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Are Ni-Cd Batteries Used in Solar Projects? [Pros & Cons + Alternatives]

The history of nickel-cadmium (Ni-Cd) batteries can be traced back to over 100 years ago, when a Swedish inventor developed a rechargeable battery using nickel and cadmium electrodes.

As a project developer or contractor, you may be aware that lithium-ion battery technology is widely adopted. But are Ni-Cd batteries used in solar projects today?

We’ll elaborate on the pros and cons of this solar battery type. Then you will have a better sense of why they are and are not a good choice as a battery solution.

Are Ni-Cd Batteries Used in Solar Projects?

What Are Ni-Cd Solar Batteries? How Do They Work?

Ni-Cd batteries use nickel oxide hydroxide as the cathode and metallic cadmium as the anode. The electrolyte contains potassium hydroxide (KOH), with a concentration ranging from 20% to 35% by weight.

When being charged, nickel oxide hydroxide (NiOOH) at the cathode of the battery is converted back into nickel hydroxide (Ni(OH)₂) while releasing electrons. Meanwhile, cadmium (Cd) at the anode oxidizes into cadmium hydroxide (Cd(OH)₂) by taking up the electrons that have traveled through the external circuit from the cathode.

During discharging, the electrical current flows in the reverse direction. 

The nickel hydroxide (Ni(OH)₂) at the cathode is reduced to nickel oxide hydroxide (NiOOH), taking in electrons. At the anode, cadmium hydroxide (Cd(OH)₂) releases electrons, converting back into metallic cadmium (Cd).

The KOH electrolyte plays no role in either the charge or discharge reactions. 

What Are the Pros and Cons of Ni-Cd Batteries?

Since the inception of Ni-Cd batteries, several new battery technologies have been introduced to the market, which in turn lines up the pros and cons of Ni-Cd batteries in today’s applications.

Cons

The reason to lift the cons above the pros in our explanation is the growing concern about the environmental friendliness of battery storage technology.

Environmental Risks

Like the lead-acid battery that was invented in a similar timeframe, Ni-Cd batteries also contain toxic elements. Nickel and cadmium are associated with various environmental issues. Although today’s manufacturing technology has tremendously minimized the leakage risk, if not disposed of properly, these elements can still bring out pollution and toxicity to marine and estuarine biota.

Environmental Threat by Cadmium in Ni-Cd Batteries

Memory Effect

The ‘memory effect’ in Ni-Cd batteries refers to the phenomenon that they tend to retain a memory of previous recharge levels when not fully discharged before recharging.

We list it as one of the top drawbacks because it poses significant challenges for the charging and discharging processes. This effect can directly restrict their applicability in solar storage projects due to the risk of diminished capacity.

Low Energy Density

Due to their chemical composition and potential material failure, Ni-Cd batteries do not provide high energy density as other modern batteries. 

The energy density typically falls within the range of around 40-60 Wh/kg. This weak point limits overall battery capacity for a given weight or volume, making them a less efficient choice than alternatives for energy storage purposes.

Lower Life Cycle & Higher Self-Discharge Rate

Ni-Cd batteries have a life cycle of 1,000-2,000 on average. It is higher than that of lead-acid batteries, yet it isn’t capable of competing with other advanced alternatives. 

Plus, many rechargeable batteries can self-discharge over time, but Ni-Cd batteries have a higher self-discharge rate of about 10% to 20% per month at room temperature. This means that they can lose a large portion of their charge gradually if they sit idle. However, since batteries are required to work in conjunction with solar panels on a daily basis, this might not be a major concern for solar projects.

Potentially More Expensive

At SolarBuy.com, we hold the view that it’s not fair to compare costs solely based on the upfront expenses of the equipment.

While Ni-Cd batteries may be cheaper upfront, the comparatively shorter lifespan, lower energy density and memory effect will produce hidden costs. These costs are not immediately apparent or easily quantifiable, but they will impact the overall cost of a project for sure.

Pros

These rechargeable batteries still possess several advantages that merit our consideration.

Rugged

The ruggedness of Ni-Cd batteries is owed to their solid construction that makes them highly resistant to physical strikes and vibrations.

This feature allows these batteries to remain functional in heavy duty industrial applications where batteries may be subjected to rough handling and mishandling.

High Discharge Rate

The low internal resistance, chemical composition and robust electrodes of Ni-Cd batteries enable them to deliver electrical power at a rapid rate. This feature makes these batteries suitable for applications that require quick bursts of energy or high power output over a short period of time.

But solar projects typically do not require batteries with exceptionally high discharge rates. Rather, they often prioritize other factors, including energy density and cycle life.

Good Performance in Extreme Temperatures

Solar projects are located at sites where solar irrradition resources are abundant. Some of these locations may have hot climates.

Attributed to their robust construction and chemical stability, Ni-Cd solar batteries are known for their resilience to temperature extremes. These batteries maintain a relatively stable discharge profile and can deliver consistent power output. 

Research indicates that Ni-Cd batteries can withstand temperatures up to 70°C. 

In cold temperatures, these batteries can also exhibit satisfying performance. While they may suffer reasonable capacity and efficiency issues in cold conditions, these batteries are less susceptible to damage compared to some other battery chemicals.

Alternatives to Ni-Cd Batteries in Solar Projects

Beyond Ni-Cd, lead-acid, lithium-ion and flow batteries are available options for the energy storage modules in solar projects.

As different projects have particular configurations and installation conditions, the decision process is a stacking-up against the pros and cons of each technology.

Battery Energy Storage Module for Large-Scale Solar Project
"Canadian Solar" (cropping) by blmcalifornia is marked with Public Domain Mark 1.0.

Lead-acid batteries are cost-effective options for various types of solar projects, especially those with a tight initial budget and those located in a region where the supply of other newer battery technologies cannot be guaranteed.

Lithium-ion batteries currently lead the global market in terms of market share, accounting for approximately 60-65% of the total energy storage market. Despite that their product cost is higher than that of Ni-Cd batteries, lithium-ion batteries outrank Ni-Cd in some critical criteria for solar projects, including energy density, charging efficiency, DoD and life cycle.

It is noteworthy that lithium-ion batteries have experienced a dramatic cost decline of about 97 percent since their introduction. With ongoing innovations in materials science coupled with economies of scale, a continued decline in prices is foreseeable.

Among the different types of lithium-ion technology, lithium iron phosphate (LiFePO4) batteries take up an expanding share. This type of battery is recognized for its superior thermal and chemical stability, exhibiting better tolerance to elevated temperatures while offering a much longer life cycle and higher energy density than Ni-Cd batteries. In recent years, the industry has witnessed a steadily increasing adoption of these batteries in light of their reputation for delivering stable and reliable performance over the long term.

Lastly, flow batteries, the youngest among all the batteries we discuss here, are renowned for their significantly longer lifespan, even when compared to lithium-ion batteries. This makes them ideal for solar projects directed at long-term smooth applications. Yet flow batteries are the most expensive battery technology so far.

Ni-MH: Another Nickel-Based Solar Battery

Ni-MH batteries were invented partly in response to the pressing need to address the environmental impact associated with Ni-Cd batteries.

These batteries are composed of nickel, cobalt, and some rare earth elements. They are more environmentally friendly due to the absence of toxic cadmium. Plus, with a less complex recycling process and improved energy density and charging efficiency, they are considered a better choice than their nickel-based counterpart.

Conclusion

We can conclude that it is very uncommon to utilize Ni-Cd batteries in solar projects. There are some better alternatives to fit the inherent needs of solar projects, entailing higher density, more stable performance and enhanced longevity.

Built upon some particular features, Ni-Cd batteries find their place in aerospace and defense, healthcare, automotive and industrial applications.
Overall, the market share of Ni-Cd batteries is merely around or below 5%. The European Commission has enacted Regulation to prohibit the use of batteries containing cadmium in certain industries.

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