Second Life UPS Mark II

Table of Contents

• 1. Introduction
  • • 1.0.1. More battery capacity
    • 1.0.2. More power
    • 1.0.3. Capacity planning for power buses
    • 1.0.4. Solid-state operation
    • 1.0.5. Modularity
• 2. Overall schematics
• 3. Build instructions
  • 3.1. Planning
  • 3.2. Enclosure preparation
    • 3.2.1. Stud removal
  • 3.3. Front panel assembly
    • 3.3.1. Mounting the main battery fuse
  • 3.4. Circuit assembly
  • 3.5. Battery assembly
    • 3.5.1. Battery cell selection
    • 3.5.2. Nickel strip thickness calculations
    • 3.5.3. Welding the cells
    • 3.5.4. Attaching voltage sense wires
    • 3.5.5. Final assembly
    • 3.5.6. Attaching thermal fuses
    • 3.5.7. Battery Management System
• 4. Final assembly photo
• 5. Bill of Materials

1. Introduction

The Secondlife UPS Mark II was envisioned as a successor to the Secondlife UPS Mark I model with the following improvements:

• increased battery capacity
• increased power budget
• circuit simplification
• modularity
• improved load connectors
• various other usability improvements (for example moving the fuses to the front of the enclosure)
• better documentation

The basic premise is to provide a short depth (~250 mm) rack mountable UPS which instead of 230V AC can provide typical low voltages suitable for powering of CCTV cameras, sensors, routers, 802.11 access points, modems, switches, embedded systems (like a Raspberry Pi) and other small devices. This approach is advantegous in terms of saving rack space where a PDU does not have to be mounted as well as potential power efficiency gains of having fewer AC <-> DC power conversion steps.

The UPS design is built using off-the-shelf or recycled components with a few custom made printed circuit boards. Most notably, the UPS is designed to be enclosed using typical 19″ Ethernet switches. These can be obtained very cheap or even given away for free when equipment is discarded. Apart from a reusability solarpunk vibe Ethernet switch enclosures provide us with ready-made cutouts in their front panels. These originally house Ethernet connectors and have typical dimensions. This fact makes it easy to build a modular UPS device with standarised parts.

Even though any Ethernet 19″ rack switch enclosure can be used for this build, this particular UPS is going to be housed in an enclosure from a damaged Netgear GS748T switch.

The UPS design is focused around two buses providing standard 12V and 5V voltages using buck converters as well as a "direct" bus (labeled later as V_rail) which is connected directly to the battery or AC power supply. The peculiarity of this bus is that the voltage on it has a significant swing - from 11.2V when operating on an almost depleted battery towards 24V when operating on AC power. The advantage of using V_rail is that its current capacity is not limited by the DC-DC buck converters and can power more power-hungry equipment - for example a Netgear RN3138 NAS.

1.0.1. More battery capacity

In order to provide more power as well as longer runtime, the UPS will contain a 4S24P Lithium-Ion battery pack built from refurbished 18650 cells having a capacity of around 600 Wh (if 2000 mAh cells are used).

1.0.2. More power

After Secondlife UPS Mark I has been deployed there have been a number of changes and additions to my home infrastructure equipment rack. Power requirements have increased. In order to come up with more detailed power requirements for each voltage I measured the amount of current draw during normal operation for each piece of equipment and came up with the following calculations for the power budget requirements of the UPS. As the voltage on the V_rail bus is not constant and changes from 11.2V when operating on a discharged battery and 24V when operating on AC its more useful to specify it's performance in terms of power not current.

Table 1: Devices connected to the UPS