Second Life UPS Mark II

After placing the batteries, AC power supply, DC-DC converters, BMS and other components some studs will need to be removed:

Some notes on the process:

• Thoroughly deburr the corners of the drilled holes with a deburring tool or a Dremel. Leftover burrs can damage 18650 cell insulation sleeves and cause a short.
• Drill with a 5mm metal drill
• Drill from the bottom of the case
• Use a puncturing tool for easier drilling. A cheap one can be bought for 2-3 EUR.
• Support the chassis with 40mmx40mm wood cubes to prevent bending

The main battery fuse socket should be selected to handle the battery current and be easily mountable in the enclosure. The most practical sockets are those that can be mounted with just round holes. The fuse selected needs to be a DC fuse rated for the voltage that the battery operates on. Those types of fuses can be often found distributed under the "solar" or PV categories. The fuse that I used in my build is linked in the BOM at the end. As the fuse socket is pretty large, care needs to be taken to drill the main mounting hole exactly on the center of the back plane.

In my socket, M4 screws were used for mounting, 22mm hole was drilled with a step drill. Remember about lubricant, my Metabo BS 12 BL Q 20 Nm cordless drill was struggling a bit but you can do it with some patience.

The 18650 cells used for the making of the battery pack can be new or refurbished from e-bike, cordless tools, old laptop batteries or other sources. Consult more specialised forums like Secondlife storage for tips on selecting, obtaining and testing reused 18650 cells. Two critical points to keep in mind are:

• always check the specifications for the cells you are using (find and read datasheets) so as not to exceed current or voltage limits
• measure and match the discharge capacity and internal resistance of the cells in a battery pack to be as close as possible

The battery will be built by welding nickel strips onto the cells in a certain pattern. The basis of the calculation of the required width of the nickel strips is the following table. Unfortunately the exact methodology of how these numbers have been obtained is unclear but it's the best data I have found so far:

Table 3: Nickel strip current carrying capacity

Strip size	Optimal [A]	Acceptable [A]	Poor [A]
0.1mm x 5mm	< 2.1	3.0	> 4.2
0.1mm x 7mm	< 3.0	4.5	> 6.0
0.15mm x 7mm	< 4.7	7.0	> 9.4
0.2mm x 7mm	< 6.4	9.6	> 12.8
0.3mm x 7mm	< 10.0	15	> 20.0

Reference

As previously calculated, the maximum battery current will be around 30A during discharge therefore the current collectors at the battery terminals need to use two 0.3mm strips. Additionally, the strips that I am using are 8mm wide not 7mm wide giving some headroom for current capacity.

The general approach is to connect the cells together with 8mm wide nickel strips using spot welding. In order to facilitate cell voltage verification and equalization within each of the 24P blocks first only one side of the block (the positive cell terminals in my case) is welded first, then cells charge is equalized and then the second cell terminals (negative cell terminals in my case) are welded together. Most welds are performed with 0.1mm and 0.12mm nickel strips but the terminal connections for the entire battery are built out of 0.3mm strips for designed current capacity.

A welding jig is used to keep the cells in place during welding. Such a jig can be purchased or manufactured by yourself as it's simply a set of four pieces of wood with a long slot cut alongside their length. The design for such a jig can be found below:

The cells for each of the four 24P blocks are inserted into 4x2 cell holders refurbished from old "hoverboard" battery packs and aligned using the jig. This is recommended as the welded nickel strips cannot be used as the only mechanical support for the cells:

Nickel strips are secured to the cells using small neodymium magnets to not move during the welding process:

The strips are welded to the cells using a kWeld from Keenlab spot welder powered by a HV Stock Spec GRAPHENE-3 7300mAh Hardcase battery - 7.6V LiPo - 130C/65C. The effect of welding both 0.1mm and 0.12mm thickness strips that connect the cells can be seen below:

The cells in all four 24P blocks are first checked with a voltmeter and if some of them have self-discharged below 4V they are charged individually. I use a DIY "charger" which connects together 5 cheap "Li-Ion charger" modules based around the popular TP4056 chip.

The strips used for terminal current collectors (providing the positive and negative for the entire battery) are longer than the length of the block to be bent together and soldered to 4mm² wires.

After all blocks are welded and equalized they are aligned side-by-side in the welding jig and connected with more 0.1mm/0.12mm strips. This process can be seen below. Care needs to be taken at this stage in order to properly orient the blocks taking into account the polarity as well as final wiring in the enclosure. A quick photo summary of these steps can be found below:

Additional tips on the cell welding process are:

• take some old cells and practice welding settings and technique with different nickel strip thickness before starting to weld the battery
• as a rule of thumb for a proper weld, when you pull off the strip from the cell there should be a hold left in the strip, in other words the weld should be stronger than the nickel strip itself
• keep the cells and nickel strip clean (degrease with isopropyl alcohol if required) and use work cotton gloves during assembly
• for best results use pure nickel strips, some places sell nickel plated steel strips which are cheaper but have more resistance

The voltage sense wires are soldered to allow the BMS to measure the voltage for each 24P block and react accordingly when voltage limits are reached:

The final assembly involves isolating and securing all exposed nickel strips and wires with electrical tape:

After this the pack is bent into a flat shape:

This allows the pack to be slided into the enclosure:

During battery assembly sense wires and thermal fuses are attached. 4 thermal fuses connected in series are 60°C Normally-Connected fuses. They are attached to the battery cells using a thermally conductive glue. Magnets are used to secure the sensors as the glue takes around 24h to fully solidify.

The BMS used with the battery pack is a EGBO Store 4S/100A based on the BM3451 made by BYD. Apart from ordinary battery protection from overvoltage, undervoltage, overcurrent, etc. it can detect the disconnection of sense wires and shut the pack down in such an instance. This feature is used to both connect thermal fuses to the battery as well as a "Battery cutoff" switch on the front panel. Both of these work by breaking the connection of one of the voltage sense wires causing the BMS to stop charging or discharging the battery.