A couple more comments that may be of use to anyone lucky enough to get their hands on NiFe cells:
To reach full charge, they need around 1.65V per cell (16.5V on a 12V battery).
When 'flat', a 12V battery sits at around 10.5V
The only problem we have is that occasionally if the bank is at full charge and holding at or above 15.5V, the inverter says 'Uh-oh! Over voltage!' and fails to start - easily remedied by flicking on a halogen (introducing a small load) which makes the voltage dip to a point where the inverter is happy.
And finally, but most important - buy yourself a new hydrometer and mark it clearly for use only with the NiFe cells.
The electrolyte is potassium chloride - you really don't want to mix even a tiny bit with sulphuric acid from a lead-acid cell (or vice versa).
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My main reason for getting them in the first place is that nickle and iron are more environmentally benign than lead, cadmium etc.