Put any notes, research, thoughts, presentations, anything here!
https://docs.google.com/presentation/d/1FwqgqGEs_yCQbeGbVTrPfB589TDN_9QRxWRRX3g2-7Y/edit?usp=sharing Anyone can add to this slide show for the knowledge share.
This was the procedure for sample cell testing.
Resources:
MSXIV Li-ion Cell Testing Procedures (for cell selection)
A note from the above source says that it would have been much more accurate to use a Source Measurement Unit that can regulate the rest time between charge and discharge. This source explains what this is. We may need to look into purchasing this. (NVM they cost too much, but we could borrow a demo unit)
https://www.analog.com/en/analog-dialogue/studentzone/studentzone-december-2017.html
The procedure and results from Bulk MS14 tests are here.
High voltage devices are dangerous so we should read up on safety before doing testing in the bay.
Notes:
Currently it seems like testing sample cells won’t give us any useful information because of the variance in cell metrics. It seems like it would be okay to trust the data sheets of the reputable cell manufacturers. However, testing sample cells will be great practice for when we need to test the cells in bulk. If we buy less variety of cells for sampling and more of each type, the test information would be more useful.
Metrics that are of interest (from Battery Testing ):
Self Discharge:
Can be measured with a self-discharge measuring unit
A single cell with a significantly higher self-discharge current will cause the module to have a larger self-discharge current (if sitting in the bay for a month), which will cause an imbalance in SoC of the cells and lead to decreased capacity and damage to cells
Over the course of competition this will not have a significant difference in SoC
Cell Weight:
There is a loose correlation between cell weight and capacity (more stuff = better)
Capacity:
Difference in capacity between cells in parallel can cause current spikes at the end of charge/discharge cycles
The cells will need to be discharged at different rates to maintain the same voltage, which will cause the pack to become unbalanced
If a pack becomes inbalanced in parallel, (say all cells are discharged except one that has a higher capacity), then the higher capacity cell will charge the lower capacity cells until the voltage is equal. This causes a fast change in current direction and decreases the pack life
Should look into how much of a difference in capacity is too much
DC Internal Resistance
measured by putting a pulse discharge current on the cell and measuring voltage drop
The voltage drop can be attributed to the series resistance in the cell
typical values are 30-80mOhm for new 18650
AC internal Impedance
similar measure to the DC internal resistance except measured with a 1kHz sinusoidal signal applied. This is typically a single point on the EIS measurement curve and is generally not too useful, as there are better methods of determining power loss and voltage drop.
need to look into
EIS Measurements
need to look into more
sweeps frequency of an applied signal across wide range of frequencies to determine cell impedance at each of the frequencies
uses algorithms to fit cell models to the measurements
Many measurements can then be derived from the model
