Hey PN you like to test and play with batteries don't you? Want to try a test I did about a year ago and post your results so we can compare results. I did this just over a year ago when I decided to switch from Top Balance to Bottom Balance LiFePo4 cells in my cart. You will have to saccrifise at least one 18650 cell maybe two because it is a destructive test. I have two 1450 mah 18650 LFP cells if you need them and the Power Resistor required. Just hop on DIY Electric EV and drop me a PM with your address. Otherwise I think you have everything else.
Material needed:
Method:
Material needed:
- 2- 3.2 volt LiFePo4 18650 cells. Any manufacture or capacity will do. They just need to be the same.
- A Hobby Charger capable of Charging/Discharging, and Capacity test. I think you have an Icharger? I used a Cellpro Power Lab 8. Both work
- 1 10-watt 2.2 Ohm power resistor. Value is not critical, all depends on what you have and how much time you want to take on discharge. C-Rate does not matter as long as you do not exceed the maximum discharge rate of the cell. IMO the faster the better.
- DMM
- Jumpers to connect components
Method:
- Run each of the two cells individually through a couple of cycles on on your hobby charger. Label them and record full capacity and Internal Resistance at 50% SOC for future reference. Do so in a room where temps are stable and cells have plenty of air circulation to keep temps consistant.
- Choose either battery, makes no difference which one, or if it is even charged up. We are going to try to kill it. Terminate it with power resistor and walk away. I did this just before going to bed for the night.
- Come back after some time and measure and record voltage with resistor still terminated. Should be ZERO volts with Resistor still terminated. Remove resistor, and take a break for a few minutes.
- Come back after break and measure voltage of dead cell. If like me you should see something just over 2 volts. That should make you scratch your head.
- Try to cycle the battery with 1 charge to 3.65 volts, 1 discharge to 2.5 volts, and then charge to 3.65 volts until current tapers to C/33 if you can. I think you can reprogram the Icharger from default C/20. If not C/20 is just fine. Record capacity and leave it charged up. What i recorded is a battery with only minor capacity loss of less than 1%. Not enough to really get an accurate measurable difference.
- Now with both batteries fully charged, discharge either one and bleed off about 10% capacity to 90% SOC. You want one battery with 10% higher AH capacity than the other
- Connect both batteries in series for 2S configuration or 6.4 volts.
- Terminate with resistor and walk away again. Let it die.
- Measure voltage with resistor still terminated and you should see ZERO volts again.
- Remove resistor and separate the two batteries, and take a break.
- Come back from beak and measure the voltage of both batteries again. One should be 0 volts (the weaker one you partially discharged before the test), and the other up around 2 volts. You should be scratching your head again and lights going off in your head by now.
- Try to cycle both individually in your hobby charger and report results.
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