Can a UPS run without batteries?

The battery is what makes a UPS "uninterruptible". Without it, even if the UPS keeps supplying power at its output, a mains outage means the devices shut down immediately, as if they were plugged straight into the wall socket.

Whether a UPS starts or runs without batteries depends on its design. Many small models will not switch on at all if they do not detect a battery. Larger UPS units, especially those with external battery strings, often keep powering the load from the mains so that the battery string can be replaced or serviced without shutting the equipment down. In any case, this situation should be temporary, and the UPS will usually show an alarm.

Practical tip: when the batteries reach the end of their life, replace them as soon as possible rather than letting the UPS run without them or with faulty ones. A common mistake is removing swollen or failed batteries and leaving the UPS in place, which gives a false sense of security. Battery replacement in large UPS units should be done by a trained technician.