When looking for the best performing battery pack the internal resistance of the cell plays a significant role. Simple Ohm’s Law shows that as we deliver a voltage we get a voltage drop equal to the current times the resistance.
The heat generation in a cell is a summation of Joule and Entropic heating. At high currents the heating is dominated by Joule heating in most cases.
The Joule heating = I2R
In most systems there is normally a required power delivery. The higher resistance cell will result in a lower battery voltage and hence higher current. This results in more heat generated in the cells. Cell ageing is strongly linked to cell temperature and hence the higher resistance cell is likely to age faster.
As we increase the capacity of a battery cell we are effectively increasing the number of cells in parallel.
Hence if we double the cell capacity for a given chemistry and cell construction we halve the internal resistance.
Therefore, often we find ourselves searching for a cell with a low resistance, as this resistance is related to cell capacity we then use metrics to take that into accout.
There are two figures of merit that we see used:
- Ohm Ampere-hour
With the Ohm Ampere-hour we are looking for the lowest value for a given C-rate.
With the Siemens/Wh (lower graph) we are looking for the highest value for a given C-rate.
A repeat of the two graphs, this time zoomed in to a maximum of 50C as a maximum discharge rate.
Now we can see the general form of these two metrics.
The data in these plots is available in the Cell Database.