Should you use Lead-Acid or Lithium-Ion Batteries for your PV system?
Most solar PV systems use a battery to store energy for use at night or during a cloudy day. The type of battery you choose can have a major impact on what you can expect from your solar PV system.
Lead-Acid and Lithium-Ion batteries are the most common types of batteries used in solar PV systems. Here is what you should know in short:
Both Lead-acid and lithium-ion batteries perform well as long as certain requirements like price, allocated space, charging duration rates (CDR), depth of discharge (DOD), weight per kilowatt-hour (kWh), temperature, and round-trip efficiency are met.
Lithium-ion batteries have a longer life cycle, work better at temperature extremes, and offer better storage capacity per unit weight compared to lead-acid batteries.
Therefore, in applications where space is a constraint, lithium-ion batteries become a better option.
In this post, we are going to look at how Lead-Acid and Lithium-Ion compare in many different areas including performance, cost, safety, and environmental impact.
What are Lead-Acid Batteries?
Lead-acid batteries are a type of large-capacity rechargeable battery found in automobiles, trucks, and motorcycles.
Because they are inexpensive and reliable, they have also been used to power solar PV systems. However, they are becoming less desirable as better battery technologies become more affordable.
Lead-acid batteries are made up of separate cells that have layers of lead alloy plates submerged in an electrolyte solution.
Small amounts of other metals, such as antimony, calcium, tin, and selenium, are also added to lead to increase its mechanical strength and improve electrical characteristics.
The electrolyte solution used in lead-acid batteries is normally made up of 35% sulfuric acid and 65% water. The energy is generated when the sulfuric acid comes into contact with the lead plate and triggers a chemical reaction.
Lead-acid batteries have a small power-to-weight ratio compared to most newer battery technologies. It means they are not going to store as much energy per pound of the battery.
Per pound or per kg of battery storage capacity is an important metric for a battery because it tells us how much total power the battery can store.
Lead-acid batteries are heavy due to their large size and high lead content. A car battery weighs 41 pounds on average, but other lead-acid batteries may weigh much more.
Because of the hefty weights, lifting, handling, and transportation require more labor and may cause injuries.
How does a Lead-Acid Battery Work?
A lead-acid battery is composed of lead plates and electrolyte solution with a voltage between two electrodes that creates an electrical double layer at the electrode surface, which causes current to flow out from one electrode to another.
A typical lead-acid battery cell uses sulfuric acid as an electrolyte, where there are positive and negative plates made up of lead and the electrolyte solution is composed of about 35% sulfuric acid.
There are many variations to this design, but it's important to understand that a battery works by having two different electrode materials in an acidic environment with ions flowing through the liquid from one plate to another.
Because pure lead (Pb) is too soft to support itself, small amounts of other metals are added to provide mechanical strength and improve electrical qualities.
The most common additions are antimony (Sb), calcium (Ca), tin (Sn), and selenium (Se). When the sulphuric acid gets into touch with the lead plate, a chemical reaction occurs, and energy is created.
Produced energy is stored in a battery as chemical energy which is converted into electrical energy when required.
The chemical energy of the battery is stored in the potential difference between the pure lead on the negative side and PbO2 on the positive side, as well as in aqueous sulfuric acid.
The electrical energy produced by a discharging lead-acid battery can be attributed to the energy released when H+ ions of the acid combine with O- ions from the positive plate to form water.
Types of Lead Acid Batteries
There are two types of lead-acid batteries: vented lead-acid batteries (spillable) and valve-regulated lead-acid (VRLA) batteries (sealed or non-spillable).
Vented Lead Acid Batteries
Vented Lead Acid Batteries are spillable and allow gases to escape from the battery. In turn, this requires vented caps on top of each cell or a vent pipe system connected directly to an external open flame protector device (flame arrestor). Battery cases may contain safety valves for pressure relief.
Valve-regulated lead-acid (VRLA) batteries
Valve-regulated lead-acid (VRLA) batteries, often known as “sealed lead-acid (SLA),” “gel cell,” or “maintenance free” batteries, are rechargeable sealed lead acid batteries that require little maintenance.
They control the entry and outflow of gas into and out of the cell, therefore they are named “valve regulated.” Because of their great power density and ease of use, VRLA batteries are the most commonly utilized battery type.
These batteries are available in a variety of forms, voltages, amperages, and sizes. VRLA batteries are deemed “sealed” since they do not allow for the addition or removal of liquid.
The term VRLA refers to the usage of safety valves, which allow pressure to be relieved when a fault condition causes internal gas to accumulate quicker than it can be recombined.
What are Lithium-Ion batteries?
Lithium-Ion batteries are the most popular type of rechargeable batteries, used in all sorts of electronics from small electronic devices to electric vehicles.
In solar PV systems, they can be ideal for both residential and commercial purposes. Unlike lead-acid batteries, lithium-Ion batteries have a longer lifespan and the production of lithium requires far less energy than lead and other metals used in lead-acid batteries.
Lithium-Ion batteries have been getting cheaper consistently over the last decade. In 2010, the price of lithium-ion batteries was $1191 per kWh of storage capacity. By 2020, the price had already dropped to just $137/kWh!
That's an astonishing decrease in price making lithium-ion batteries far more appealing.
Since lithium-ion batteries are getting cheaper, they're quickly becoming the battery of choice for solar energy storage systems.
How do Lithium-Ion batteries Work?
Lithium-Ion batteries use lithium ions to move between the two electrodes in a charged state. When discharging, lithium ions move from anode (negative electrode) to cathode (positive electrode).
When charging, the movement of lithium ions is reversed. The chemical reactions involved in both the charge and discharge processes are highly reversible, which means that Lithium-Ion batteries have a high round trip efficiency.
How are these batteries compare to each other?
Cost
Lithium-ion batteries cost $300-$400 per kWh storage, while lead-acid batteries cost $80-$100 per kWh storage. Although lithium-ion batteries cost about three times the cost of lead-acid batteries, they last longer and are more efficient.
Another reason lithium-ion batteries are being more expensive is the number of industry applications they can be used. Lithium-Ion is used in smartphones, tablets, and laptops. While lead-acid batteries are used mostly for vehicles and solar PV systems primarily.
Lifespan
Lead-acid batteries generally have a shorter lifespan than lithium-ion batteries. In fact, lithium-ion batteries can last up to five times longer than lead-acid ones. Most lead-acid batteries have a lifespan of between 500 and 1200 cycles without significant capacity loss.
On the other hand, lithium-ion batteries can charge and discharge between 500 to 5000 times before there is a significant decrease in battery life or power output.
Roundtrip efficiency
Roundtrip efficiency is a measure of how much energy a battery can store, and is the ratio of discharge from a fully charged state to its charge.
Round trip efficiency is a function of the battery’s internal resistance and its ability to dissipate heat, which in turn is represented by a number called ‘C-rate’ or discharge rate in amperes per hour.
The lower this value, the better it is for batteries as they can be discharged deeper, and can thus be charged more times before the capacity begins to drop.
A typical lithium-ion battery will lose only 5% of its energy round-trip (at 95% efficiency), compared to 20-25% losses in lead-acid systems.
Lithium-ion batteries have a low internal resistance, making them ideal for high C rates. Lead-acid, on the other hand, has a very high internal resistance that increases with the age of use which reduces its ability to dissipate heat during charging. This means that lead-acid batteries are best suited for low or medium C rates.
Also, lithium-ion batteries have a higher power density than lead-acid which means they can produce more amps of current under the same size battery, making them ideal for high ampere applications like solar PV systems.
Lithium-ion also has no memory effect meaning that it does not need to be fully discharged before recharging.
Weight
Lithium-ion batteries are about half the weight of lead-acid batteries, but this may vary between battery types and chemistries. That means fewer raw materials used in manufacturing batteries, and less weight in transportation costs.
Durability
Lithium-ion battery cells can be stored at a wider range of temperatures (-20° to 45° Celsius) than their lead-acid counterparts which are typically limited to temperatures of 0° Celsius.
Also, lithium-ion batteries are less prone to problems with overcharging, and overheating makes them more durable than lead-acid batteries.
Charging time
The overall charging time of lithium-ion battery systems is much shorter because they take less power to charge up to full capacity while using a lower voltage for the entire process.
On the other hand, lead-acid batteries require a higher voltage to charge their batteries and so take more time to charge.
Maintenance costs
Lithium-Ion Batteries are typically longer lasting because they do not have any memory effect unlike Lead Acid batteries which need regular deep discharge cycles in order to maintain their optimal performance
Lithium batteries self-discharge at a much slower pace than lead-acid batteries, which is why they are preferred for solar energy storage solutions.
This means less maintenance and effort on the part of owners looking to keep their battery systems up and running efficiently long term.
Since Lead Acid batteries have been around longer, they tend to come with lower prices on them which can make them appear like a better deal at first glance.
However, when you take into account that Lead Acid batteries need to be replaced every five years on average, the initial price advantage diminishes.
Cycle life/Disposal costs
Lithium Batteries have cycle lives that can be up to five times longer than Lead Acid batteries. This means that you’ll have to buy and dispose of your batteries less which can save thousands in the long run.
Space availability
Lead-acid batteries are larger than Lithium-ion batteries which can be problematic in applications where you don’t have much space for them to fit.
Lithium-ion batteries are much smaller than Lead Acid, which makes them easier to fit into small spaces and can even allow you to install more of them in the same area if needed.
Also, lead-acid batteries are often sized at a 50% depth of discharge in order to extend battery life. This means you're taking up twice the space and spending twice as much money, neither of which are efficient solutions.
Installation position
Lead-acid batteries must be installed at a certain position provided by the manufacturer or else they may be damaged.
Lithium-ion batteries, on the other hand, don’t need to be installed in a specific position and can even be installed upside down if needed since there is no acid within that may leak.
Protection Circuit
Lithium-ion batteries come with a protective circuit that prevents them from draining. While, with lead-acid batteries, you have to purchase a separate protection circuit that will prevent your battery from draining.
There is no need to use an external charger to charge your lithium-ion battery. The alternator will charge the new lithium-ion battery in the same manner as it charges the lead-acid batteries.
Furthermore, with lithium-ion batteries trickle charging is not necessary. Unlike lead-acid batteries, which lose power when not in use, lithium-ion batteries do not require any protection circuit.
Reliability
Lithium-Ion batteries have a smaller change in performance as they age. The difference between the capacity of new and old lithium-ion batteries is typically around 20% over their lifetime, which is less than half that of lead-acid batteries.
Lithium-ion cells will also continue to operate at higher voltages for longer periods of time before they become unusable. They also can be discharged deeper without damage or loss in performance providing they remain charged up.
Storage Capacity
Lead-Acid batteries have a much lower energy density than Lithium-Ion batteries. The specific energy of a lead-acid battery is around 35Wh/kg whereas that of lithium-ion batteries is up to three times higher at 100 Wh/kg.
In general, you can expect your lead-acid solar PV system to store roughly half the amount of power as that stored in a lithium-ion system.
Charging time
The lithium-ion batteries will typically take around four hours to fully recharge, which is faster than lead-acid batteries that can require as much as 20 hours.
Charging time is, therefore, a major factor in reducing your time spent maintaining and servicing solar PV systems.
Charging efficiency
Lithium-ion batteries have been shown to charge more efficiently than lead-acid batteries, with charging losses as low as 5% compared to 15 – 20% for standard flooded lead-acid equivalents. Charging losses are the power lost during recharging which is lost as heat.
Safety
Lithium-ion batteries are safer to use than lead-acid batteries even in extreme temperatures. They can operate from -40°C to 50°C, while lead-acid batteries will begin deteriorating in the range of 0°C to 30°C.
There is always a risk that a leak may occur with any battery, however, the hazard is much higher for lead-acid batteries.
Unlike lithium-ion batteries which contain no free liquid electrolyte and are therefore not flammable, lead-acid batteries have a high sulphuric acid content in their electrolyte solution. This can be very dangerous if spilled or leaked from the battery.
Environmental impact
Lead-acid batteries are made from a mix of lead, cadmium, and sulphuric acid. This can be harmful to the environment as these metals cannot be disposed of in landfill sites safely due to their toxicity.
In addition, recycling lead-acid batteries are time-consuming and costly as it needs trained professionals to do so not something that most households have available to them.
Lithium-ion batteries are also environmentally friendly which means they do not contain any toxic heavy metals like lead does, making them easy and safe for recycling or disposal compared with their lead counterparts.
Should you choose Lead-acid or Lithium-ion batteries for solar PV systems?
Both battery options are used in solar PV systems since can serve the goal of storing energy for when it is needed.
Lead-acid batteries are cheaper but they take up more space and are also time-consuming to maintain. On the other hand, lithium-ion batteries take up less space but they are expensive compared with their lead counterparts.
The choice between lead-acid or lithium Ion batteries for solar PV systems depends on your needs as an individual consumer considering the size of your storage requirements and how long you can afford to have your power outage.
Lithium-ion batteries are more durable and require less maintenance making them the clear choice for people who want to have peace of mind.
However, if you want to maximize energy storage capacity with minimal installation costs, then lead-acid batteries may be a better option.
Furthermore, lead-acid batteries degrade slowly over time, but lithium-ion batteries tend to die all at once when they reach the end of their cycle life.
However, this is not a major issue because lithium-ion batteries feature a Battery Management System (BMS), which means you will almost never encounter a dead battery situation.
Having lead-acid batteries you may want to consider adding up more batteries at some point to keep up with your storage demand.
Can I replace the lead-acid battery with a lithium-ion battery?
Yes, you can replace the lead-acid battery with lithium-ion batteries. However, it is not recommended. Because of the voltage difference between lead-acid and lithium-ion batteries, you will need to adjust the voltage of your solar PV system or get a new inverter.
If you are not comfortable with this kind of upgrade, it is best if you leave the lead-acid batteries in place and add up more at some point instead.
Can you combine Lead-Acid Batteries and Lithium-Ion Batteries?
You can combine lead-acid and lithium-ion batteries, but again it is not recommended to do so. Because combining battery types and chemistries can cause a slew of problems.
Firstly, the load characteristics of lithium-ion batteries are different than those of lead-acid. This means that you will need to adjust the voltage, current, and several other factors when using both types of batteries simultaneously.
Depending on your system design, this can be a complex process. If you're not sure about how it works or don't have time for extensive engineering changes, you should stick with one type of battery.
If you are planning to use lithium-ion batteries or lead-acid batteries together you can create groups of the same type of batteries by connecting them together. By doing so, you can manage your solar PV system easier.
Recommending to use the same type of battery is not always possible due to different factors, including cost and availability. Therefore, plan in advance to ensure that you'll keep the same type of batteries throughout your solar PV system.
Conclusion
The best choice for your solar PV system will depend on many factors including the budget, available space, and how often you need to cycle/dispose of batteries.
Lithium-ion batteries are more expensive than lead-acid batteries, but the difference in price is quickly offset over time because of their longer lifespan and lower maintenance costs.
Lithium-ion technologies have become much cheaper since they were introduced to the consumer market around 2010, while lead-acid has not changed in cost for decades.
The comparison of lead-acid vs. lithium-ion solar batteries favors lithium-ion batteries on almost every metric except initial cost. However, lead-acid batteries can still be a good option if you want to save money and have no space constraints.
