Introduction
The objective of this innovative project is to repurpose outdated and used laptop batteries, integrating them into a robust backup power solution housed within a military surplus box. This approach not only promotes environmentally sustainable practices but also offers a cost-efficient alternative for reliable energy storage, capable of delivering 1.72kWh of power. The rugged military surplus container ensures durability, portability, and suitability for outdoor or emergency applications, making this a versatile and eco-friendly energy project.
Step 1: Essential Materials
To successfully build this backup battery system, several key materials are required. Firstly, sourcing high-quality lithium-ion 18650 cells from discarded laptops is crucial; these can often be obtained from old devices or purchased from electronic recycling centers. You will also need 4×5 and 3×5 cell holders to organize and securely mount the batteries. An ammo can serves as the sturdy enclosure for the entire setup, providing protection and portability.
Managing the charging and discharging cycles safely necessitates a reliable 40 Amp Battery Management System (BMS). This component safeguards the batteries against overcharge, over-discharge, and imbalance, thereby extending their lifespan. To connect the cells and the BMS, a spot welder is essential for fusing nickel strips onto the batteries. Additional insulating and protective materials such as heat shrink tubing and Kapton tape are used to ensure safe electrical insulation and durability of the connections.
Step 2: Designing the Cell Configuration
The initial step involves determining the number of lithium-ion 18650 cells that can fit within the chosen ammo can, which in this project is designed to hold two packs of 91 cells each, totaling 182 cells. The cells are assembled into two 7×13 arrays, which are then configured to achieve a 24V nominal voltage. Connecting seven cells in series (7S configuration) yields approximately 25V, considering each cell’s nominal voltage of 3.7V.
By grouping the cells into this 7S arrangement and connecting multiple such groups in parallel, the system can achieve the desired voltage and capacity, making it suitable for backup power needs.
Step 3: Assembling the Cells
The selected cells are Samsung ICR18650-28A units, each with a capacity of 2800mAh. For safety, each cell is rewrapped with heat shrink tubing, and an insulator disk is added to the positive terminal to prevent accidental shorts. The assembly begins with connecting 26 cells in parallel, ensuring their polarities are correctly oriented. The next 26 cells are connected with opposite polarity, then linked in series with the previous group, forming a robust 7S 2P configuration.
This configuration results in a maximum capacity of approximately 72.8Ah (26 x 2.8Ah), providing a reliable and high-capacity power source suitable for backup applications.
Step 4: Creating Series and Parallel Connections
To establish safe and efficient connections, a four-wide fused nickel strip is placed over the first four cells and spot-welded using a sunkko spot welder. Each cell is individually fused to prevent cascade failures in case of a short circuit. The nickel strips are carefully placed and welded on both sides of the pack, ensuring that the cells are connected in the desired 7S 2P configuration.
Special attention is given to avoid shorts by positioning the strips and welds appropriately, especially over the negative terminals. Smaller 0.15mm nickel strips are used to connect the positive terminals at the ends of the pack, which are then linked to a copper busbar for connection to the BMS.
Step 5: Connecting Multiple Battery Packs
Once individual packs are assembled, they are aligned and stacked carefully. A fused nickel strip connects the positive terminal of the first pack to the negative terminal of the second pack, effectively configuring the system in series to reach the targeted voltage of approximately 25V.
To secure the connection, the strip is bent at a 90-degree angle and spot-welded onto the respective terminals. A piece of Kapton tape is added over the connection to insulate and prevent accidental contact. The packs are then held firmly together using a 1/16-inch ABS plastic sheet positioned between them to distribute pressure evenly.
Careful folding and wrapping with Kapton tape ensure the entire assembly remains stable and insulated. Additional nickel strips are used to connect the unconnected terminals, completing the series wiring and ensuring reliable electrical continuity.
Step 6: Identifying and Connecting Terminals
To establish the main power outputs, a THHN copper wire is soldered across the terminal ends of the battery pack. The connections are secured by folding and soldering the nickel strips over the wire, ensuring a solid, low-resistance connection. An XT90 connector is then attached for high-current connections, offering a secure interface to external devices or systems.
Step 7: Integrating the Battery Management System (BMS)
The next critical step involves connecting a 7S 24V BMS with specified charge and discharge currents (20A and 40A respectively). The BMS protects each series-connected group of cells from overcharging, over-discharging, and imbalance, thereby prolonging the battery’s lifespan.
Eight sense or balancing wires are connected from the BMS to each series cell group, starting from the most negative terminal, progressing through each series group, and ending at the most positive terminal. All connections are insulated with heat shrink tubing to prevent shorts. This comprehensive management ensures the safety and longevity of the entire battery pack.
Step 8: Preparing the Ammo Box
To accommodate the battery assembly, a hole is drilled at the rear of the ammo can for cable pass-through, ensuring safe and organized wiring. A small piece of ABS plastic is placed at the bottom of the box to support and insulate the battery pack, preventing movement and potential damage during operation.
Step 9: Final Assembly and Installation
The assembled battery pack is carefully lowered into the ammo box, followed by placing the BMS on top. The XT90 connector and balance wires are connected from the battery to the BMS, with the B- terminal on the BMS linked to the main negative terminal of the pack. The charge/discharge lead from the BMS is connected to the external system via the XT90 connector.
To enhance insulation and prevent movement, two pieces of insulating ABS sheets are attached to either side of the battery inside the box. The lid is then secured, sealing the entire assembly. This meticulous process results in a durable, portable, and efficient backup power solution built from recycled laptop batteries, ready for practical use in various emergency or off-grid scenarios.