Hi,
Check with the battery manufacturer but typical charging voltages for 12V lead acid batteries is between 13.2V and 13.8V.
This 13.2V ... 13.8V is the so called float charge voltage .. just to keep the batteries charged.
But indeed it´s not very healthy if the batteries are not "discharged" from time to time due to their memory effect.
*****
There are many good and reliable informations about how to charge a lead acid battery. Nothing is hidden, everything is free to download.
So most battery manufacturers provide informations on how to charge (and discharge) them. ..depending on their use case.
* for fast charging
* for high lifetime
* for high cycle count
* for high capacity usage
* for high efficiency
* for long "lazy time" like in an USV, where it may be unused for years (kept charged)
* for high current draw like for a car starter
* for different temperatures ...
and so on.
And for sure not every lead acid battery is the same. There are different ones, optimized for the one or the other use case.
So it´s on the designer to decide on what to focus. And according this .. what charging type and parameters to chose.
And to keep the life time high .. you even my discharge the batteries from time to time , maybe even with ratherh igh current pulses.
There are ready to buy chargers. Some are cheap and drity, some are really advanced.
There are dedicated charger ICs
***
If I would design my own one.. it surely would be an advanced one with a microcontroller and switch mode technology. Not only to adjust charging voltage and charging current, but also to discharge the batteries (partly) and to monitor the health state. Also with alarm to change batteries before they are completely dead. And an alarm before they are discharged .. and an undervoltage disconnect for them not to be killed ... and so on.
I personally would not spend my time to build the above charger which wastes a lot of energy into heat ... and has no benefit against simple, cheap, ready to buy ones.
Again: my personal way...
***
But we don´t know why the OP wants to build one instead of buying one.
Klaus