The cell phone in your pocket has become the default communication device in our modern era. No longer does the patchwork of phone cables to the handset in your hand carry the signal to the person on the other end. OR DOES IT? Cell phone towers, those funny looking trees, antennaes on the sides of tall buildings, and towers a top mountains in every neighborhood around the world all run on battery backup systems. Each of these installations has enough battery power on site to run the system for at least several hours. These battery systems ensure adequate communication from your personal, portable, phone to the network of phone cables, fiber channels, and telecom networks the world over.
Similar to the battery backup system under your computer desk, a cell phone tower has the battery systems to take AC power from the power grid, and charge a battery with DC power. By charging a battery at it's "FLOAT" voltage, just like under your desk, the battery is charged at the "FULL" voltage, no more. Because the batteries float, they are always ready in case the charger is removed from the system.
Routinely these batteries are discharged, or exercised, by maintenance staff. Often remotely initiated and monitored, battery exercising requires the battery chargers to be turned off, the batteries to be discharged a bit (say 25% - 30%), and then recharged at the ABSORPTION voltage. The abosorption voltage (higher than the float voltage), forces the batteries to full charge and ensures a long life installation.
When building cell phone towers, engineers often request DC systems, running on 12, 24, 36, or 48 volt systems to reduce the cost of inverting the current from AC - DC to clean, and back to AC for equipment compliance. By eliminating the second conversion, the system is 20%+ more efficient.