I am making an assumption that the battery is of the "SLA" type
these are still "wet" batteries, but the electrolyte is soaked into porous blankets between the plates, which also stops them buckling and shorting against each other and stops the paste crumbling to a heap at the bottom of the cell and shorting the plates that way.....now, somehow the battery has to be filled and vented.....there is normally a cover-plate which is bonded/stuck with a well-concealed joint (sometimes under the brand-label)...Under the cover are a series of "chimneys"..one per cell..... each is closed by a soft vinyl/rubber cap....sliding the cap off, reveals the cell which is probably bone dry adding a few teaspoons of distilled/deionised water, restores conductivity and the correct specific gravity.....in use, overcharging causes the water to decompose ..the oxygen/hydrogen mix will push out past the closure cap at very low pressure and will vent. cooling or any other cause of negative pressure within the cell will just "suck" the cap onto the chimney more tightly.
This drying out means many a serviceable battery is junked....'cos they are sealed units and maintenance-free, innit?
starting off a "slave" battery makes good sense.
deep-cycling a battery not specifically built for the job, shortens it's life....a constant float charge, minimal surface-discharge and immediate recharge will lengthen it's life....conversely, a laboured start followed by the alty cramming in a heavy fast recharge, kill them!