You cannot charge the lead-acid battery well, using 12V.
For the battery to be fully charged, you need to bring it up to "float" voltage which is about at least 13.7V.
There are various algorithms of charging that employ constant current, or constant voltage, or both, and then trickle-charge/floating of fully charged battery. But all of them eventually will end at about 13.7-13.9V to float the battery.
So, if using 12V, you will only partly charge the battery, and this eventually is detrimental to its life/quality.