Charging Lead Acid battery using SMPS - unexpected voltage measured at battery
What you are currently doing exactly opposite of what the article you linked to suggests charging lead-acid batteries.
The article says to use CCCV method which means constant current constant voltage. It means that the charger first limits the charge current to constant predefined level and the charging voltage is at the battery voltage level, until the battery voltage has risen to the predefined voltage level, so the charge current then starts decreasing.
Your SMPS is a constant voltage source, it will try to push as much as current it possibly can into your battery because battery has less voltage. Fortunately the resistance of wires has limited the current because there is voltage drop in them, but the battery or SMPS or wires may not handle the current if it is too much.