Balancing LifePo4 cellS

2022-05-08 Off By Jonny

Our new cells arrived 6 months after order.

The cells are CATL 302Ah 3,2V Lifepo4 with dimensions H 207mm x W173mm x D71,65mm. They where ordered directly from China from Docan. The QR code was not readable via iPhone camera but the code seems legit and also stamped directly in the Aluminium.

This is how I performed the top cell balancing. I strongly advice reading up on Lithium SOC as the Voltage curve is very different from a lead battery. I’ll show you my readings and how long the Voltage stays the same until it all of a sudden rises quickly.

The Cells placed prepared for busbar installation. All 24 of them are placed in parallel.

Busbars CATL 302Ah 90x20x2 m6

Auto grade(cheap) DC measurement

Measuring the initial cell voltage with a normal cheap Auto multimeter.

I was a bit suprised with that measurement as I was expecting them to be around 2,92. I therefore went out and bought another instrument that has a higher accuracy 0,05% as well as 5 digit display capability.

High grade DC measurement

Now it shows a more realistic value, hwn measuring all 24 cells they where all within 3mV difference except one that was at 3,288 so 5mV difference. That is excellent readings.

Parallel connection

All cells are connected in Parallel and hooked up to the charger. The charger was first adjusted to 3,650V and should according to specification be able to deliver 120A, in my test it only max produced 65A.

Charging

Start Voltage 3,292 with all cells in parallel.

Day 0 2022-04-27 22:08, charging started

Day 1 2022-04-28 21:37 3,347V 60A current

Day 2 2022-04-29

  • 07:45 3,365V 51A current
  • 11:37 3,366V 54A current
  • 18:00 3,366V 48A current
  • 22:45 3,370 51A current

Day 3 2022-04-30

  • 07:45 3,370-3,380 41,4 A
  • Disconnected charger 3,362
  • 21:41 3,370 -3,389 45,5A

Day 4 2022-05-01

  • 09:00 3,390-3,403 39A
  • 19:00 3,577-3,581 16.0 A
  • 22:10 3,628-3,630 7,2A
  • 22:50 3,634 22:50 6,5A
  • 23:44 3,641 4,50A

Day 5 2022-05-02

  • 00:24 3,646 4,2A
  • 01:50 3,650 4,0A

30h later with cells left in parallel without charge. 3,473V

Separation of cells and 3 days later almost all of the cells are within 10mV difference and lowest and highest cell is 50mV from eachother.

I did notice a very slight swelling of cells after charging when separating the busbars. I will do a compression fixture for final installation.

CategoryThinking
TagsLifePo4 Lithium