installing a perko battery selector switch

i want to install a second battery this winter but i dont quite understand the idea on how the second battery will tie in. i know on the switch theres 1 , 2, and all, but say theres either one or two selected... does that mean only 1 or 2 will be charging depending on which is selected. and on all both are being used and charged? Will this help the electrical performance having it set at all or is having the second battery just a back up?

If battery 1 is selected, battery 1 will receive the charge. If battery 2 is selected, battery 2 will receive the charge. If all/both is selected, both with receive the charge. You won't see any performance gains by using it on both/all and if you let them run too low with it set to both/all, you've defeated the purpose of having a second battery when you can't start the boat.

Personally, I run battery 1 one day/outing and then battery 2 the next to keep both charged and ready to go if one of them were to fail.