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Nickel Iron Batteries for Solar PV Systems

PV systems rely on batteries to store the energy generated by solar panels and deliver it during peak demand, low production situations.

Several energy storage systems exist for solar power and it can be challenging to pick the right one.

We understand this and want to help you find the best storage option so that you can get the most from your photovoltaic system. Throughout this article, we examine nickel-iron batteries and their suitability for use in solar power systems.

Interested in finding out if this is the right energy storage solution for your new or existing solar PV system?

We’ve compiled everything you need to know about nickel-iron batteries in this article.

What is a Nickel Iron Battery?

A nickel-iron (NiFe) battery is a flooded battery that stores electricity through oxidation and reduction reactions.

A battery that is ‘flooded’ is one that has a flowing electrolyte. Nickel iron batteries usually contain an alkali electrolyte and water as the aqueous components.

Nickel iron batteries were invented in the early 1900s– they were among the first batteries to be used in electric cars. The use of these batteries grew rapidly in the 20th century, but their scope expanded into renewable energy in the 21st century.

Today, nickel-iron batteries are being integrated into solar PV systems to store solar energy for standby and off-grid use.

Components of Nickel Iron Batteries

Negative plate

The negative plate in NiFe batteries is iron oxide. This material is contained in rectangular, nickel-plated steel tubes (retainer pockets).

To provide contact of the active material to the plate, the tubes are perforated on the sides. The retainer pockets are also pressed during mounting to provide contact to the grid,

Positive plate

NiFe batteries use nickel hydrate as the positive plate.

The nickel hydrate is packed into steel tubes with nickel flakes in alternating layers to make the positive plate. The role of the nickel flakes is to improve the contact of the nickel hydrate with the steel tubes improving conductivity.

Electrolyte

The electrolyte can be potassium hydroxide or a combination of potassium hydroxide and lithium hydroxide.

A potassium hydroxide-lithium hydroxide electrolyte is preferred as it offers the following benefits:

  • Extended cycle life (10,000+ cycles)
  • Great energy efficiency, and
  • Wide operating temperature range.

Battery cells

This is where the chemical energy in nickel-iron batteries is stored waiting for conversion to electric energy. Several cells are connected to make a battery unit of the needed voltage. Battery cells combinations may yield voltages such as 12V, 24V, 48V, etc.

The standard nominal voltage for most nickel-iron battery single cells is 1.2V.

Battery enclosure

Charging nickel-iron batteries release hydrogen gas which is a combustible gas. An enclosure helps contain this gas and vent it safely.

So, when buying your nickel-iron batteries, you should make sure they come with enclosure battery boxes. A vent fan should definitely be part of the enclosure and the top of the battery container should have a cap or valve for water refilling and electrolyte top-up.

Tip: You always keep your NiFe batteries far from other electronics and ignition points to avoid fire risks.

Battery busbars

Battery busbars are the components you use to interconnect several nickel-iron batteries to each other.

While these components may not come with your battery unit, you may need them when you want to scale up your solar energy storage.

How do Nickel Iron Batteries Work?

Charging and discharging NiFe is basically moving oxygen from one plate to the other.

Charging

During charging, iron oxide on the negative plate is reduced to metal iron. The released oxygen moves to the positive plate where it oxidizes nickel raising its oxidation state to nickel III.

On a fully charged nickel-iron battery, the positive plate is said to be super oxidized.

Discharging

Before discharging begins, the iron at the negative plate is in a spongy deoxidized state– Fe(OH2).

During discharging, the positive plate deoxidize, releasing oxygen. The oxygen travels to the negative plate and oxidizes the spongy iron.

Reaction at the positive plate:

2 NiO(OH) + 2 H2O + 2 e− ⇔ 2 Ni(OH)2 + 2 OH−

Reaction at the negative plate:

Fe + 2 OH− ⇔ Fe(OH)2 + 2 e−

Left to right reactions represents discharging, while the right to left reactions represents charging.

In the charging and discharging cycles, water is electrolyzed, releasing oxygen and hydrogen gases. Nickel iron battery hydrogen should be vented from the battery unit since it’s ignitable.

Nickel Iron Batteries: Characteristics/ PV Suitability Analysis

NiFe Battery Overview

Specific energy19-25 Wh/kg
Power density65-90 Wh/kg
Energy density20-45 Wh/kg
Self-discharge rate20-30% per month
Nominal cell voltage1.2V
Cycle life10,000+
Lifespan30 years
Temperature range-40°F to 114.8°F (-40°C to 46°C)
Battery cost$0.09 per kWh

Let’s discuss some of these crucial factors below.

Tolerance to overcharge

You might be surprised to learn that nickel-iron batteries perform better when they are charged aggressively.

Well, this makes nickel-iron batteries ideal for solar applications where solar power is produced intermittently, and storage is needed to be done in the same way.

Tolerance to over-discharge

The over-discharge performance of nickel-iron batteries is among the best among energy storage systems. A discharge rate as high as 100% is still fine for these batteries and has a low chance of affecting their efficiency and/or cycle life.

Even so, it’s good to know that while you can completely discharge your nickel-iron batteries, manufacturers recommend that you go up to 80%.

Operating temperature range

The average operating temperature range of nickel-iron batteries is -40°F to 114.8°F. Nickel iron batteries are more suitable for solar energy storage since they can tolerate temperature fluctuations.

Please note: Extremely low temperatures may slow down nickel-iron batteries, but apparently, not much solar power is produced in cold temperatures.

Inverter compatibility

Nickel iron batteries are compatible with most solar inverters in the market today. This means you shouldn’t find problems integrating these batteries in your solar PV system.

It’s still a good idea to inquire about the inverter compatibility of the specific brand of nickel-iron batteries you intend to purchase to be sure it will function with your existing system.

Cost

Nickel iron batteries aren’t the cheapest to manufacture, even though they are a fairly developed battery technology. The good thing is, these batteries offer long cycle life, so they compensate for this cost.

Taking into account the lifespan, nickel-iron batteries should cost around $0.09 per kWh.

Pricing obviously varies with the amp hour rating and voltage of the battery. Here are some typical examples:

  • A 24V, 100 Amp-hour nickel-iron battery costs around $2000
  • A 48V, 100 Amp-hour nickel-iron battery costs around $4000
  • A 48V, 200 Amp-hour nickel-iron battery costs around $9000

When you install solar panels, it’s obvious that you spend quite a lot of money. Thus, you wouldn’t want to dig deeper into your pockets to invest in energy storage systems. Therefore, you may want to look for reasonably priced nickel-iron batteries that fit your budget.

Manufacturers

Nickel iron batteries are available from most battery manufacturers, but you should do your research to find the best one. Nickel iron batteries from reputable manufacturers are of good quality and come with stellar after-sale services. Just a few customer reviews will tell you whether to trust a manufacturer.

Also worth mentioning is that nickel-iron batteries aren’t priced the same. So, you should compare a few prices and choose one that suits your needs.

Lifespan

Nickel iron batteries are extremely durable and can last for 30 years, so they last for roughly the same amount of time as solar PV systems.

Nickel iron batteries have a long lifespan thanks to stable and non-degradable nickel plates that do not change state or dissolve into the alkali electrolyte.

The electrolyte doesn’t undergo a chemical reaction during charging or discharging, so it also remains unchanged for a long time.

With a lifespan of 30 years, nickel-iron batteries usually have a warranty of at least 10 years.

Pros of Nickel Iron Batteries

Exceptional performance even with extreme charging/discharging

NiFe batteries can be charged up to 118% of their rated capacity, so a loss in one of the cells has little impact on the battery’s capacity.

In the same way, the battery likes to be overworked during discharging, so the more load you connect to it, the better its performance.

Nonetheless, nickel-iron batteries shouldn’t be left sitting around for too long without being used.

High operational safety

Nickel-iron batteries do not contain any acidic materials or lead; therefore, therefore they’re entirely safe to use. Nevertheless, the gaseous products of electrochemical processes must be appropriately vented to prevent fires.

Reliable and dependable

Whether you need solar power for stationary use or for on-the-go power, nickel-iron batteries are a perfect choice.

Although slow, NiFe batteries will capture solar power from your MPPT and provide the power on demand. Furthermore, these energy storage systems are resistant to temperature fluctuations, so they can be used in a wide range of climates and regions.

Materials are widely available

To be effective, solar PV batteries must be made from widely available materials.

Nickel and iron are abundant materials, and their manufacturing technology is well-established and optimized for cost savings. At least with these batteries, the manufacturing process won’t be hindered by depleting materials anytime soon.

Cons of Nickel Iron Batteries

Relatively pricey battery technology

Nickel-iron batteries aren’t the most costly solar batteries, but they can be pricier compared to conventional energy storage systems.

These batteries come with substantial upfront costs and can prove to be expensive if you’re looking to install them as the only storage system for your solar PV system.

The good thing is, NiFe batteries have a decent lifespan, and you can be assured you’ll get all the money you invest in them before the end of their useful life.

High weight to charge ratio

Nickel iron battery cells have a nominal voltage of 1.2 volts. Therefore, you would need to stack a lot of these cells together to achieve an average battery capacity. Due to their heavier weight, nickel-iron batteries are used primarily in stationary applications.

NiFe batteries’ high weight to charge ratio also complicates other processes, such as transportation and installation.

Low energy and specific energy

NiFe batteries fail when it comes to storing energy. These batteries have a slow charging rate and slow delivery of the stored power.

To have standard storage and backup solution for your solar PV system, you need several nickel-iron batteries due to these characteristics.

The energy inefficiency coupled with the high weight to charge ratio makes nickel-iron batteries inconvenient for large-scale applications.

Frequently Asked Questions

Are nickel-iron batteries suitable for solar PV systems?

Because of the deep charge and discharge characteristic, wide temperature range, and assured operational safety, Nife batteries are suitable for solar PV systems. The initial cost of these batteries should not put you off since you have at least 30 years to use them.

As with solar panels, NiFe materials are still being improved, and their cost and overall performance will likely improve in the future.

What are the maintenance practices for nickel-iron batteries?

The following maintenance practices will enable your nickel-iron batteries to continue to perform optimally:

  • Replace the electrolyte every 10 years – The electrolyte degrades over time due to carbonate buildup, so replacing it every 10 years can help recover this loss.
  • Add distilled water to your batteries every 1-3 months- Water in nickel-iron batteries is also lost during electrolysis as oxygen and hydrogen.
  • Charge and discharge the battery regularly- Nickel iron batteries are designed for intense cycling, so you should always be charging and discharging them.

Can nickel-iron batteries be used on a daily basis?

NiFe batteries perform better when they’re cycled daily. Additionally, they’re tolerant to varying depths of charge and discharge, so you don’t have to worry about the power output coming from your MPPT or the load consumption.

Conclusion

Nickel iron batteries have been around for more than a century now, and their development has resulted in reliable, robust, and long-lasting solar PV storage systems.

Nickel iron batteries are among the best options for solar energy today, and they are well worth the investment, as shown in this article.

As with any other energy storage system, nickel-iron batteries can have some drawbacks, like high costs and low specific energy, but these disadvantages are outweighed by their benefits.

Nickel iron batteries are a leap ahead of other solar batteries, so they’re worth considering if you’re in the market for a solar PV energy storage solution.