Are Nickel-Cadmium batteries good for solar PV systems?
Batteries are an essential component of any solar PV system. Because they store the energy produced by solar panels for use when the sun is not available. Therefore, selecting the right batteries for your solar PV system is critical.
The most common types of batteries used in solar PV systems are lead-acid and lithium-ion batteries. Although these batteries have their own use cases and benefits, new battery technologies have been developed over the past decade.
One of these technologies is nickel-cadmium batteries, which have a long history of use in other applications such as consumer electronics. Nickel-cadmium batteries have recently begun to be used in solar PV systems.
It is possible to use Nickel-Cadmium (Ni-Cd) batteries in solar PV systems. There are companies manufacturing Nickel-Cadmium batteries specific to solar PV systems.
However, is it the best choice? Should you be them instead of lead-acid or lithium-ion batteries?
Here is what you should know in short:
Nickel-Cadmium batteries have traditionally poor charging efficiency and require a higher voltage charger, which is more expensive. They are subject to memory effect, are difficult to define the state of charging, and have safe disposal restrictions that increase the initial costs.
Generally speaking, Nickel-Cadmium batteries are not designed for solar PV systems.
However, more and more companies are working on this battery technology to make it suitable for solar use. Assuming Nickel-Cadmium batteries can be used only for consumer electronics would be a mistake.
Because there are certain characteristics of Nickel-Cadmium batteries superior to lead-acid batteries that really make us think if we should revisit our opinions about their use in solar PV systems.
How does a Nickel-cadmium battery (Ni-Cd) work?
A nickel-cadmium battery converts chemical energy to electrical energy when discharged and electricity back to chemical energy when recharged.
In a fully depleted NiCd battery, the cathode contains nickel hydroxide [Ni(OH)2], whereas the anode has cadmium hydroxide [Cd(OH)2]. The chemical make-up of the cathode changes when the battery is charged, and nickel oxyhydroxide forms from nickel hydroxide. Cadmium hydroxide is converted into cadmium in the anode. As the battery discharges, the process is reversed, as stated in the following formula.
Cd + 2H2O + 2NiOOH —> 2Ni(OH)2 + Cd(OH)2
Nickel-cadmium batteries are less efficient than lead-acid batteries, but they have greater mechanical resistance makes them useful for many applications that lead-acid batteries are not.
Using Nickel-Cadmium batteries for solar PV systems
Constant current
Nickel-Cadmium batteries provide a constant current during the whole cycle. They are ideal for uninterruptible power supply and generator starting applications.
In fact, the world's second most powerful battery bank is a NiCd type at Golden Valley, Fairbanks, Alaska. In an electrical-island operation mode, the 27 MW rated battery at this station can supply 40 MW for 7 minutes and is used for spinning reserves and grid stabilization.
Battery Memory Effect
The battery memory effect, also known as the lazy battery effect occurs when a battery is frequently charged before all of its stored electricity has been used up. As a result, the battery will remember its shorter lifecycle and may not be used to its full potential in the future.
Ni-Cd batteries have a battery memory effect problem. If you use a solar PV system with Ni-Cd batteries, the amount of power that can be stored in a battery will be gradually reduced over time.
Unless you discharge the battery every time down to zero charging level (which is very impractical for solar PV applications) a certain part of the battery will die over time.
Low charging efficiency
The charging efficiency of Ni-Cd batteries is low. This means that a lot of energy can be lost during the charging process, and it takes longer to charge them as well.
Charging efficiency is calculated using the below formula:
Charging efficiency = (Energy input – Energy output) / Energy input
Most lead-acid batteries have a charging efficiency of 85% – 90%, whereas Ni-Cd batteries have a charging efficiency of 70% – 75%, indicating that a significant amount of energy is lost during the charging cycle.
Number of cycles
Nickel-cadmium batteries have a long operational life, as determined by the number of charge/discharge cycles or years of service life.
Nickel-cadmium cells have a long, trouble-free life, whether they are actively utilized by charging and discharging repeatedly or are kept on charge in a ready-to-serve condition. Sealed nickel-cadmium batteries will typically provide hundreds of charge/discharge cycles or run in a ready-to-serve standby mode for many years.
The energy density of a nickel-cadmium battery is 50 Wh/kg, whereas that of a lead-acid battery is 40 Wh/kg. Also, a nickel-cadmium battery can reach up to 2000 cycles at 80% discharge, whereas a lead-acid battery can only reach up to 1800 cycles. It is very common for a nickel-cadmium battery to achieve 8000 cycles at 15% depth of discharge.
NiCd batteries can technically be stored in a fully discharged condition for years and yet function normally. However, their lengthy lifespan requires adequate and regular care, which adds to the cost.
Charging difficulty
NiCad batteries are among the most difficult to charge. Unlike lithium-ion and lead-acid batteries, which may be controlled by simply setting a maximum charge voltage, nickel-based batteries do not have a “float charge” voltage.
As a result, charging is accomplished by driving the current into the battery. The voltage required for this is not set in stone, as it is with the other batteries.
This makes charging these cells and batteries in parallel extremely challenging. This is due to the fact that you cannot be certain that each cell or pack has the same impedance (or resistance), and hence some will accept more current than others even when they are full.
What that means is you have to use a separate charging circuit for each string in a parallel pack, or balance the current in another way, such as by utilizing resistors with a high enough resistance to dominate the current control.
Tolerance to overcharging
Nickel-Cadmium batteries are very tolerant to overcharging. This is one of the aspects that they are superior to most other battery types, which are irreversibly damaged by overcharging.
If you would overcharge lead-acid batteries, they would release explosive hydrogen gas, and if not monitored, a fire or explosion can start. However, this is less likely the case with nickel-cadmium batteries.
The most efficient way to charge a nickel-cadmium battery is to charge it at C/10 (10% of the rated capacity each hour) for 16 hours.
For example, a 100 mA battery would be charged at 10 milliamperes for 16 hours. This method does not require an end-of-charge sensor and guarantees a full charge.
Cells may be charged at this rate regardless of their pre-existing charge condition. The minimum voltage required to get a full charge varies with temperature, but it must be at least 1.41 volts per cell by 20 degrees Celsius. It is recommended to use a timer to prevent overcharging for more than than 16 hours.
Some nickel-cadmium batteries are designed to be “quickly chargeable.” This is a timed charge at C/3 for 5 hours or C/5 for 8 hours, respectively.
This technique might lead to difficulties since the battery should be entirely discharged before it can be recharged.
If the battery is still at 90% capacity when the timer starts, you have a high chance of venting it. One approach to avoid this is to have the charger automatically drain the battery to 1 volt per cell, then switch on the charger for 5 hours. This technique is useful because it eliminates the possibility of battery memory.
Stability in the deep discharge curve
Nickel-Cadmium batteries have a very flat voltage profile during their usage cycle, which makes them great for off-grid solar PV systems.
Because they provide more even power production over time. In addition, when they are discharged below their nominal voltage, they are much more stable than lead-acid batteries.
Since nickel-cadmium batteries have very little self-discharge current when not in use, it is possible to store them for longer without having to recharge them.
Battery cost
Because of the high cost of Ni and Cd, Ni-Cd is significantly more expensive than lead-acid. Therefore, Ni-Cd is expected to create no threat for lead-acid batteries for the foreseeable future.
A typical nickel-cadmium battery has an energy density of around 50 Wh/kg, whereas a lead-acid battery has an energy density of around 40 Wh/kg.
Difficult to determine the state of battery charge
The way Ni-Cd batteries charge and discharge is more complicated than lead-acid batteries. It is also harder to determine the state of charge for a Ni-Cd battery.
This is due to the fact that, unlike lead-acid batteries whose voltage drops quickly during discharge and rises slowly while charging, nickel cadmium’s voltage remains almost constant until it gets very low.
The detection of full charge in sealed nickel-based batteries is more complicated than in lead-acid and lithium-ion batteries. Low-cost chargers frequently employ temperature sensors to terminate the fast charge, which can be unreliable.
The center of a cell is several degrees warmer than the skin, where the temperature is monitored, and the resulting delay causes over-charge. The temperature cut-off for chargers is 50°C (122°F).
However, a modest overshoot is fine as long as the battery temperature drops immediately when the “ready” light goes on.
Requires a charger with a higher voltage
Because of the higher resistance, Ni-Cd batteries need a higher voltage charger (compared to a lead-acid battery charger).
While these chargers are still available, they are becoming increasingly expensive. A fully charged 12V battery bank can easily reach 17V, causing inverters to shut down or, worse, DC lamps to blow.
Temperature tolerance
Because nickel-cadmium batteries are very resistant to extreme temperatures, they are an excellent choice for applications requiring a constant power supply in low or high temperatures.
A typical Nickel-Cadmium battery can work in temperatures ranging from -20 °C to +50 °C (but can withstand extreme temperatures ranging from -50 °C to +70 °C) with no significant loss of performance or capacity.
No hazardous fumes
Ni-Cd batteries don't release hazardous fumes like lead-acid batteries do when they are being charged or discharged, so they can be safely used indoors. Although it is less of a concern today due to lithium-ion batteries, it is still worth mentioning.
Environmental Impact
Nickel-Cadmium batteries were invented in the 19th century, well before environmental concerns existed. Nowadays, due to legislation that requires the recycling of both nickel and cadmium products at the end of their life cycle, Ni-Cd batteries are considered hazardous waste.
Cadmium is extremely toxic to the environment, and cannot be disposed of simply by throwing it in the trash. It is a known carcinogen and must be disposed of through a specialized recycling process.
Although nickel is abundant in the environment, its functional role as a trace element for animals and humans has yet to be discovered. Nickel exposure can result in a number of negative health impacts both for animals and humans.
Whilst recycling Ni-Cd batteries are encouraged on both an environmental and health level, the process of recycling is so specialized that there are very few companies who can actually recycle Ni-Cd batteries. Also, most Ni-Cd batteries around the world still end up in landfills.
Also recycling facilities processing Ni-Cd batteries can only recycle up to a certain percentage while significantly increasing the carbon footprint through recycling.
Ni-Cd batteries have long been regulated in developed countries. In the United States, part of the battery price is a fee for its proper disposal at the end of its service life. Also, they can only be utilized for a few specific applications in the European Union.
NiCd battery sales fell between 1995 and 2003. This is both due to the increased environmental controls for toxic cadmium, such as the European Union's directive on batteries and accumulators in 2006, which banned NiCd batteries in September 2008, or to new battery developments that do not justify the cost of NiCd batteries (1,000 /kWh) for specific applications.
Conclusion
Nickel-Cadmium batteries offer certain great features like constant voltage at the low level of discharge and high power density.
However, they also have major disadvantages such as memory effect, need for regular maintenance, and specialized charging equipment, the difficulty of monitoring the charging state, and toxic cadmium content.
Although some companies manufacturing Nickel-Cadmium batteries and claim their products to be “maintenance-free”, and a better alternative to lead-acid and lithium-ion batteries, it is still not the best solution for solar PV systems.
Therefore, using Nickel-Cadmium batteries in solar power systems is not a good idea unless a new emerging Ni-Cd technology in the future can resolve all of these issues.
