I think I for some applications most if not all these disadvantages can be null
I'll post the entire article and leave a comment as to what circumstance negates the con
Disadvantages of Nickel Iron Batteries
22 November 2017
Category:
Renewable Energy
Nickel-Iron (Ni-Fe) batteries, also known as Nickel-Alkaline or Edison batteries are rechargeable batteries with a long life expectancy, high Depth of Discharge (DoD) and a reputation for durability. The battery can withstand overcharge, overdischarge and short-circuiting and yet last 20 years or more.
The disadvantages however outweigh the advantages.
Cost:
The initial cost is at least 30% over a high-end Lead Acid battery of comparable size (considering usable energy) and also still a lot dearer than Lithium batteries.
If you can build your own using common materials, it may be possible that they are far cheaper. I will have to get price quotes from pottery store and metal suppliers, but I expect it could be a fraction of the cost. For people on a set budget and strong DIY means, this could make solar an option when otherwise it may not be. Needs more investigation.
Efficiency:
Nickel-Iron batteries have lower energy density and lower specific power compared to lead-acid batteries (or in layman's terms are less efficient). The cells take a charge slowly, and give it up slowly (cannot supply sudden large power spikes). This means one would need more batteries and more solar panels to achieve the output of a 'standard' lead-acid based power system. In addition, Ni-Fe batteries have a significant self-discharge rate of 1% per day.
I'm assuming that home built battery will be substantially cheaper, and as a result you may be able to build a very large one, so capacity can increase a lot if you are able to build on a large scale with cheap materials. The fact that they have low energy density is irrelevant in stationary application where you have enough space to accomodate the cells. Low specific power is also not a concern if you are building a large capacity bank. The overall power you can draw increases as you increase the capacity, so you make up for the specific power being lower. In this type of application we aren't really concerned about weight.
Ventilation:
They produce a lot of hydrogen, daily gassing is required to get the expected performance. Hydrogen gas is explosive, therefore good ventilation is imperative.
The type of application of have in mind is stationary, and would have the cells housed in a shed or a very simple roofed structure. Simply ensure there is plenty of ventilation for the structure. There is no need to prevent cold temperatues.
Compatibility:
The characteristics of Ni-Fe batteries are not supported by most solar equipment. The voltage window is so wide that standard inverters are likely to shut themselves down well before the battery is fully discharged. Hence claims like "100% usable capacity" are exaggerated which will further add to cost, size and maintenance.
This is the biggest real concern. We may be able to deplete the battery to 0v but this is no use if we require AC power from an inverter that cuts off at 10V. I am interested to finding out what percentage of the capacity is calculated from within the 10v to 14v range or essentially what fraction of the capacity is able to be converted to AC power by a standard DC to AC power inverter. Something like a graph of Capacity vs DOD would be helpful.
Size and weight:
The cell voltage is 1.2V, so you need 40 cells to form a 48V battery bank. Even smaller battery banks easily weigh a ton. This further adds to the cost for shipping, storage (battery box/room dimensions), installation and maintenance.
It would not be feasible to ship these cells filled with electrolyte, but in a DIY manner the materials relatively light to ship. The bulk of the weight coming from steel and water added in electrolyte
Maintenance:
Most batteries sold these days are classified "maintenance-free". Wet cell Lead Acid batteries are still available and possibly even better than dry cell technology but RPC does not recommend them as time has shown that at some point maintenance will be neglected. While Ni-Fe batteries may withstand such treatments without damage, their performance will most certainly drop. Ni-Fe batteries should be checked and topped up weekly. Inspecting and filling 40 cells is time consuming and tedious. Furthermore, every so often the electrolyte solution has to be completely replaced - a messy and labour intensive exercise.
If build well these batteries are said to need electroyte replacement every 10 years. That's fine. Making a project to deconstruct, clean, and refill the electrolyte every ten years is less of an expense to purchasing new batteries every 10 years(or more). The fact that they can be put back into service after 10 year maintenance is actually a plus. Also, while they do require a lot of topping up, this can be made simpler by allowing a larger headspace above the cells. If you have plates 10 inches tall, and have electrolyte filled to 20 inches, you have a 10 inch of headroom before you really need to top up electrolyte, so you could easily design so that topping up is done only every few months. This is common for lead acid anyways and is as simple as pouring in some distilled water. No big deal.
Conclusion
While not as bad as
Ni-Cd batteries, RPC strongly advises against Nickel Iron batteries for home solar systems. The initial cost is unlikely to pay off unless maintenance is conducted meticulously - for decades. If you need high quality deep cycle batteries, take a look at these
Lead Acid or Lithium batteries.
To be clear I am not advocating anyone use this cell technology. I am just suggesting that there may be potential here for people in specific circumstances. A tried and tested method for constructing DIY nickel iron batteries that is feasible still needs to be developed. I'm suggesting that if someone with a strong interest in pioneering this were to develop a good method, that it may be a valuable thing for people in the future. 
Every Watt matters here since I don't have enough sunlight to even make it through worst winter even with my 10kW array.
