Lithium iron phosphate batteries.

[adam2]

Take care not to confuse lithium IRON with lithium ION batteries, they are very different types of battery but sound similar in the spoken word.

Lithium IRON phosphate batteries are reported to be lower risk than other types of lithium battery and are available in large sizes.
They have a slightly higher voltage than lead acid batteries, the "12 volt" version being about 12.8 volts, a "24 volt" version is available and averages about 25.6 volts. This voltage difference is minor and should be fine for most applications.
They are claimed to last for about 3,000 cycles, and unlike lead acid batteries can fully discharged without shortening the life.
A standby life of ten years is claimed.
They can not be safely charged if very cold, a few types have internal heaters and divert the charging supply into these heaters until the battery is warmed enough to accept a charge. If regularly used, then the small but real losses in the battery should keep it warm enough.

They can be charged from SOME TYPES of lead acid charger, but a purpose made battery charger is recommended.

I have seen prices as low as £3 per ampere hour, but £5 is more typical. The weight is about one third of a lead acid battery for the same capacity.

All the reputable brands of these batteries incorporate a battery management system to protect against over charge, over discharge, over or under temperature and internal faults.

In my view two drawbacks exist. Firstly despite the reported lower risks I still do not trust ANY lithium battery indoors. Secondly the battery is useless without the built in battery management system which seems unlikely to survive an EMP event. There are also restrictions on series connection to obtain higher voltages.

[Ralphw2]

These are the same as LFP (LiFeP) batteries that have been available for a few years now. They are the preferred chemistry in home/domestic solar batteries and are found in an increasing number of EVs including many standard range Teslas, and the new citreon C2 ev. Among their supposed benefits are their tolerance of being fully charged and discharged in each cycle. LiIon batteries are recommended to keep in the 20-80 percent charge level in each charge cycle to maximise the battery life.

Our house battery is LFP