to enact its own safety precautions. If you try to overload a battery inverter, you can insert reliability issues into the equation. We need a better understanding of how battery inverters actually work.  A solar inverter is one way. If you get rid of the batteries, it just takes all the power from the solar array and dumps it into the grid.  The grid does not back feed the solar inverter and charge up the solar array.

In other words, a battery inverter is more complicated than a solar inverter. It’s more expensive.

There’s actually two sides to a battery inverter as well. There’s the grid side and the backup side, which creates its own signal. The grid side synchronizes to the signal but guess what?  If the grid goes out, the synchronousness side goes out with it. The asynchronous side remains running, but what this means is that the load on the asynchronous backup side, regardless of the mode of operation of the inverter, regardless if the grid’s up or the grids down, you can’t overload this backup side of the inverter.  Otherwise it’s going to lose power and drop power.

 That actually is a National Electric Code violation, so National Electric Code says if you’re installing a standby source of power, your power supply has to be able to power the load that it’s connected to, especially in a backup circumstance or an emergency circumstance, because the customer is relying on that power. 

So where you can run afoul of your design is looking at your usage data and if you’re fortunate enough to have a utility account with a smart meter, you can go and see the 15 minute or 30 minute interval data.

The interval data is telling me that this is not going to exceed 14 kilowatts of power, and so you can put that on a 16 kilowatt battery inverter and be just fine, even have a little bit of headroom.

The reason why that can result in design errors is that loads are not spiky. Loads are not 15-minute spikes, they could be 5 minute spikes and if that 5-minute spike is averaged over a 15-minute interval on that 15-minute interval data it can show up as a much smaller load than what it actually is.  Install a consumption monitor on your electric service panel and get one minute data or five second data, and you’ll see a much different picture. 

The fact is, it gets really complicated, and if you read too far into it and if you try and push the limits of your design too far, you’re going to run into mistakes. A big example is that battery inverters have what they call surge modes.  Here’s a 10 second surge mode, here’s a 0.1 second surge mode.  You can see this battery inverter can surge. It’s a 8 kilowatt inverter but it can surge to 16-25 kilowatts. We’re talking about a matter of seconds, not a matter of minutes. The fact is if you have an 8 kilowatt battery inverter and it runs for more than one minute at above eight kilowatts it’s going to turn off.

 At that point, everything on the backup side, everything on the asynchronous side of the inverter, is going to turn off with it, and those are, in fact, your critical loads.  Those are the loads that you don’t want turning off, so what is this burst mode is really referring to, and the thing is, there are certain loads that have very quick charge and discharge, one of those loads is the inverter systems.

The charge controllers themselves, because they have to be capable of supplying this very rapid burst, (and that means they’re also capable of sucking in that much electricity very rapidly) so when you turn on the inverter for the first time these capacitors suck up all the energy, they’re connected too and that can that can result in that burst or surge rating. So, if you really wade into the spec sheets.

You can find the burst mode for the battery and the burst mode for the inverter and do your sizing from that but as long as you stick to this kind of “down and dirty” two-hour sizing principle, you’re going to be in the clear. 

Here’s another quirk of solar battery design. There’s not much of a cost difference between a four kilowatt battery inverter and an eight kilowatt battery inverter.  So, we see a lot of solar installers opting for the 8-kilowatt battery inverter.

What you need to remember is that when you increase your battery size, say, you double your battery size, you’re also going to increase the minimum battery size required to go along with that battery. Not all off-grid customers go for the minimum battery size, but if you’re only planning to have your solar array and battery there for two-hour long power outages, or even longer if it’s during the day, you don’t want your battery bank to be too small. If you take the inverse size of that battery inverter and multiply it by two, running that battery inverter for two hours will to give you the minimum kilowatt-hour rating of the battery bank to have good reliable grid electricity.

Is 8 kilowatts of battery inverter, which is a very popular battery inverter size, enough to power a whole house?  The answer is no. Eight kilowatts is not enough to run air conditioning units. People think having an energy efficient heat pump that only runs at 3 kilowatts, and it doesn’t run at10 kilowatts, I know it’s on a 50 amp breaker but it’s only running at 20 amps, so they’ll never get near the maximum breaker capacity.

Sadly, there’s no such thing as a free lunch.  If an electrician put that air conditioning unit on a 50 amp breaker there’s a reason for it, probably because the manufacturer recommended it. Breaker size heat pumps and air conditioning units, even if they’re very efficient they can be very inefficient for a very short period of time.

This isn’t about the operation of the heat pump itself, but some auxiliary circuits, such as turning on the defrost inside the unit. So, if your unit’s working too hard and it ices up, it has some electric heat tape inside of it that turns on to rapidly fall out the system so that it you can run it really hard again. These heat pumps can use the same electric tape to provide a short burst of electric heat in the winter. If you want instant heat, the heat pump can help out with that, and then it’ll transition a more energy efficient mode of operation.

Most heat pumps have 10 kilowatts of heat tape inside them and so one air conditioner unit even if it’s only running at 3-4 kilowatts under normal operation is still too large for 8 kilowatts battery inverters. You’re not going to have your air conditioners on it. Prioritize other loads, which we’ll get into in a minute.  So, with 8 kilowatts, think about the loads that you absolutely need when the power goes out. Medical equipment, main bedroom, your entry and exit to the house, internet and television and you don’t want the the food in the fridge to go bad. 

True critical loads are the most basic functions of a home, without air conditioning or hot water heating.  But people want more than that.  The air conditioner is going to be something that the customer wants to have on during a blackout.  For most large homes in the US, you’re going to need more than 10 kilowatts of battery inverter. 

What if you use two battery inverters that are 8 kilowatts?  The critical loads we just talked about run on one eight kilowatt inverter, but the second inverter can”t always handle the air conditioning and hot water. A dozen days a year your air conditioner unit runs a 10, and so if you’re not controlling these loads and they all turn on at the same time, that is well above an eight kilowatt inverter, so an unmanaged load you really have to choose between hot water or air conditioning.  You can’t have both at the same time.  If you want both, you need three 8 kilowatt battery inverters.

Remember: every single time you increase the size of your battery inverter you’re also going to increase that minimum size of your battery bank. 

There are load management devices that you can implement to make sure your hot water tank doesn’t run at the same time as your air conditioner.  In fact, I love electric tank water heaters on solar battery systems because the tank itself can use be used to store the hot water.

So, if you have a load management system that can turn on your hot water tank during off-peak times and then turn it off when the air conditioner is running, then you might be able to squeeze more onto that backup load panel.  Just keep in mind the solution that works has to work when it’s grid tied as well as when it’s off grid! The minute that backup load panel goes above the inverter output rank rating, the system will turn off.

Smart home and smart appliances are exciting from an energy management perspective.  An electric tank water heater uses about 40 amps of power and you can find 40 amp programmable smart controllers to put in line to turn those tanks on and off.  Really, all you need to do is talk to your home consumption so that during your electric tank and then you can turn it back on again during off peak.  Solutions are available on the market. The real question is reliability.

Lumen makes a energy manager or basically they have a lot of disconnects. They wire them to all your major circuits right next to your electric service panel so they can turn them on and off based on the power usage of those individual circuits. Installers generally operate on the KISS principle and want the smallest possible configuration andthats understandable. 

Obviously, in certain markets, you don’t need to have air conditioner on a backup panel, so location makes a big difference. A Texan would rather have all the food in the fridge spoil than to go without air conditioning, but in Maine, not so much.  You have to compare the implementation cost of your energy management system as well as its reliability. We aren’t trying to do a commercial for Lumen but a hardwired system like theirs is going to be more reliable than a wireless system.  Trying to hodgepodge together a variety of smart home devices is tricky and might cost more.  In the end, the client has to decide.

As a final note, and maybe we’ll talk about this more in the next class, you couldn’t put an undersized battery that violates that two hour rule because this is 18.5 kilowatt hours on 16 kilowatts of inverter capacity. You would put all of these devices onto two 8 kilowatt inverters so that during grid tied operation you have that reliability.

Another design rule:  Don’t neglect generators!  Generators can go very far to reduce your minimum battery bank sizing.  

Here is a graph where we’ve taken the month to month and divided it into day-to-day energy use of the building. Every day the building uses power, every day the solar array charges the battery, so we’ve just taken the building consumption, using 12-month electric bills, taken the PVWatts solar production figures, and we’ve charted it out for every day of the year.

Assuming we’re not running a generator, we need to make sure we don’t run out of electricity, and so our minimum battery bank requirement is huge. If we have a generator turn on to cover this interval, this is only for extreme weather systems like a summer hurricane, with 4-5 days of cloudy conditions, you can either buy 4-5 days worth of batteries or you can buy a little bit more than one day’s worth of batteries for off-grid, and let the generator take over.

In the past, the solution for off-grid without a generator was to buy four days worth of energy storage to avoid generator usage.  Now installers have to decide between four days worth of lead acid or one day’s worth of lithium iron phosphate.

Keep in mind that you really can’t eliminate the generator entirely in an off-grid scenario, and many customers are designing for the minimum battery size for protection during short power outages.  During the day, save the generator for outages that are longer than two hours. The size of the battery bank when you’re grid tied in particular shrinks down even further.

The last thing I want to talk about are battery management systems. They do more than just monitor voltage! 

A quality battery management system will disconnect the battery during failure state modes like overcharge, discharge, extreme temperatures, or when those inrush currents are too high.  

Battery management systems for home hobbyists are weaker than battery management systems for a whole house. For small appliances, they’re only using a couple of amps like a server. A whole house 48 volt battery on the DC side can get up to well over 100 amps. The traditional way to manage your energy and a battery management system is to use these what’s called a MOSFET, which stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. This is also called as IGFET meaning Insulated Gate Field Effect Transistor, and they’re great for low amperage solutions but they do not withstand the high current that’s required for nearing whole house backup!  Battery management systems use embedded processors instead on their own circuit boards and are designed to handle that high current.

Lastly, a battery management system can communicate directly with an inverter to manage charge settings. Most installers think this means you do not have to program inverter charge settings – this is not true. Settings still need to be programmed, because the inverter needs a fall back in case communication fails, and also some settings are site specific, such as if the homeowner wants to use the battery to its greatest extent or reserve some capacity for an unplanned power outage. 

The greatest advantage of closed loop communication is being able to undersize the battery bank, such as combining battery combinations which may in fact violate the 2 hour sizing principle we discussed at the top of the hour. So, closed loop communication is most advantageous when the project has a limited budget, and the battery is not relied upon for daily use. In an offgrid system, the battery banks need to be larger and the precise controls that come with closed loop communication are less advantageous.

Well, thank you all for coming, we’re out of time today but for next time, we’re going to discuss installation pain points and I’ll talk more more about battery management systems and their features and functionalities.  If you have any questions, ask them on the survey and I will be happy to answer them.  I hope you will come join me in the next video! At Fortress Power, we know how valuable your time is and we sincerely appreciate you taking time to come learn with us.