This project goes into the build of a 1.72kwh emergency backup battery system out of old and used laptop batteries and an old military surplus ammo box. The materials you need to build this project are lithium ion 18650 batteries from old laptops, 4X5,3X5 cell holders, ammo can, 40 Amp BMS or Battery management system, spot welder, fused nickel strip, heat shrink, kapton tape.
First we determine how many batteries that we can pack inside the ammo box. In our case, we have 2 packs of 91 18650 cells , a total of 182 cells. We take the 4 X5 and 3 X 5 cell holders and connect them to make a couple of 7 x 13 cell holders.
To make this 24V lithium ion battery , we need a 7S ( 7 cells in series connection) combination . A single lithium ion cell has a nominal voltage of 3.7V . To make a single long 7S configuration battery , we connect 7 groups of 26 cells in series to get the 25V nominal voltage.
The cells used in the build are Samsung ICR18650 – 28A with a capacity of 2800Mah .The cells are rewrapped with heat shrink and added an insulator disk at the positive side of the cell for safety. The cells are installed on the holder in such a way that the first 26 cells are in a same polarity and are connected in parallel. Next 26 cells are installed with opposite polarity and is the connected in series with the first 26 cells . The rest of the cells are connected in the similar way to make a final 7S 2P ( 7 series and 2 Parallel ) configuration with max capacity of 26 X 2.8mah or 72.8 amp hours.
A four wide fused nickel strip is used to connect the batteries in series. The nickel strip is placed over the first 4 cells and spot welded in place using the sunkko spot welder. Each cell is individually fused in case there is short circuit or malfunction. The nickel strip connects the first two rows in parallel and then connects the next two rows in series .
Similarly, to complete the series connections, the nickel strips are placed and welded on the opposite side of the pack in such a way that it wont short out the connection by coming in contact with the most negative side of the battery.
We take 0.15mm standard nickel strips to connect the positive ends of the whole pack together. The last 2 rows of most positive end are connected together using the nickel strip . Small pieces of nickel strips are placed across these two rows to connect them in parallel . The pieces are bent so that it can be connected to a separate copper busbar.
The separate 2 battery pack of 7 X 13 cells are connected together by the nickel fuse strip in such a way that one of the pack is flipped on top of the other. The nickel strip that connects the first 3 connection on the 1st pack is bent to connect the 4th connection on the other pack. A 90 degree bent on the last row of the 4p fused nickel strip is welded on to the first battery pack.
A piece of kapton tape is placed over the nickel strip to insulate it and hold it together.
The bent nickel strip on the first pack is placed on the other pack is such a way that the fuses are perfectly aligned . Then it is welded using a spot welder. A one sixteenth inch ABS plastic is placed in between the two packs. The second pack is now slowly folded over the top of the first pack. The whole pack is then wrapped around with the kapton tape so that it doesn’t move around.
The last three unconnected terminals on one side of the battery pack is connected to the single row of unconnected terminal on the other side with help of four nickel strips. The 4p fused nickel strip is cut to connect the 3 sides and the other side.
To connect the main negative and the positive tabs , we attach a THHN copper wire across the both the terminal ends. The extended nickel connections are folded across the wire to hold it into place and soldered . The terminal wires are then connected together with XT90 connector.
Next step is to connect the BMS or Battery management system to the pack . This is a small circuit board which is used to protect each cells of the battery pack from overcharging and becoming unbalanced and getting damaged. It stop the over draining when the cells are fully charged. The BMS used here is a 7S 24V with charge current of 20A and discharge current of 40A. It has two negative leads, one connecting the battery and the other for charging and discharging.
The BMS also has 8 sense or balancing wires which are connected to each series connections on the battery. The black wire is connected to the most negative terminal of the battery. The first red wire is connected to the first series connected group of cells, the second red wire is connected to the second series connected cells and so on. The last red wire is connected to the main positive terminal of the battery. A heat shrink is wrapped around the whole battery pack for added safety .
Before inserting the battery pack into the ammo box, a hole is drilled on the back side of the ammo can to allow the cables from the battery to pass through. Also a small piece of one sixteenth inch ABS plastic is placed at the bottom of the ammo box as a support and insulation.
The battery is slowly dropped into the box . The BMS is placed on the top and is connected to the XT90 connector and the balance wires from the battery. The B- terminal on the BMS is connected to the XT90 connector on the battery. The black wire on the BMS is the charge and discharge lead. To provide extra insulation between the battery pack and the the ammo box ,we attach two pieces of the insulting ABS sheet on either side of the box.
The lid is put back on the box and the battery build is complete.