For most simple peak power calculations we will be interested in the Direct Current Internal Resistance (DCIR) value for a new cell at 50% SOC (25°C, 10s), Open Circuit Voltage (OCV) and minimum voltage. Note: this is just an estimation and this needs to be compared to the manufacturers specifications and tested values.
However, as an estimate it is a good starting point.
If we have an OCV of 3.7V @ 50% SOC and an internal resistance of 0.025Ω and we draw 10A from the cell the voltage will drop 0.25V This is simply Ohms Law.
V = 3.7V – 10A x 0.025Ω = 3.45V
Hence the voltage of the cell under a 10A load will be 3.45V
We can also calculate the maximum current we can draw taking the cell down to the minimum voltage:
2.5V = 3.7V – I x 0.025Ω
I = (3.7V – 2.5V) / 0.025Ω = 48A
These numbers are quite typical of a 5Ah NMC cell. Peak discharge is around 10C.
This post has been built based on the support of and sponsorship from: Quarto Technical Services, Eatron Technologies and About:Energy
If we want more power then we need more voltage or more current. We could:
- use a large battery cell
- put more cells together in series / parallel
We can extend this to look at peak power vs SOC if we have the OCV and DCIR values versus SOC. This simple calculation is now a sheet in the Battery Calculations Workbook.
The interesting result is that the OCV dropping and the DCIR increasing means that the power capability of the cell drops significantly below 20% SOC.
It must also be noted that DCIR increases with the age of the cell and that it changes versus temperature.
Relationship Between DCIR and Peak Discharge C-Rate in Lithium-Ion Cells
Correct management of performance parameters in lithium-ion cell technology plays a critical role in energy storage and fast discharge processes.