How Does a UPS Battery Backup Work?

A UPS (uninterruptible power supply) sits between the wall outlet and your equipment. While power is normal, it passes electricity through and keeps an internal battery charged. When the power fails, the UPS switches to the battery within milliseconds and uses an inverter to turn stored DC power into the AC your devices need. That gives you time to save work and shut down, or to ride out a short outage.

The Main Parts of a UPS

• Rectifier or charger: converts mains AC to DC to charge the battery.
• Battery: stores energy. Most units use sealed lead-acid, and newer ones use lithium-ion.
• Inverter: converts battery DC into AC power.
• Transfer switch: moves the load between mains and battery.
• Control circuit: monitors voltage, triggers the switch and sounds alarms.

What Happens During a Power Cut

1. The UPS detects a drop or loss of mains voltage.
2. The transfer switch moves the load to the inverter in a few milliseconds, or instantly on an online UPS.
3. The UPS beeps to warn you that it is running on battery.
4. Connected software or the USB or network connection can tell a PC or server to shut down safely.
5. When mains returns, the UPS reconnects and recharges.

Types of UPS

• Standby (offline): simple and low cost, for basic home use.
• Line-interactive: adds voltage regulation, suited to home offices and small business.
• Online (double conversion): always powers the load from the inverter, for servers and critical equipment.

What Does a UPS Protect Against?

A UPS covers blackouts, brownouts (low voltage), surges and spikes, and electrical noise. Different types handle these to different degrees. Protection from data loss and hardware damage is the main benefit.

What Are Back-UPS Units?

Back-UPS is APC's range of home and small office battery backups. The name is often used loosely to mean any battery backup unit. Other ranges such as Smart-UPS target servers and networks.

What Is a UPS Server?

The term usually refers to a UPS that protects servers, or to a UPS with a network management card that lets servers monitor it and shut down automatically. It does not mean a server that provides UPS service.

How Long Will It Run?

Runtime depends on battery size and load. Light loads such as a router can run for hours, while a PC may get 10 to 20 minutes. Check the runtime chart of the model.

Frequently Asked Questions

Can I plug a surge protector into a UPS?

No. Plug devices directly into the UPS outlets. Daisy chaining can overload the circuit.

Does a UPS save electricity?

No. It uses a small amount of power to keep the battery charged and the electronics running.

How do I know my UPS is working?

Run a self-test using the unit button or software, and watch for the replace battery indicator.