Electrical – CV or CC battery charging via SMPS module

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.

You're confused about charging times while you should not be. It is actually quite simple because you're using a SLA battery. You can charge such a battery simply until it reaches 13.8 V. This can be done simply by connecting it to a 13.8 V voltage source, which is your SMPS.

Do make sure there is some form of current limiting in place so that the current does not get out too large when the battery is empty and 11 V for example. I expect your SMPS to have proper current limiting but if it current limits at a high current, check that the battery is allowed to be charged with such a current. If not you may want to add a power resistor between the SMPS and the battery to limit the current.

When the battery is full it's voltage will be around 13.8 V and charging will stop automatically because battery and source will be 13.8 V. There is no need to keep track of charging time. Most Uninterruptible power supplies have SLA batteries and they charge them this way because it is simple and effective.