What does cold start mean on a UPS?

Normally a UPS switches on when mains voltage is present, and the battery takes over only if the mains is lost later. With cold start (or DC start) you can switch it on directly from the battery, even when there is no power at the socket at all.

This is useful in specific scenarios. For example, if the outage happened while the UPS was off and you need to save files or shut down a server properly, or if you want to power a router or an alarm system for a while until the power returns. On many models you start it by holding down the power button for a few seconds; the exact procedure is described in the manual.

Practical tip: the available time depends on how charged the battery is. If the UPS has already covered a long outage, there may be almost no energy left. A common mistake is repeatedly cold-starting while the battery is nearly empty, because deep discharge stresses lead-acid batteries. Once the mains returns, let the UPS charge fully before relying on it again.