Assembling Lithium Ion Battery 12.8V 300Ah For Your Sail Boat! Step By Step!

LiFePO4 batteries have been widely used in our daily lives, such as RV, camping, solar, and wind energy storage. Compared with the lead-acid battery, the LiFePO4 battery can help you stay on the water for a longer time. With high energy density and a longer lifespan, more and more people are likely to upgrade the sailboat battery using LiFePO4 battery.

There are two types of batteries we used in a boat. One is the starting battery, which is used to start the engine, and the other one is the storage battery, which can power the devices on our boat. Our 12V 300Ah is the storage battery. It’s used to power the trolling motor and other accessories.

As a DIYer, I decided to build a 12V 300Ah LiFePO4 battery for my sailboat. I wrote the full prismatic battery pack assembly process down and share it in the following post. Hope it can help you.

*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.

Material Used:

  • 3.2V 300Ah LiFePO4 battery cells (4 pieces)

There are currently three common shapes of LiFePO4 batteries: cylindrical, prismatic, and pouch. Different shapes of batteries will have a certain impact on performance. At present, the most suitable battery DIY enthusiasts are the prismatic LiFePO4 batteries, which are very suitable for both performance and operational difficulty. 

We recommend you purchase the Grade A battery with a high quality and reliable warranty.

  • BMS (Battery Manage System, 1 piece)
  • Connectors (About 4pieces)
  • Others: EVA cotton, screws, ribbon cable, plastic pipes, etc.
prismatic lithium cells

Material Used:

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

Assembly Methods: 4 Series And 1 Parallel

  • Series(S): The way of connecting the positive and negative electrodes of two different cells is called a series connection. As a result, the voltage of the battery pack will increase while the capacity remains the same.
  • Parallel(P): where the positive and positive electrodes of two different cells are connected in parallel. The result is that the capacity of the battery pack increases while the voltage remains the same.

Assembly Process

1. Install Signal Acquisition Wires in Sequence

The connecting piece has been connected to the battery cell by laser welding. Laser welding is a process used to join together metals or thermoplastics using a laser beam to form a weld.

Before we connect this group of battery cells, we should use the multimeter to check their voltage of them to ensure consistency. It is better if you have a testing device to check the internal resistance. Without testing the consistency, the battery pack may cause an accident.

Then we place them in order and fix them with fiber tape. (Made of hot melt adhesive and forms a stronger tape that is resistant to tears and has better temperature tolerance than regular  tape.)

2. Cut the Signal Line to An Appropriate Length

Tidy up the messy lines, then cut them into an appropriate length.

3. Soldering Signal Lines With Lead-free Solder

Generally, leaded solder is composed of tin and lead. The advantages of using leaded solder include: easier to bring to working temperature, shock resistant, and fewer internal flaws in the structure after cooled. 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.

4. Check the Signal Lines in Correct Order

The wrong sequence may cause BMS to burn out.

5. Put Battery Pack into Shell and Fill EVA Cotton

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

6. Secure BMS with Thermally Conductive Tape

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 thermal management of high-powered LED’s which can run at high temperatures, thus increasing the efficiency and reliability of the system.

7. Connect B- of BMS to Negative of The Battery Pack

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.

8. Connect the Positive and Negative Power Line to The Cap

9. Plug signal acquisition wires’ port into interface

Battery Tests

1. Test 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

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 300Ah should provide 300A for one hour.

3. Discharging Test

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.


Thanks for reading the post about assembling the 12.8V 300Ah LiFePO4 battery! It is not hard for us to build a LiFePO4 battery. Would you like to have a try?


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