Although batteries have chemical energy enclosed in a mechanical housing, we monitor them electrically while they are in use. The discharge characteristics are an important parameter to be monitored while energy is being taken from the battery.
Above is a graph of what discharging profiles for a Lithium-ion cells looks like. This is an example of how cell voltage drops with discharge capacity.
It’s important to note that the cell voltage shown here is the terminal voltage of the cell. If you are holding a cell in your hand, you can measure the terminal voltage by connecting a multimeter’s positive probe to the positive terminal of the cell and the negative probe to the negative terminal of the cell.
When the cell is in use however, the terminal voltage can differ from the ‘true’ or ‘open circuit’ voltage of the cell. The magnitude of this difference depends on the cell’s internal resistance.
If you take a look at the graph above, there are three distinct regions to be noticed. The first is between 0 – 100mAh where the cell voltage and discharge capacity have a non-linear relationship. The second is between 100 – 2100mAh where the cell voltage and discharge capacity have an almost linear relationship. The third is between 2100mAh and 3000mAh where the cell voltage and discharge capacity again have a non-linear relationship.
This graph and similar graphs give a lot of quick information on the cell being used. For example, you can tell that this cell from LG Chem has a capacity of around 3000mAh. You can also tell that the cell has a nominal voltage of around 3.5 – 3.7V with a max voltage of 4.2V and a minimum voltage of 2.5V. This means that if you are designing a battery management system to monitor these cells, its undervoltage should be set to 2.5V and overvoltage to be set to 4.2V.
Another thing to be noticed here is the relationship of current with the discharge profile. The graph at 3A (shown in red) of current looks different than the one at 30A (shown in brown). If you take a look at the mid-point around 1500mAh or 50% SOC, the voltage of the cell when being discharged at 3A will be around 3.55V and at the same 50% SOC, the voltage of the cell while being discharged at 30A will be 3V.
These numbers tell us that at higher discharge currents, the terminal voltage of the cell is lower at the same SOC %. This is also how the Battery Management System (BMS) knows how much current it can take from the cells at any given point in time! A current demand that makes the cell voltage drop to below 2.5V in this case will be violating the undervoltage rule of the cell, hence it will be rejected by the BMS. In other words, when calculating how much current the cells can deliver at a certain point in time, it is useful to look at what the cell voltage will drop to if that current were to be taken from the cells at a specific SOC %.
You can see how much information one graph can tell us about the cells and how to design the BMS around it. There are many more graphs to look at when looking at the discharge characteristics of cells, but the one shown above is definitely an important if not the first one to look at.