Lithium-ion battery phosphate (LiFePO4 or LFP) is the safest of the mainstream li-ion battery types.
The nominal voltage cell is 3,2V (lead-acid: 2V/cell). A 12,8V Li-ion battery therefore consists of 4 cells connected in series; and a 25,6V battery consists of 8 cells connected in series.
They required external Victron Energy BMS control