Batteries for Solar Applications – Chemistry, Care, and Terminology

What do I need to know about batteries for solar applications?

Understanding how different batteries work, the maintenance involved, and understanding basic formulas all help you make the best decision for your specific needs. Our team of application engineers is here to make it easy, but we know some people really want to have a good understanding of their options before they decide to make this kind of investment.

Common battery chemistries for solar applications


What are lithium-ion batteries and how do they work?

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 renewables industry is the Lithium Iron Phosphate (LiFePO4) battery. Compared to other lithium chemistries, iron phosphate promotes a strong molecular bond that 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. 

How do you maintain lithium-ion batteries? 

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. While the operating temperature range of these batteries is vast, it is required to charge lithium batteries above freezing (32°F/0°C). Most batteries provide low-temperature charge protection. But thermal regulation would be necessary for applications that see routinely low charging temperatures (such as a battery heater).

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

Their extremely high energy density makes them small and lightweight. They are often a quarter the weight 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 caustic spill hazard. As a result, they don’t require special enclosures or venting considerations. They are completely maintenance-free and offer the highest cycle life of any battery. The average rated life span with ranges anywhere from 6000-10000 cycles depending on the manufacturer (typically based on 80% depth of discharge). Most lithium battery manufacturer’s warranty their batteries for up to ten years.

Lithium batteries are extremely tolerant to abuse and will tolerate periodically being completely discharged as well as being discharged and charged at extremely rapid rates (when compared to lead acid). They are also very 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 70-80%. For systems lacking significant solar power during winter, the fuel savings from generator-charging lithium batteries can be tremendous. The absorption charge stage of lead acid batteries is particularly inefficient, resulting in efficiencies of 50% or even less. Lithium batteries don’t require an absorption charge, the charge time from completely discharged to a nearly 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 scaled to relatively limitless and dynamic capacities. A lithium battery bank can be expanded over time allowing consumers to start smaller and scale up to meet their expanding needs...

Other considerations:Lithium batteries with integrated BMS should not be series configured for cyclic renewable applications because it could result in a potentially hazardous installation with reduced life span and unnecessary maintenance. 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 a battery heater if the batteries are subject to charging conditions 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 for lithium batteries.  Please consult a qualified solar engineer before purchasing equipment that needs to be integrated with an older system.


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 is 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 solar power 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 that under normal charge/ discharge cycles easily recombines into the electrolyte. During discharge, 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 the 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 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 compared to other options. 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 charged regularly to minimize sulfate build-up. Using a hydrometer to check the specific gravity (SG) of the electrolyte is the best way to confirm 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 with an equalizing cycle. Equalizing is a controlled overcharge that causes a significant amount of hydrogen production. The rapid production of hydrogen bubbles through the battery and 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 a series to attain the correct capacity and battery voltage. This configuration results in multiple connections. It’s imperative that good low-resistance connections are maintained by cleaning the connections and checking proper torque 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 of the cells since often the furthermost positive and negative batteries get most significantly cycled.

What are the advantages and limitations of flooded lead batteries for solar 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 which can cost consumers in the long term.

Lead acid batteries can function in a wide range of temperatures. Although, they prefer to be maintained at or around 77°F (25°C). Lower temperatures 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, loss of active material, 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 and old 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.


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 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 to check terminal torque periodically and 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. It’s very important to periodically inspect of connections and monitor for corrosion also help ensure good conductivity and minimal resistance, guaranteeing 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 for overcharging. If the batteries become over-pressurized by excess gassing of hydrogen, valves will release the excess pressure. This reduces the 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.

Battery Calculations and Terminology

Our solar application engineers are here to help you wade through the information and make decisions. Below are some common terminology and calculations used to determine the right battery solution for your solar power needs.


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 s 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 (Wh) or kilowaat-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-houror 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 current based on Peukert's law. 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” ndicate that the battery is discharged steadily at a consistent current 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). A battery with a 100Ah rating (20-hour Rate) would be discharged at a consistent rate of 100Ah/20h = 5A. Currents greater than this would result in less delivered capacity.

Batteries can be measured in watt hours or amp hours they are kind of the same except Ah isn’t accounting for voltage. 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. Battery capacity calculations can get very confusing otherwise. For example, one may think a 400Ah 12V battery bank is much larger than a 100AH 48V battery bank (when comparing Ah) but in fact they are identical in capacity: 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. Keep it simple and calculate using energy via Watt-hours (Vbattery x Ahbattery = Whbattery) then you can add up all the energy storage and keep your calculations accurate.

Depth of discharge (DOD) s how much of the available charge has been used compared to 100% state of charge (SOC) 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 should not be discharged below 50% SOC. Lithium batteries can be discharged completely but are typically designed to consider a cyclic depth of discharge of 80%.

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

Batteries connected in parallel means that all the positive (+) terminals are connected, 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.

AC Batteries explained

Some manufacturers are now combining the inverter and battery together as one component and this combined solution is what we refer to as an AC  Battery. 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.

AC batteries may be one of the easiest ways to add battery backup to an existing grid-tied solar electric system or battery backup 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 island (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 solarelectric 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(isolate) 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). ESS systems may also be configured to charge from grid during low electricity cost to then use the battery during high electricity cost, as a form of arbitrage.

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.

How much energy storage is needed?

How much storage in amp hours, do you need? This will vary significantly with every 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. We can help you determine your energy storage needs.