Because the air within a submarine provides buoyancy, the large mass of the lead in lead-acid batteries helps pull the vessel underwater when it is time to submerge. It is possible to use lighter weight batteries, but likely the submarine would then still need lead ballast to help maintain its neutral buoyancy when submerged.
Advances have occurred in lead-acid battery technology to increase storage density, extend usable service life and improve cold weather performance at comparatively lower cost than modern battery technologies. As a result, derivatives of lead-acid battery technology still have multiple applications in modern boating.
In compact, limited volume of a sub. Safety: Unlike lithium or any light metal, lead is non-flammable, thus poses no issue of flammability aboard. Cost: Lead acid batteries are absurdly cheaper, 3x less than Lithium.
Lead-acid batteries are the heritage batteries used in nuclear powered naval submarines. Figure 1 shows a U-boat lead acid battery. Although, they have low energy density they are mature in technology and cost considerably less than lithium-ion alternatives.
Two main types of battery are used on board ship: the lead—acid and the alkaline type, together with various circuits and control gear. The lead - acid battery is made up of a series of cells. One cell consists of a lead peroxide positive plate and a lead negative plate both immersed in a dilute sulphuric acid solution.
Lead Acid batteries have been around far longer than most alternatives - there were no lithium batteries when submarines relied so heavily on batteries. But lead acid batteries also tolerate abuse quite well. The Wikipedia article on the Royal Navy's first submarine, HMS Holland illustrates this.