Batteries and Battery storage

How did batteries come about?

In the early 1800s Alessandro Volta discovered that mixing galvanic materials with various electrolyte solutions resulted in a continuous flow of electrical power. The devices he created were known as voltaic cells, more commonly referred to as batteries. In the mid-1800s, the lead acid battery was invented by Gaston Planté—with a potential difference between a negative pure lead plate and positive lead oxide plate with a sulfuric acid electrolyte solution in between. To this day, nearly all voltaic cells (batteries) are made with a similar principle, an electrochemical reaction that creates a flow of power.

What types of batteries are used in renewable energy?

The renewable industry is dominated by three common battery types:

Flooded lead acid batteries are lead batteries with a liquid electrolyte solution. They have a low upfront cost but require various levels of maintenance and can be sensitive to abusive high demanding applications.

Absorbed glass mat batteries (seal lead acid) are lead acid batteries with the electrolyte absorbed in a fiberglass mat. These are a sealed recombinant battery. They do not require maintenance nor do they give o ff gas hydrogen. However, they are sensitive to over discharge and partial state of charge just like flooded lead acid batteries.

ithium batteries have a high upfront cost, however, they are extremely safe and offer the best levelized cost of all solutions for most renewable energy applications. They are extremely resilient to abuse, can accept fast charging/discharging and aren’t affected by partial start of charge. They also can be expanded over time, which is ideal for most dynamic growing applications.

Learn more about each type of battery below

FLOODED LEAD ACID

What are flooded lead acid batteries and how do they work?

This is one of the oldest battery conventions. Because flooded lead acid batteries can offer high power output at a relatively low cost, they are quite common all around the world and are used in multiple industries from vehicle starting to large energy storage. The nominal cell voltage of a flooded lead acid battery is 2.1V/cell and are commonly seen in multi-cell configurations resulting in 4V, 6V and 12V batteries. These can be joined further into series to create the 12V, 24V, and 48V configurations we use for our systems.

During discharge the electrochemical reaction between the lead oxide and pure lead plates, facilitated by the sulfuric acid solution and ionic exchange, results in power delivery and the creation of lead sulfate on the plates. Lead sulfate is an insoluble salt which under normal charge/ discharge cycles easily recombines into the electrolyte. The electrolyte loses much of the sulfate ion and becomes mostly water. A significantly discharged battery will have a very low specific gravity (close to that of water). During charging, electrons are forced into the cells and push the sulfate back into solution. This process also causes electrolysis of water, resulting in the off-gassing of hydrogen gas. The hydrogen bubbles out and results in a loss of water in the battery. It is imperative with flooded lead acid batteries to maintain them by replacing the lost water with distilled water. The off-gassing of hydrogen and loss of water is a normal process and ensures that the batteries are fully charged. Batteries that are not off-gassing are likely not fully charging. Over time or during significant discharge events, the lead sulfate can create a stable crystalline structure that no longer will recombine, which results in the loss of active material necessary for the electrical power delivery. The buildup of sulfate crystals and loss of active material will cause battery failure.

How do you maintain flooded lead acid batteries?

Flooded lead acid batteries require a significant amount of maintenance. As addressed above, maintaining a suitable electrolyte level is very important. Monitoring the electrolyte level and adding distilled water when necessary is critical to getting a long life from your battery bank. It’s also important to assure that the battery is fully charging regularly to minimize the sulfate criticization. Using a hydrometer to check with the specific gravity (SG) of the electrolyte is the best way to confirm a significant amount of the sulfate is recombining with the electrolyte and therefore the batteries are fully charging. As the battery undergoes multiple charge cycles, it’s possible for the heavier ions to stratify. Recognizing stratification is very important, and with frequent SG testing one will see a deviation from normal readings as the heavier sulfate ions concentrate near the bottom. It will appear as though the batteries aren’t fully charging. Just like a salad vinaigrette, once it’s stratified it’s necessary to “agitate” the solution but it’s not as easy as shaking up the battery. We do this typically with an equalizing cycle. Equalizing is a controlled overcharge that causes a significant amount of hydrogen production. The rapid production of hydrogen mixes up the electrolyte, homogenizing the solution. Excessive equalization can cause a loss of active material, so it’s recommended to do this only when necessary.

Battery banks are often made of several batteries in series to attain the correct capacity and battery voltage. This configuration results in multiple connections. It’s imperative that connections are maintained with the least resistance possible, and to do this, it’s necessary to clean the connections regularly. At least once a year the batteries should be rotated in their series configuration with the outermost (furthermost positive and negative) batteries being moved to the center of the string and the center to the outer. This helps to balance the consumption on the cells, since often the furthermost positive and negative batteries get most significantly consumed.

What are the advantages and limitations of flooded lead batteries for RE applications?

Flooded lead acid batteries have by far the least upfront cost of all the batteries available in the renewable energy industry. However, by their nature they require the most maintenance and can be very sensitive to over-discharge and improper recharge. Lead acid batteries can function in a wide range of temperatures. They prefer to be maintained at or around 77°F (25°C). Lower temperature will result in less usable capacity and higher temperatures will shorten lifespan due to accelerated degradation. Flooded lead acid batteries can deliver extremely high discharge currents, however with a relatively high internal resistance it results in high voltage drop across the bank. Compared to alternative chemistries, lead acid batteries charge at a slow and regulated rate. It’s also critical that they do not get over-discharged regularly. A more significant discharge than the industry standard limit of 50% could cause accelerated sulfation and premature failure. An established bank should not be added to or expanded since it could cause premature failure of the bank and accelerated aging of the new batteries. These batteries are great for low cost applications where there is an abundance of charging time and where performing regular maintenance is not a problem. Avoid flooded lead acid batteries for applications requiring no maintenance, significant deep cycling or the need for expandability.

SEALED LEAD ACID

What are sealed lead acid batteries and how do they work?

Sealed lead acid (SLA) batteries are valve-regulated recombinant lead acid batteries that often have an electrolyte contained in a solution or substance that significantly increases the viscosity. They are often referred to as non-spillable. There are two common forms of SLA batteries, absorbed glass mat (AGM) and gel. As the name implies, AGM batteries have their electrolyte absorbed in a fiberglass mat. These are electrolyte-starved batteries—if punctured, there’s very little to no acid electrolyte that will leave the battery. On the other hand, gel batteries have an electrolyte that’s mixed with a fumed silica, making it a jelly immobile solution. Both types of sealed lead acid batteries do not necessarily need to be kept in an upright position, and because they are valve-regulated recombinant batteries, there is virtually no maintenance. The chemical reaction within these batteries is identical to that of flooded batteries and thus they do produce hydrogen gas during the charging process. However, unlike flooded batteries the recombinant nature of these batteries is achieved by maintaining a pressurized vessel. The gas is contained in the battery and reabsorbed into the solution.

How do you maintain sealed lead acid batteries?

Unlike flooded lead acid batteries, SLA batteries require very little to no common maintenance. The only practical maintenance would be on a bi yearly or yearly basis to rotate the batteries—taking the outermost batteries on a string and rotating them inwards towards the center. The outermost positive and negative batteries tend to see the most cycling, so by rotating the batteries periodically you minimize the long-term effects of the increased cycling. Plus, periodic inspection of connections and monitoring for corrosion also help to ensure the longest life possible for your batteries.

What are the advantages and limitations of sealed lead acid batteries for RE applications?

SLA batteries have very similar characteristics and tendencies to that of flooded lead acid batteries. They are typically a median cost battery but having no maintenance requirement gives them advantages for applications where maintenance is very difficult or undesirable. Because they’re sealed, there’s not a lot of forgiveness to overcharging. If the batteries become over-pressurized by excess gassing of hydrogen, the valves will dissipate the excess pressure. Immediately this reduces any risk of catastrophic failure, however long term this is very bad for overall battery health. Thus, AGM and especially gel batteries require conservative charge values with accurate and reliable charge control. AGM batteries are particularly tolerant to higher charge and discharge currents (common in solar applications) but gel batteries can be damaged due to excessively high currents. Gel batteries are typically not recommended for photovoltaic applications where it’s necessary to gain a full charge by day’s end—they prefer long and slow charging.

LITHIUM ION

What are lithium ion batteries and how do they work?

This is one of the newest technologies to hit the industry. Lithium ion batteries offer the highest energy density and have proved to have the best levelized cost compared to all other battery options. The term “lithium battery” is rather ambiguous. The most common form of this chemistry for the RE industry is the Lithium Iron Phosphate (LiFePO4) battery. This battery is assembled with a naturally safe cathode material (iron phosphate). Compared to other lithium chemistries, iron phosphate promotes a strong molecular bond, which withstands extreme charging conditions, prolongs cycle life and maintains chemical integrity over many cycles. Iron phosphate also provides superior thermal stability, making it extremely safe and reliable.

Lithium batteries are often offered as a complete factory assembly composed of many cells. Like lead acid batteries, these cells are configured in series strings to provide batteries with suitable nominal output voltages. The lithium iron phosphate (LiFePO4) cells have a nominal voltage of 3.2V— that’s over a volt higher than that of lead acid cells (2.1V). Therefore, to achieve a 12V battery you’ll typically have four cells connected in a series. This will make the nominal voltage of a LiFePO4 12.8V. Eight cells connected in a series make a 24V battery with a nominal voltage of 25.6V and sixteen cells connected in a series make a 48V battery with a nominal voltage of 51.2V. These voltages work very well with your typical 12V , 24V, and 48V inverters.

A LiFePO4 cell will be permanently damaged if the voltage of the cell ever falls to less than 2.5V. It will also be permanently damaged if the voltage of the cell increases to more than 4.2V. Thus all assemblies utilize an integrated battery management system (BMS). The BMS monitors, evaluates, balances and protects cells from operating outside the “safe operating area.” The BMS protects against under or over-voltage in each individual cell, as well as over-current and under or over-temperature. Another essential responsibility of the BMS is to balance the pack during charging, guaranteeing all cells reach full capacity without overcharging. The cells of a LiFePO4 battery will not automatically balance at the end of the charge cycle. There are slight variations in the impedance through the cells and thus no cell is 100% identical. Therefore, when cycled, some cells will be fully charged or discharged earlier than others. The variance between cells will increase significantly over time if the cells are not balanced. In lead acid batteries, current will continue to flow even when one or more of the cells is fully charged. This is a result of electrolysis taking place within the battery—the water splitting into hydrogen and oxygen. This current helps to fully charge other cells, thus naturally balancing the charge on all cells. However, a fully charged lithium cell will have a very high resistance and very little current will flow. The lagging cells will therefore not be fully charged. During balancing the BMS will apply a small load to the fully charged cells, preventing it from overcharging and allowing the other cells to catch up.

How do you maintain lithium ion batteries?

Unlike flooded lead acid and other battery chemistries, lithium batteries do not vent explosive gases. That means they can be stored in confined areas without venting requirements or the risk of explosion. Also, there’s no danger of exposure to caustic electrolytes such as sulfuric acid or potassium hydroxide. Due to their integrated battery management system, these batteries are a completely maintenance-free solution. The BMS assures the battery cells reach a full charge and stay balanced throughout the battery bank. While the operating temperature range of these batteries is vast, it is required to keep lithium batteries above freezing (32°F/0°C) when charging. Most batteries provide low temperature charge protection. But for applications that see routinely low charging temperatures, thermal regulation will be necessary.

What are the advantages and limitations of lithium ion batteries for RE applications?

Lithium batteries have by far the highest upfront cost of all the batteries offered, but they also have some of the most significant advantages. Their extremely high energy density makes them small and relatively portable. They often weigh ¼ that of a similarly sized lead acid battery bank, and they take up a fraction of the space. They’re extremely safe, do not off-gas and do not pose a spill hazard. As a result, they don’t require special enclosures or venting considerations. They are completely maintenance free and o ffer the highest cycle life of any battery. The average rated life span with a cyclic 80% depth of discharge ranges anywhere from 6000-10000 cycles depending on manufacturer. Most manufacturers even warranty their batteries for up to ten years.

Lithium batteries are extremely tolerant to abuse and will tolerate periodically being completely discharged. They are also extremely efficient. The round-trip energy efficiency of a lithium iron phosphate battery is upwards of 95-98%, compared to that of lead acid which is about 80%. For systems lacking significant solar power during winter, the fuel savings from generator charging can be tremendous. The absorption charge stage of lead acid batteries is particularly inefficient, resulting in efficiencies of 50% or even less. Considering lithium batteries don’t require an absorption charge, the charge time from completely discharged to a nearly completely full battery can be as little as two hours with enough charging power. Some manufacturers allow even faster periodic charging. By far one of the most significant advantages is that lithium batteries can be a relatively limitless and dynamic solution. Unlike lead acid, a lithium battery bank can be expanded over time. So a customer can start smaller and build a battery bank up to a suitable size.

While lithium batteries offer a lot of advantages, there are also some limitations. Batteries with integrated BMS should not be series configured because it could result in reduced life span. Batteries should be used at their nominal capacity. There are many manufacturers that make 12V , 24V, and 48V batteries suitable for nearly all applications. Applications that do not have a thermally regulated storage location may require some sort of battery heater if the batteries are subject to charge below freezing. Because lithium batteries have significantly different charging and discharging characteristics to that of lead acid, some older power equipment may not be suitable with lithium batteries.

Battery Bank Capacity

Calculations and industry terminology

Ohm’s Law is the fundamental electrical formula used in calculating the relationship between voltage, current and resistance. It’s often used to determine the total energy storage of a battery bank.

E=I x R, or voltage = current x resistance, or volts = amps x ohms

One can easily rearrange this formula with relation to power:

W=V x A, or power = voltage x current, or watts = volts x amps

Watt is a term used to measure total power. It is amps multiplied by volts. 120W is the same as 120 volts @ 1 amp which is the same as 12 volts @ 10 amps. A battery that can supply 100AH at 12 volts can provide 1200 watt-hours . Watt-hours or kilowatt-hours (kWh) is a unit of energy. How many watts times the number of hours. This can be energy provided or energy consumed depending on the observation.

An ampere-hour or AH is a unit of electrical capacity this tells you how much energy the battery will store. Current multiplied by time in hours equals ampere-hours. A current of one amp for one hour would be one amp-hour; a current of 3 amps for 5 hours would be 15 AH. It’s similar to the “gallons per day” measure of water. Amp-hour ratings will vary with temperature, and with the rate of discharge. For example, a battery rated at 100 AH at the 6-hour rate would be rated at about 135 AH at the 48-hour rate. Ampere-hours (AH) designates the storage capacity of the battery. Terms such as "6-hour rate” or “20-hour rate” indicate that the battery is discharged steadily over 6 or 20 hours, and the amp-hour capacity is measured by how much it puts out before reaching 100% depth of discharge (DOD).

Depth of discharge is how much of the available charge has been used compared to 100% SOC, or state of charge, which is how much is left. Most deep cycle batteries are considered to be at 0% SOC, or 100% DOD, when cell voltage is 1.75 volts. For lithium batteries this is close to three volts per cell.

Lead acid batteries are often configured in banks of either parallel, series, or combination both parallel series connections.

Batteries connected in parallel means that all the positive (+) terminals are connected together, and all the Negative (-) terminals are connected together. Batteries wired in parallel supply the same voltage but higher current. The amp-hour ratings add for each battery, but the voltage stays the same.

Batteries connected in series have the positive (+) terminal of one battery tied to the negative (-) terminal of the next battery. Power is taken from the two terminals at the end of the series string. Batteries wired in series supply the same current, but the voltage is higher. For example, four six-volt batteries in series will supply 24 volts.

Batteries can be measured in watt hours or amp hours—they are the same. At a given known voltage, let’s say 12V, a 100AH battery is the same as a 1200Wh battery. It’s recommended to use watt-hour calculation whenever one is calculating the sum of battery capacities. It can get very confusing otherwise. For example, one may think a 400AH 12V battery bank is larger than a 48V 100AH battery bank but in fact they are identical 12x 400 = 48x 100. The nominal voltage of the battery bank is very different and the AH capacity of each is very different, yet they both are 4800Wh.

How much energy storage is needed?

How much storage in amp hours, do you need? This will vary significantly with each application. As a rough rule for off-grid solar electrical systems, the total battery capacity (in watt hours) should be 2-3 times your daily usage for lead acid and could be significantly less for lithium. For backup power systems (battery backup systems), the total capacity should be enough to cover about twice (for lithium this can be equal in energy to) the longest anticipated outage. Significant autonomy is not recommended without taking into consideration additional charging resources. Energy storage is a tough concept to master and it’s best to speak with an expert.

AC Batteries

As battery storage becomes increasingly more popular for self-consumption and grid independence, the industry is presenting emerging battery technologies. Up until recently most ESS (Energy Storage System) used low voltage DC coupled battery solutions, newer ESS solutions may use low voltage or high voltage DC batteries. These DC batteries are coupled with an inverter system to create the Energy Storage System and the battery and inverter scale are usually independent components. So, one can scale the energy storage (battery) independent of the power output. Some manufactures are now combining the inverter and battery together as one component and this combined solution is what we refer to as an AC Battery.

AC batteries may be one of the easiest ways to add battery back up to an existing grid-tied solar electric system or simple battery back up to a home. The term AC battery may be a little misleading, the battery does not produce alternating current. AC Batteries are essentially batteries coupled internally with an inverter/charger as a single solution. This keeps the installation easy and the footprint small. These units are usually mounted on a wall or on the ground against a wall and they are often combined with a MID (Micro-Grid Interconnection Device) which is used to isolate dependent loads from the grid during a grid outage. AC Batteries usually have a slim design, so they do not take up too much space and they use some variant of lithium-ion chemistry.

This type of battery is designed to be AC coupled with a grid-tied solar electric systems (interactive grid-tied inverters). This type of configuration allows the solar array to act as a power source to the home and a charging source to the batteries, even in a grid-down situation. The MID can island the system from the grid in the event of a grid failure. AC Batteries usually have intuitive control and programming options, so one can set up specific configurations. Typically, renewables do the charging of the battery, and the system can use the stored energy from the battery to offset energy consumption during peak hours (Time of Use).

These energy storage systems are usually expandable by adding more AC batteries. Expansion will increase the energy storage capacity as well as the Inverter’s power output.

Most AC batteries can be used with a Power Control System (PCS) to manage loads when the system is in back up mode (islanded). These smart load controllers can shed unnecessary loads to keep the power and energy consumption within the sustainable limits of the system.

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