Backup Battery Pack

In this project, I’ve built a Li-ion battery pack that is capable of charging directly from a solar panel. The battery pack has 75 cells in a 3-cell configuration which results in a 12V output. I used an external inverter to convert the DC output of the battery to 230VAC.

Battery pack with the external inverter

Since I had planned on charging the battery using a solar panel, I used a Maximum Power Point Tracking (MPPT) module. The maximum charging current of this unit is 5A. A Battery Management System (BMS) oversees the charging and discharging cycles of the battery and, it is capable of a maximum current draw of 100A.

Spot Welded battery pack

I arranged the batteries in a 3-cell series and 25-cell parallel(3s25p) configurations to get a 12V output. The cells were spot-welded using the Spot welder, which I built during my last project. You can find more information about the Spot Welded on its project page. The 12V output of the battery can be connected to an inverter to convert the DC voltage into mains AC voltage. The following diagram shows the interconnections of the components in the system.

CAD drawing of the enclosure

I used Fusion360 to design the enclosure. When making the enclosure, first, I designed all the parts inside the CAD program and logically arranged them. After doing that, I designed the enclosure around it. Placing the parts beforehand allowed me to optimize the component placement in a tight space.

Internal view of the battery pack

All the parts were arranged before designing the enclosure

Due to the limited space, steady airflow is critical for cooling the modules during operation, hence the 6mm cooling fan on the top.

Parts I used for this project

I used the following parts and components to build this project.

  • 1 x 18V Solar Panel: 100W
  • 1 × Solar MPPT Charge Controller: BQ24650
  • 1 × Li-ion Battery Management System: Maximum discharge current 100A
  • 1 × Digital Voltmeter / Ammeter Panel: 0-100VDC / 0-10A
  • 75 × 18650 Li-ion cell: 3.7V
  • 2 × XT90 connectors
  • 1 × Inverter: 12VDC-230VAC 1000W
  • 1 × Cooling fan: 12V-6x6mm

Interconnections of the components

Design Files

You can find all the CAD files of this project on my GitHub page.