Log Entry // July 11, 2026

sysadmindiy

Replacing Batteries in a Rackmount UPS Backup System

A complete step-by-step guide and video log of replacing dead lead-acid batteries in a server rack UPS.

Uninterruptible Power Supplies (UPS) are the unsung heroes of any home lab. They protect your hypervisors (like ESXi or Proxmox) and storage pools (like TrueNAS or Synology) from sudden power crashes.

However, lead-acid batteries inside them degrade and typically fail every 3 to 5 years. In this log, I document the process of replacing the batteries in my rackmount UPS system.

Video Walkthrough

I filmed the entire teardown, terminal wiring, and battery swap process. Watch the video below:

Tools & Safety Precautions

[!CAUTION] Safety First: UPS systems store highly lethal electrical currents, even when unplugged from the wall. Never touch internal circuit boards or metal contacts inside the unit. Only work with the designated battery wiring terminals.

Parts List

• Replacement batteries (ensure you match the specific model, e.g. HR1234W or matching size/amperage).
• Philips head screwdriver.
• Insulated needle-nose pliers (useful for tight terminal clips).

Step-by-Step Swap

1. Graceful Shutdown: Shut down all server equipment and VMs before working on the UPS. Unplug the UPS from the wall mains.
2. Access the Pack: Remove the front plastic bezel and unscrew the metal battery security bracket.
3. Slide Out the Pack: Carefully pull the heavy battery tray out. Disconnect the high-amperage quick-disconnect plug first!
4. Wiring Harness Swap: Take note of the wiring setup (usually in series to combine 12V batteries into 24V or 48V). Swap the jumper wires to the new batteries one-by-one.
5. Reinstall: Slide the pack back in, connect the plug, secure the bracket, and turn the system back on!