Disadvantages of Nickel Iron Batteries
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.
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.
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.
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 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.
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.
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.
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.