Use of Nickel Iron Batteries with Solar Power Systems



 

Purpose of this Information:

Whilst Nickel Iron (NiFe) batteries have been around for well over a century, they have not been widely used for solar power applications. In fact only in recent years they have been “re-discovered” after this technology had been forgotten after the original and last factory closed in 1979.

Whilst there are many battery types out there, most rely on nasty acid or harmful chemicals. Further to that many types suffer when exposed to temperatures above 35C°,  freezing conditions, and / or deep discharge.  

So after being involved in solar power technology for over 30 years and replacing my Battery Bank with Nickel Iron batteries I’ve decided to pass on my gained knowledge.


 

System Setup and Specs:

Why Nickel Iron Batteries?
 


 

Disadvantages of Nickel Iron Batteries:


 

Detailed Findings:

At this stage had Nickel Iron batteries in use for less than 4 years. Hence not all the details are known yet nor can any remarks be made in regards to lifetime under such usage conditions.

However, it is a well known fact when it comes to Nickel Iron batteries that in the inevitable event of poor performance the electrolyte simply needs to be replaced rather than the whole battery. This of course much cheaper and much more environmentally friendly then replacing a whole battery bank.


 

Below information is based on 24V system with 600Ah batteries:

As mentioned in the specs the Nickel Iron Batteries have a large voltage range. Hence finding / using suitable equipment can be challenging. The equalisation charge can be as high as 34V (or even slightly higher if required), yet the low voltage point can be around 19V.

The other challenge is that the batteries appear to have a memory effect. So in daily usage it was found if the batteries had been charged with only C5 (30A) the voltage would rather drop quickly when a current draw of over 100A was required.

On the other hand if a larger charge current was used – i.e. C10 (60A) then a large current drawn from the batteries would only cause a slow voltage drop.

So in practical terms and in my experience it appeared after regular slow & low charging (charging via solar panels at around 30A) the system would drop out due to low battery voltage at around 80% when a current of over 100A was required. (Given that SP-Pro will turn off when voltage drops to below 20V. - This is much better than the usual 22V limit most other inverters have.)

But if batteries been charged with 60A, then a 100A current drawn from batteries wasn’t a problem and could be maintained at short duration even when battery was as low as 65% SOC.

With lesser currents (not exceeding 30A ) the batteries could even be used until a level of around 50% SOC.

Below is a graph indicating slightly higher low battery voltage after a higher charge current. (Yellow line)  Thus preventing early inverter shutdown due to low voltage.

Battery Performance Graph
 

Conclusion/ Recommendation:


When using Nickel Iron Batteries for Solar Power application the following points should be considered:

  1. Can the system cope with the unusual voltage range ?

  2. What is the relationship of intended battery capacity and available charge current?

  3. When calculating required battery capacity consider the effects of voltage drop as well as required usable battery storage.

  4. When a larger charge current (at least C10) can not be achieved then a larger capacity battery bank should be chosen to prevent fast voltage drop under load.


 

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Copyright: Karl Schoelpple 2020