They don’t have any toxic heavy metals, they last a lifetime, and they could be cheaper than lead acid. Potentially you could even make your own.
It says they can have charge/discharge efficiencies of 85% there. Where are the batteries that can do that, is what I want to know. Who sells them or how do you make them?
If they can only achieve 60-65% that is not as good, but do not write them off altogether. Lead acid batteries are only something like 80% efficiency. So you lose ~40-35% vs. 20% of the energy that went into the battery, but if only half of the energy from the solar panels goes into the battery, with the other half being used as soon as it is produced, using nife instead of LA will increase the size of the solar collector array by only 15 to 20%, which might be altogether manageable.
If the cost of the solar collector is only 50% they system cost that is only 7.5% to 10% increase in cost. Then, remember, these batteries last much much longer than lead acid, which wear out fast enough that they have a substantial “running cost”, of roughly 8% the cost of the battery, per year, whereas NiFe do not.
So all things considered, the capital cost of the system as a whole might be more expensive, might be cheaper, depends on if you can get the better batteries, but by eyeballing it, I think they have a lot of potential to actually be cheaper over the long run.
Plus, no environmentally harmful lead, which is nice, and yet they are I think probably cheaper than lithium ion.
Also, there are sources like this one that indicate that in photovoltaic systems, lead acid batteries have other issues too, and depending on their usage patterns, they may have even lower effective charge/discharge efficiencies.
Some more links:
http://www.changhongbatteries.com/Ni-Fe_battery_for_Solar_&_wind_appliances_p53_m2.2.1.html says charge efficiency is actually quite high at room temp, but is not the same as round trip efficiency, but maybe the loss during discharge is pretty low and determined mainly by resistance. I wonder how much these batteries cost and if they are actually for sale.
BTW just a bonus, I discovered that if you search ebay for “thundersky lithium” there are some relatively cheap lithium iron phosphate batteries going on there.