Stopping criterion of lead-acid battery charging operation with SMPS
Lead-acid batteries are fairly forgiving.  Unless the battery is quite low, it can be charged by holding its voltage at the "float charge" level.  That is usually 13.6 to 14.0 V for a 12 V (6 cell) battery.  This is basically what is done in automobiles.  When the engine runs, it turns the alternator, which has a regulator around it so that the "12 V" line is really kept at the float charge voltage.

For a really depleated lead-acid battery, you have to be careful to limit its charge current.  Just applying the float charge voltage would cause too much charging current until the battery charge level gets higher.  Since you have your own power supply in there managing things, adding this protection is actually quite easy.  What you want is a supply that is both voltage and current limited.  Make sure that is only puts out the float charge voltage, or the maximum allowed charge current, whichever comes out to less output power.