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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)
do we need to test this if we test capacity?
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
might not be as important as we think? i think this only applies in parallel but not parallel and series connections
DC Internal Resistance
useful in determining power loss and voltage drop?
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
need to look into
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.
shows early sign of weakness in cells
useful in determining power loss and voltage drop but there are better ways to do this.
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
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