[madaxe_again]

That’s because you get ~2.1v out of a single charged lead acid cell, and an old tractor battery contains three. Your car battery has six.

So the real answer to “why” is because of the electrochemistry of lead.

Kinda similar to how a lot of our world is structured around the dimensions of two horses side by side.

[foobarbecue]

Also the electrochemistry of sulfuric acid (the other half-cell)

[psychphysic]

It'd be very hitchhiker-esque if OP got the answer because 3x(1.69 - -0.36) ~ 6.3