Assembling Lithium Ion Battery Pack 24V 200ah for Off-Grid Household Solar System

The 24V Lifepo4 Battery Pack is ideal for off-grid household solar energy storage systems. When we install an inverter, a LiFePO4 battery pack, and several rooftop solar panels, a simple off-grid solar system is done. It is a reliable power backup and it works independently of the grid. The 5kWh solar power system can not only save our energy bills but also make lots of benefits to our environment.

If you want to try the off-grid solar system on a small scale, you can start with a 25.6V 200Ah LiFePO4 battery. Let’s check how to assemble a 25.6V 200Ah LiFePO4 battery！

*Note: It is recommended that you learn some basic knowledge about LiFePO4 batteries before assembling the battery pack. Keep safe when you are assembling the battery pack.

• 3.2V 200Ah LiFePO4 battery cells (8 pieces)
• BMS (Battery Manage System, 1 piece)
• Connectors (About 8 pieces)
• Others: Epoxy boards, EVA cotton, screws, ribbon cable, plastic pipes, etc.

Tool Used:

• Spot Welder
• Spot Welding Pen
• Soldering Iron
• Wire Cutter
• Wire Stripper
• Multimeter

The connecting piece has been connected to the battery cell by laser welding

First, we should check the voltage, brands, and other parameters of each battery. Make sure the parameters are consistent. Then place them neatly, and stack the battery cells properly with separators.

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. Rather do it firmly but gently.

When connecting the voltage collection lines (equalization lines), do not connect the external protection board to avoid accidental burning of the protection board.

After installing signal acquisition wires in sequence, we can cut the lines to an appropriate length.

Generally, leaded solder is composed of tin and lead. The advantages of using leaded solder include: being easier to bring to working temperature, being shock resistant, and having fewer internal flaws in the structure after cooling. However, lead material is harmful to the body as it’s readily absorbed. We’d better choose lead-free solder to protect our health and environment.

The wrong sequence may cause BMS to burn out!

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.

EVA cotton can be shockproof, fireproof, and insulated, which can protect the LiFePO4 battery cells well.

Thermally conductive tapes are designed to provide preferential heat transfer between heat-generating electronic components and cooling devices such as fans, heat sinks, or heat spreaders. They are also used for the thermal management of high-powered LED’s which can run at high temperatures, thus increasing the efficiency and reliability of the system.

We can use the multimeter to check the voltage of the 25.6V LiFePO4 battery. A multimeter is an electronic device that can measure the current, voltage, and resistance. The multimeter is also known as a voltage-ohm-milliammeter abbreviated as VOM. Two types of multimeters exist the analog multimeter which uses a moving pointer to display readings and a digital multimeter with an LED display that shows accurate readings.

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 24 volts battery will indicate about 25.6 volts when it is fully charged.

The battery capacity, or the amount of energy a battery can hold, can be measured with a battery analyzer. If you’re doing a capacity test, be sure to charge the battery until the battery reaches 100%. Then discharge the device until the battery is fully depleted. The charge and discharge rates of a battery are governed by C rates. The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at 200Ah should provide 200A for one hour.

The discharging test of the battery is very helpful 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 or not 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 battery.