How to Build a 12v Lithium Battery from old Laptop Cells. Then put inside a UPS to charge from home, or a Solar panel. In order to run your 120vAC, 12vDC, and 5vDC devices off-grid.

Building a 3S5P (3 in series, 5 in parallel) 12v Lithium battery pack from old 18650 laptop battery cells. 

To put in an old UPS, and use as a solar charger and inverter to run 5v, 12v, 24v, and 120v devices.  Charging either with 120v AC, or a 15v-21v solar panel.  


  • 15x 18650 cells
  • 30x cell holders
  • 4x copper bus bars
  • 30ga copper wire 
  • Doublestick smartphone stuff
  • 1x 3S BMS board
  • 2x spade battery connector male
  • 2x 4” of 16 AWG solid copper wire
  • 1x 4 wire connector with wires (JST-XH or Dupont for balance charging)


  • UPS for AC charging, and powering 120v AC devices.
  • Cigarette jacks for 12v and 24v devices.
  • USB jack for 5v devices.
  • DC 5.5mmX2.1mm jack for 12v devices, and charging from solar.
  • MPPT controller for solar charging lithium 3s battery
  • Solar panel with output of 18v?-21v


  • Heat shrink tubing
  • High temp soldering iron with large tip (I used 800 F temp setting)
  • Non corrosive flux no clean
  • Rosin core lead free solder
  • 18650 cell charger or holders for the Imax B6 (For charging the cells)
  • Lithium Charger/discharger/balancer (Imax B6)
  • Flush Cutters
  • Wire strippers
  • Heat gun (For shrink tubing)
  • Glue gun and glue 

Find, test, and sort cells

  1. Visually inspect for damage. Corrosion, punctures, deformed, and insulating sleeve damage. 
    1. Rewrap, or recycle damaged ones.
  2. Check voltages. > 3v is ok to charge
    1. Recondition, or recycle low voltage ones 
  3. Charge, and see if they get hot. > 100F
    1. Recycle hot ones.
  4. After full charge check voltages. Should be ~4.16v. 
    1. Discharge recharge again, or recycle low voltage ones.
  5. Wait 1-4 weeks, and test voltage again. Should not be lower than ~4v. 
    1. Discharge recharge again, or recycle low voltage ones.
  6. Discharge and check capacity. 
    1. Sort into 3 groups ( Low: < 1000 mah Mid: 1001-2000mah high: > 2000mah ) Highest you’ll find is around 3500mahs
    2. Choose the 15 you’ll use in this 12v battery. I chose 12x 1200mah@5v and 3x 1550mah @ 5v.  
  7. Sort those 15 cells by capacity using
  8. Calculate capacity of finished battery. i.e. (Amp hours @ 12v), and KWHs (Kilowatt hours)
    1. The final pack configuration is designated as 
    2. “3S5P pack” with a final specification of 
    3. 11.1V nominal and 
    4. ~8.4v-12.6v output range
    5. ~5000mAhs @ 12v (loosely based on 15x 1200mah 3.7v cells) and 
    6. 60 Watt hours. 
    7. 0.06 kWh pack

Connect cells and BMS

  1. Install cells in cell holders
  2. Flux and Tin every battery positive and negative terminal.
  3. Cut bus bars to length
  4. Doublestick busbars to battery
  5. Flux and tin every busbar in 5 spots 
  6. Solder each cell to the busbars. 
  7. Solder power pigtail to BMS
  8. Solder bus bars to BMS
  9. Solder balance wires to BMS

Thank you!:…

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