A battery is essentially a chemical process inside a box. The battery has chemical energy and this is converted into electrical energy when needed.
Electrons flow from one electrode to the other in the battery. This flow produces an electric current. This current flow is the current you use to power equipment.
No-load voltage #
One of the properties of a cell is that the voltage depends on the amount of remaining energy – the state of charge (SoC). The more energy, the higher the voltage when no load is applied. This no-load voltage is called the open-circuit voltage (in short OCV). However, the exact relation between open-circuit voltage and SoC depends on the chemical characteristics of the battery. For example the LiFePO4 has a fairly flat curve, especially between 40% and 80%, while NMC and NCA chemistries have a steeper slope.
Example of the LiFePO4 OCV vs SoC slope:
Internal resistance #
Each battery cell has it’s own impedance. To keep it simple, we will only discuss the DC resistance. The internal resistance of the battery cell depends factors like battery type, manufactering process, age of battery and temperature. In general you want a resistance as low as possible. Less resistance means less power loss and problems. A BMS keeps the battery as healthy as possible, to keep the internal resistance low.
Voltage drop #
The internal resistance has effects on the measured voltage. The voltage drop over the cell resistance can be calculated with the well-known formula:
U = I x R
This means that the voltage drop is the discharge current multiplied by the cell resistance.
Let’s take a generic LiFePO4 90Ah cell which has an internal resistance (with DC currents) of about 1mΩ at 20°C. When you draw 40A from this cell, the voltage drop is:
40A x 0.001Ω = 0.040V
In other words, when you discharge with 40A, the cell voltage will directly drop to 40mV below the no-load voltage. If the no-load voltage was 3.30V, the cell voltage will directly drop to 3.26V. When the load is removed, the drop disappears and the voltage will go back to the no-load voltage.
Higher voltage when charging
The voltage drop is reversed when charging. Instead of a lower voltage, the cell voltage will become higher when charging. The higher the charging current, the higher the measured voltage over the cell. Following the same example as above, A 40A charge current makes the cell voltage rise to 40mV above the no-load voltage. The cell with 3.30V no-load will instantly rise to 3.34V when charging with 40A.
Influences on resistance #
One of the biggest determining factors on the internal resistance is the capacity of the battery. In general: the bigger the capacity, the lower the internal resistance. A smaller resistance means that you are able to charge and discharge with higher currents.
Another factor is the temperature. The optimal temperature is between 20-40 degrees and gives the lowest resistance. Below 10 degrees, the resistance inceases fast. This is the reason why electric cars first need to warm up the battery before supercharging!
Time to rest #
Ever noticed that when you stop charging, the cell slowly drops back to a lower voltage, even when the charge current was remove some time ago? This is because of the chemical process. A cell needs time before it is fully at rest. For LiFePO4, this can even take up to 24h.