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Alcad, a leading battery manufacturer have introduced a new type of wet nickel cadmium battery.
The "vantex" range. These are similar to the classic flooded alkaline batteries but have the merit of charging at much lower voltages.

A traditional 10 cell alkaline battery needs as much as 17 or 18 volts to fully charge it, higher than that supplied by most battery chargers or charge controllers, and also too high for most 12 volt loads if these are to remain connected during charging.

The new batteries are said to eventually fully charge at 13.9 volts for a 10 cell battery. Faster charging is possible at the moderately higher voltage of 14.4 volts.

They are maintenance free if charged as directed, but are NOT sealed and CAN be topped up with purified water if non optimum charging reduces the electrolyte level.

A shelf life of 2 years, filled and charged is claimed together with a service life of at least 20 years.

These batteries are primarily intended for industrial standby power in cases where cost is secondary and reliability is paramount.
They are hugely expensive.


Not perhaps ideal for domestic RE purposes, but I mention them in the interests of completeness.
Might be worth considering for long term doom.

I have no connection with the suppliers.

Thanks for the information. Like any new product we can expect further development and price reductions over time. The twenty year service life will be very attractive to preppers with the means to have the best.

Flooded nickel cadmium batteries are a mature technology and I am not very hopeful that prices will fall. INCREASES seem likely due to the toxicity of cadmium and therefore growing restrictions on handling and use.

These new cells are a forward step as regards lower charging voltage, the 20 years or longer service life is the norm for this type of cell.

Certainly worth considering as a long term prep, but not in fact ideal for off grid use.

Advantages
20 year life, at least, perhaps double that in good conditions.
Maintenance free under ideal conditions, and reduced maintenance under adverse conditions.
Tolerate much higher and much lower temperatures than lead acid.
The full nominal capacity may be used, unlike lead acid which should not be regularly deep discharged.
Not damaged by storage whilst discharged, within reason.

Disadvantages.
Very expensive.
Very slow to charge at 13.9 volts, might NEVER fully charge from PV with a charge controller set to that voltage.
Not very efficient in terms of charging input to energy delivered on discharge.
The difference between charge and discharge voltage, though reduced in the new cells, is still greater than with lead acid cells.
Don't accept low charge rates. Example a one amp charge into a 500 AH lead acid battery will eventually charge it though this takes weeks.
One amp into a similar sized alkaline battery will achieve nothing whatsoever.