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Second Life UPS

A UPS using refurbished ("second-life") lithium-ion 18650 cells.

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I have decided to build a UPS that would power my home network equipment rack using lithium-ion 18650 cells.

Second Life UPS

Table of Contents

1 Design overview

I have decided to build a UPS that would power my home network equipment rack using lithium-ion 18650 cells.

thumb-rack.jpg

First let's lay out some basic design considerations

  • should fit the equipment rack in a 1U format
  • it should provide 12V DC, 5V DC as well as 230V for the switch

The current list of devices that need to be powered by the UPS together with their nameplate voltage and current consumption information:

Device Voltage [V] Current (max) [A] Power (max) [W] Notes
Router 19 2 38 A small form factor PC
16-port switch 230V AC 0.1 22.5 Rated at 22.5 W in the technical specs
AP 24 0.25 6.5 Rated at 6.5 W (see datasheet), passive 24V PoE Mode B (230V/0.02A)
Raspberry Pi 5 2.1 10  
GPON modem 12 0.5 6  
Total     83.  

As can be seen there is a large amount of different voltages required in order to power all of the devices, therefore some modifications and compromises will have to be made:

  • The UPS will provide stabilized 12 V and 5 V power rails
  • The 19V for the router will be provided by a dedicated step-up dc-dc converter based on the very popular XL6009 chip from aliexpress
  • The 230V AC will be temporarily provided by a small car 12V -> 230V inverter
  • The PoE injector is scheduled to be replaced by an PoE injector which can be powered from DC
  • The router will be replaced in the future with a Mikrotik Routerboard or a PFsense appliance from https://teklager.se/en/pfsense-hardware/ that both can be powered from DC
  • The switch will be modded to have a DC power supply

2 Component layout

The 1U case that I used for this project is a case from a ipTime ZC-SW2405 switch:

thumb-switch-case.jpg

I have removed everything from the switch case including the power supply as it was only providing 3.3V and did not have enough power. The case has 1U dimensions, to be exact 16 cm x 44 cm x 4.4 cm. I have also drilled out all of the studs holding the original PCBs to the case in order to fit the battery and AC PSU. This has proven to be a hindrance however as I had to remake some of them later to mount my custom boards.

The case size puts a limit on the size and shape of two biggest internal components - the battery pack and the 230V AC power supply. I have devised the following placement for these components.

thumb-basic-layout.jpg

From the basic layout the following elements are COTS components:

Brand Model Parameters Notes Link
KAYPW Switching Power Supply 24V 5A 120W max   https://www.aliexpress.com/item/33042313383.html
  40A 16.8V Balance REV 2.3 4S 40A   https://www.aliexpress.com/item/4000026248298.html
  XL4015-B CC/CV module 5A CC/CV, 75W max Li-Ion charger https://www.aliexpress.com/item/4000383898441.html
  XL4015 buck dc-dc converter 5A max Provides 5 V and 12 V outputs https://www.aliexpress.com/item/4000796650052.html

3 The battery

The battery for the UPS was built using 18650 Li-Ion cells recovered from hoverboard battery packs. These are cheap no-name cells with not a lot of capacity left in them (around 800 mAh) each but I figured that a UPS would be the perfect idea to utilise them. They will operate under low current load per cell so their internal resistance shouldn't be a problem. The UPS will be mostly sitting idle and charged so the cells won't get further capacity fade from cycling. The cells have been screened for self-discharge and excessive heat generation during operation. As the space in the case is pretty tight heat might be a problem therefore two thermal fuses tripping at 75 °C have been fitted in the battery as a safety feature. The battery will be protected with a 20A car fuse and paired with a 4S 40A BMS.

As a further step to increase the lifetime of the cells the top charging voltage will be limited to 4V per cell. This...

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  • Design refinements #2

    Enki04/25/2021 at 15:51 0 comments

    I did another round of design refinements. This time I replaced the LM2596 converter supplying the 5V outputs to another XL4015 which has more current capacity.


    This way I can reliably power the raspberry Pi as well as a fan colling the entire rack. I have added some stickers on the front panel as well:

    All changes were as usually reflected in the project details.

  • Design refinements #1

    Enki04/18/2021 at 14:31 0 comments

    Today I have performed a number of design refinements:

    - I rewired the output board to provide 6 x 12V output and 2 x 5V output as the 12V is much more versatile

    - I improved the way that the AC 230V power cable attachment to the device, I have an electrical tube fitting instead of the cable just dangling out

    - I added a 7812 V regulator to power the AC PSU Fan, powering it from the DC rail directly was working but the high-pitched sound was annoying

    All of these have been reflected in the project details.

  • SVG not rendering in project details

    Enki04/15/2021 at 08:56 0 comments

    It looks like I can't add SVG to the project details page.

    I have added a PDF version of the schematics as well as uploaded the Kicad project.

  • Energy density

    Enki04/13/2021 at 13:39 0 comments

    I did some back of the envelope calculations on the energy density of my setup compared to lead-acid AGM batteries often used in UPS applications.


    My setup

    a 4S10P battery made from junk cells each having around 800 mAh gives me 14.8 V x 8.315 Ah nominal capacity of 123.062 Wh (calculated by https://www.repackr.com/ battery pack builder). As the battery size is 27 cm x 10 cm x 4 cm this gives a volume of 1080 cm^3 and a volumetric energy density of 0.11 Wh/cm^3.

    Lead-acid

    A random "flat" AGM battery (CSB HRL 634W F2) used in a 1U rack UPS I found laying around in the hackerspace has a nominal capacity of 6 V x 9Ah = 54 Wh with the size of 15.1 cm x 3.4 cm x 9.4 cm without leads. This means that the volume is 482.596 cm^3 and the volumetric energy density of also 0.11 Wh/cm^3.

    Not bad considering my lithium cells are essentially on the level of junk not reusable for anything else.

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Discussions

Kosma wrote 04/15/2021 at 10:44 point
is possible using old acumulators? and charging separately one by one not every in this same time?

  Are you sure? yes | no

Enki wrote 04/15/2021 at 21:07 point

Yes, the cells I used were old ones, they had around 50% nominal capacity. What do you mean by charging separately? If you connect the cells in series they charge and discharge at the same time. If you don't connect the cells in series you will get very low voltage which is not practical to use.

  Are you sure? yes | no

Kosma wrote 04/16/2021 at 13:00 point

pity. I have old cells. some working 50% some 100% some 3%.

If I dont charge separate cells module i cant charge it efective. Many power will lost many power are never get back.

second trouble I canot replace failed cells in any time.

Sorry but this charging scenario is not good for old cells

  Are you sure? yes | no

manta103g wrote 04/16/2021 at 12:17 point

I have built an intelligent charger for Li-Po cells, charging them indiv idually, in parallel, than switching parallel to serial to get higher rated voltage output

I am using mechanical switch: parallel 2 serial

  Are you sure? yes | no

Enki wrote 04/16/2021 at 15:56 point

Yeah that could work. If you can share the details to @Kosma that would be helpful. I'm interested as well to see how you exactly did this.

  Are you sure? yes | no

Stephen Johnson wrote 04/15/2021 at 01:10 point

Sorry, your browser does not support SVG. !?!?

This project is useless without the schematic for the controller board!

Where is it?

  Are you sure? yes | no

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