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Lead-Acid vs. Lithium-Ion: Deciding the Best Fit for Solar Projects

Battery energy storage systems (BESS) are an integral part of the solar energy ecosystem, complementing solar by mitigating its intermittency and enhancing both resilience and grid stabilization.

Rechargeable battery technologies like lead-acid and lithium-ion are widely adopted in the solar sector. Beyond differences in chemical makeup, what are other attributes that set them apart? And which is the best fit for your solar project?

Let’s dive in.

Solar Battery Comparison

Technology Overview: Lead-Acid vs. Lithium-Ion

Invented by Gaston Planté in 1859, lead-acid was the first rechargeable battery for commercial use.

These batteries typically comprise two primary lead-based plates (electrodes) in a grid structure. The positive electrode is coated with lead dioxide and the negative counterpart is made of sponge lead. Both plates are immersed in an electrolyte solution of sulfuric acid and water.

Since its invention, lead-acid has been constantly refined, and its improved version, sealed valve-regulated lead-acid (VRLA), has been widely adopted. Gel lead-acid batteries, a variant of VRLA technology, have become a good choice for solar energy systems and other off-grid applications. Unlike traditional flooded lead-acid batteries, these batteries are less likely to encounter liquid leakage and require less maintenance.

Solar Lithium-Ion Battery

The history of lithium-ion technology can be traced back to the 1970s when M. S. Whittingham and his colleagues invented the first ‘rechargeable lithium cell.’ 

Today, the positive electrode in a lithium-ion battery is made from a metal oxide or phosphate while the negative electrode commonly uses lithium cobalt oxide (LiCoO2) or other materials. Unlike lead-acid technology, the electrolyte in a lithium-ion battery usually consists of a lithium salt dissolved in organic solvents, allowing lithium ions to flow between the positive and negative electrode.

Performance Comparison for Lead-Acid vs. Lithium-Ion

The variation in chemical composition results in unique traits that affect the real-world performance of these batteries. Major criteria include energy density, charging efficiency, depth of discharge, cycle life, size and weight, cost, and more.

Energy Density

Energy density is the amount of energy stored in a battery in relation to its size and weight. 

The gravimetric energy density of lead-acid batteries range from around 30 to 50 Wh/kg while that of lithium-ion batteries is about 150-250 Wh/kg. That is to say, the energy density of lithium-ion batteries is approximately 5 times greater than that of the lead-acid, supplying much more energy per unit mass.

Charging Efficiency

Charging efficiency refers to how much of the charging current is turned into energy stored in the battery. This measurement is particularly crucial for battery use within the solar sector because of the intermittency of solar which prays for maximum energy to be converted and stored within limited function hours.

The former generations of lead-acid batteries hold a poor efficiency of about 50%. Despite decades of progress, maximum efficiency has not yet steadily exceeded 85%. That translates to much energy loss during the charging process.

In contrast, lithium-ion batteries provide a more than 90% charging efficiency with some top-notch-technology-equipped products approaching the figure of 99%.

Monitoring Solar Battery Charging

Depth of Discharge (DoD)

In short, DoD is a measurement that refers to how much capacity can be safely drained from the battery relative to its total charged capacity.

The DoD of most lead-acid batteries is only about 50%. In other words, users will need to recharge the battery after consuming around half of the charged capacity; otherwise, it will pose a negative impact on its lifespan.

For lithium batteries, the DoD is 95% or more, which means you can efficiently utilize the energy inputted and stored.

Cycle Life

This measurement is the number of full charge cycles a battery can go through without a significant reduction in performance. One single cycle is counted when a battery goes from a full charge to a completely discharged state.

On average, a lead-acid battery has a lifespan of 300 to 1500 cycles, which can be equal to 1 to 3 years of usage. Lithium-ion batteries are well-known for their long lifespan, providing a cycle life of about 2,000-5,000.

Cost

Another critical measure to evaluate between these two batteries is their cost.

Lead-acid batteries typically cost about $75 to $100 per kWh, while lithium-ion ones cost from $150 to $300 per kWh.

Some will be thinking that lead-acid batteries pop up as an ideal choice for projects with tight budgets. But always, the cost should not be simply counted. The per-kWh cost here is the initial cost of a battery. You should factor in other expenditures associated with battery use, especially in the case of solar applications.

Battery Maintenance

Last but not least, you should also consider the maintenance details of these batteries.

Sealed lead-acid batteries, the principal type of lead-acid batteries adopted in solar projects, require monitoring of their charging cycles and regular checks on ventilation.

However, lithium-ion batteries require much less maintenance once put into operation. Most of these batteries today are furnished with a battery management system (BMS) to ensure they are not overcharged and operate within normal temperatures.

Charge time and battery weight and size are not covered in the above discussion. They actually correlate to other measurements such as energy density, charging efficiency, and DoD.

Rounding Up: Which Is the Best Fit for Solar Projects?

After a quick revisit of the discussion above, you will find that lithium-ion batteries excel better over lead-acid in most aspects except cost. Wait… to be more precise, it is the INITIAL cost.

The Payback Time of Solar Projects

The lifespan of solar panels is generally 25 years, with some premium products lasting up to 30 years. Broadly speaking, the designed lifetime of a solar project, whether C&I or residential, can be at least 10 years, right? Stepping back, some superb solar projects can press down their payback time to around 5 years. That implies that they will require the presence of solar batteries for the same duration.

A quality lithium-ion battery can last for about 10 years. Even though the initial cost for lead-acid batteries is low, the accumulated cost will stack up; not to mention associated labor and accessory expenditures.

After-Sale Services and Supply Chain

We’re not talking about the quality of after-sale services, but the hidden cost behind them. 

Exceptional after-sale services, like any other, take time. In addition to equipment cost, the replacement of end-of-life lead-acid batteries, as part of your project operation, follows certain procedures within the organization and consumes time to communicate and coordinate.

For those projects located in a region where a reliable local solar supply chain has not been established yet and the local logistics system is inadequate, it is ideal for operators to go for reliable products with a longer lifetime to maintain a smooth operation for the project.

How About the Thermal Runaway Issue?

You may learn that there’s a thermal runaway issue tied to lithium-ion batteries, which is a chain reaction generally caused by redox reactions (Feng et al., 2019) and high ambient temperatures. 

Having undergone many years of improvement, the occurrence of thermal runaway in these batteries is minimized, attributed to the use of optimized materials and battery structure as well as advanced BMSs. 

These BMSs monitor and manage various parameters of the battery beyond charging and heat distribution, such as cell voltage, current, state of charge, and state of health. They help to ensure the battery achieves excellent performance under controllable and safe conditions.

You may still be concerned about the use of lithium-ion batteries in regions with a hot climate, such as many African or Middle Eastern countries: Nigeria, Egypt, Israel, Saudi Arabia, etc.

Rest assured. Over recent years, the lithium iron phosphate (LiFePO4) battery, a newer member within the lithium-ion family, has been demonstrating its distinctive advantages.

These batteries are known for their excellent safety due to their thermal and chemical stability while offering longer life cycle, reinforced stability in high-heat environments, and lower environmental impact.

They can be more cost-effective over their life cycle, despite a higher initial cost than traditional lithium-ion batteries. 

Conclusion

It’s evident that lithium-ion batteries provide more benefits than lead-acid batteries.

For short-term projects, lead-acid may potentially outrank their peers for their lower price points. But this is definitely not the case for solar projects, which bear in mind sustainability and long-term well-being of people.

Remember, lifetime cost is foremost and more practical than initial cost.

Ongoing innovation in the lithium-ion battery segment is making it more competitive. Constantly driving down product costs is undoubtedly one top subject within researchers' work scope.
By the way, if it is of lower quality and/or not properly disposed of, the lead element in lead-acid batteries will pose big risks.

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