13.8V is the nominal voltage used to "maintain" a sealed lead acid (AGM or RG or VRLA) 6-cell battery. Such a battery, when fully charged, will measure ~12.85V at room temperature, 24 hours after being disconnected from the charger, provided it is isolated from any load. During recharging, the voltage can rise to as much as 14.7V.
13.8V is too low to recharge a flooded-cell lead acid battery, and is too high to float the battery for long periods of time. Most commercial chargers float at ~13.2V. Automobile charging systems typically hold the battery at 14.2V to 14.5V while the engine is running.