I recently found this interesting article about Nickle-Iron batteries and wanted to write my notes down for future reference.

Comparison of battery chemistries

  • Lead Acid Batteries: cheap but degraded by high temperatures and being stored in partially discharged state (you shouldn’t fully discharge it).
  • Lithium Ion Batteries: expensive and degraded by high temperatures, deep cycling, and when stored at a high state of charge (keep your Li-ion powerbanks at 50% for long term storage).
  • Nickle-Iron Batteries: sort of expensive (more expensive than lead acid, less expensive than lithium ion), low specific energy (4-10x less energy storage per weight than lithium ion), low round trip efficiency (50-80% VS Li-Ion at 80-90%) and high discharge rate (~20-30% per month) which increases at high temperature. But ultra durable (20-25 year avg service life, max service life up to 50 yrs). Nickle-Iron batteries are very tolerant of abuse (overcharge, overdischarge, short-circuiting).

Mini grids

So if you want to go off-grid (maybe you live in Texas and are fed up with their stupid republican leadership) the most expensive part of your system is the battery. Also it’s scary because condensing that much energy into a small location means fire risk, and if some cells in your power wall decides to malfunction after 10 years, you better have money to pay up for the next version / a replacement.

Ni-Fe batteries, aside from being very robust and abuse tolerant, turn out to have another interesting property: you can use them to do electrolysis (you can use the battery as an electrolyser). While you’re charging the Ni-Fe battery, charge is divided between charging the cell and producing gases. Once they’re fully charged, all the charge is used solely for electrolysis / making hydrogen & oxygen (if you’re electrolyzing water; you can electrolyze other liquids too though).

One use case here is if you have a solar or wind farm – by definition you generate very peaky energy. You could save some of that electricity (say enough for most of your current demand) in the batteries, then turn the overflow energy into hydrogen through electrolysis.

The authors of the article call this combination battery/electrolyzer a “battolyser” and say if you run a mini-grid you can use the hydrogen for things like cooking & heating, and the battery electricity for lighting and electronics. Hydrogen is a sustainable alternative to LP gas for cooking.