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DIY Lifepo 12 Volt Car Battery


40 Amp-hour 12 V DIY Car battery - pre-assembly

23 July 2020

My Nissan Leaf is a 2011 model so the 12 V battery is getting a bit old and it's capacity will have faded over the last 9 years. Other Leaf owners have reported problems with there 12 V battery so I'm worried mine could die soon.

Also, I've got a bunch of 5 Amp-hour LeFePO4 cells from a few old e-bikes that would be perfect for building a replacement 12 V battery. So that's what this project is about.


3D model of battery box and lid

Parts:

  • I've measured up the existing Leaf battery and designed a new box to 3D print. (The main box base took 40.5 hours to print!)
  • Bus bar separator
  • I'll pick the best recycled 40 cells I've got to create a 40P4S LiFePO4 pack with a nominal voltage of 12.8 volts and a capacity of somewhere around 40 Amphours - which is what the original Lead Acid battery had.
  • I've bought a BMS rated at 80 Amps (Deligreen BMS 4S 12V 80A BMS for 3.2V LiFePO4 battery pack)
  • And a battery monitor meter (DC 10V~100V Lifepo4 Capacity Indicator 12V 24V 36V 48V 96V)
  • FR4 PCB material (from old ebike batteries) for cell group separators and to separate the BMS from the cells
  • packing/padding foam
  • 8 AWG wire for leads and bus "bars"
  • 5 Amp fuse wire
  • crimp lugs for the main external connections

One of the advantages of this battery over the installed Lead Acid one is it'll be a lot lighter - my guess is 7 kg (16 lbs) lighter, or about half the weight).


Battery box after 40.5 hours printing (0.5mm nozzle)

Todo:

  • weigh the old and new [6.9 kg] batteries to compare
  • capacity test the existing battery to see how's doing
  • print the battery box base (it'll take 2.5 days to print!)
  • capacity test all my LiFePO4 cells, and select the 40 best ones
  • solder the cells together
  • attach two sets of balance wires (the second set is for cell level testing prior to sealing the battery)
  • add the BMS
  • add the battery level monitor
  • add the main output wires
  • test it
  • pack the cells in with closed cell foam to dampen vibration
  • screw down the led
  • install in the car (I want to do this in a way that keeps 12 V connected to the at all times so the computers don't loose data/settings. I could (should?) plug a 12 V power-bank in to the 12 V power socket with the car turned on to ACC - that should keep the 12 V rail powered up whilst I swap out the lead acid battery.)
  • more testing
  • if all goes well - completely seal the box lid with silicon sealant.

Cheers, Paul



Page last modified on 23 July 2020 at 08:01 PM