Submitted by QH-Technology t3_10o0n2b in DIY
Hello, folks.
Several months ago, my AGM travel trailer battery is broken. After comparing YouTube and Google, I decided to build a 12V 120Ah LiFePO4 battery. I buy the battery cells and the kits online. After preparing all the tools and materials, I start to build it and finally got it done. I'd like to share the full process.
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Materials Used:
- 3.2V 120Ah LiFePO4 battery cells (4 pieces)
- BMS (Battery Manage System, 1 piece)
- Connectors (About 4 pieces)
- Others: EVA cotton, screws, ribbon cables, plastic pipes, etc.
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Tool Used:
- Spot Welder
- Spot Welding Pen
- Soldering Iron
- Wire Cutter
- Wire Stripper
- Multimeter
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Assembly Process
- First, place four battery cells on the work table. Before we assemble the battery pack, we should check the quality and ensure consistency, which means they are produced by the same brand with the same voltage, and capacity.
- Next, we can put the epoxy boards between the batteries. The epoxy board is made of alkali-free E-glass cloth impregnated with epoxy resin by processing under heat and pressure. Which have extremely high mechanical strength, Insulation, heat resistance, electrical properties, and good moisture resistance properties.
- We place them in order and fix them with fiber tape. The fiber tape is made of hot melt adhesive and forms a stronger tape that is resistant to tears and has better temperature tolerance than regular tape. It is explosion-proof and leak-proof, providing high-strength insulation protection.
- Connect the cells using the connectors in series while connecting the indicator cables to the CPM and the cells. Don’t tighten connectors and nuts too tightly.
When connecting the voltage collection lines (equalization lines), do not connect the external protection board to avoid accidental burning of the protection board.
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- Terminal blocks are available as rows, but each terminal connects to only a single wire, which can prevent the loosening of signal wires.
- After soldering the wires (the balancing leads and the charging-discharging cables), we can tidy up the messy wires.
- We can not put the battery pack into bare condition. It is an excellent choice to put it into a plastic shell. Compared with the aluminum shell, the plastic shell has a good insulation performance and is cheaper.
Most of the cases of lithium batteries are mass molded manufacturing, production of high cost, and personal customization of the case is difficult. You can look for the case on sale online and pay attention to the size.
- The thermally conductive tape for battery packs keeps the BMS in place and protects them from the shock of extreme temperature changes.
Besides, these tapes provide electrical insulation, which prevents short circuits and provides a barrier to BMS.
- A BMS is one of the most important elements in a LiFePO4 battery, like the brain of the battery pack. It calculates the State of Charge (the amount of energy remaining in the battery) by tracking how much energy goes in and out of the battery pack and by monitoring cell voltages, which can prevent the battery pack from overcharging, over-discharging, and balancing all the cells voltage equally.
There are two main sets of wires we need to install, the thick wires and the thin wires. The thick wires are your charging/discharging wires and the thin wires are your balance wires. Not every BMS is the same, but most are similar. Your BMS will likely have 3 thick wires or 3 pads to solder on your own heavy gauge wires. These are the B-, P-, and C- wires (or pads for adding wires). We usually start with the B- wire. We can connect the B- of BMS to the negative pole of the battery pack.
- If you connect the negative to the ground first then the current will flow through the positive terminal to your body when you connect the positive terminal but when you connect the positive terminal first then while connecting the negative terminal the current will pass through the negative to ground wire instead of your body to ground as the resistance of your body is greater than the resistance of wire path and current will always flow through the lowest resistive path. Thus always connect the positive terminal first.
- If you connect the negative to the ground first then the current will flow through the positive terminal to your body when you connect the positive terminal but when you connect the positive terminal first then while connecting the negative terminal the current will pass through the negative to ground wire instead of your body to ground as the resistance of your body is greater than the resistance of wire path and current will always flow through the lowest resistive path. Thus always connect the positive terminal first.
- Measure battery temperature with the probe. Use tape to secure it near the battery pack.
- The total input and output ports of the battery pack are assembled. The assembly of this battery pack is almost done.
- EVA cotton can be shockproof, fireproof, and insulated, protecting the battery pack well. It can reduce the expansion force of the battery and improve the service life of the battery.
- Check the signal lines in the correct order, or the wrong sequence may cause BMS to burn out!
- The signal acquisition technology can provide accurate parameters for battery balancing, SOC estimation, and BMS centralized monitoring to meet the actual requirements of battery packs.
Using the signal acquisition technology, the voltage signal, temperature signal, current signal, etc. In addition, data from the battery pack CAN be transmitted to the central control unit.
Battery Tests
1. Test The Voltage Of The Battery Pack
In this step, we can use a multimeter to check the voltage of the whole battery pack. Attach the multimeter probes to the positive and negative battery terminals. Then we can check the voltage on the screen. The multimeter’s red probe must be connected to the positive terminal, while the black probe must be connected to the negative one.
A fully-charged battery must indicate a slightly higher voltage than the voltage listed on the battery. For instance, a 12 volts battery will indicate about 12.8 volts when it is fully charged.
2. Charging Test
Through the test, we can check that the charging value is normal and that the battery is abnormal during the charging process. We need to pay attention to the battery charging time and charging current.
3. Discharging Test
The discharge test of the battery is beneficial to the battery cycle life and discharge performance evaluation. We can use a professional device(Such as a Programmable DC Electronic Load) to check whether the battery works well during the discharging process, which can protect our battery and devices for further daily use.
When testing, there are three factors we need to pay attention to the port voltage of the battery, the resistance of the wire between the battery and the electronic load, and the temperature of the storm.
cdude t1_j6chkez wrote
Sure it's a good build, but is this really a DIY? The images are screenshots from a video that showcases their lithium battery products and business. OP's name is obviously their business, which you can look up with a quick search.