Lead Acid batteries with 6 cells is 14.2v max. (Car Batteries).
The 12v is the nominal voltage. All battery chemisties you can over charge.
But saying 12v is easier than saying 14.2v.
10v is flat any less than that then all the cells are borked and will not work or there is a bad cell.
Alternator should probably put out a little more than 14.2v to take into account DC losses through resistance in the wires.
Lead batteries are old **** really. Heavy, not very powerful. Lose charge over time and have about a 2-3 year shelf life.
The only reason they last over that is because they fit huge ones to cars and they are used daily.
But there cheap and easy to manufacture.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts