We can measure the voltage of a battery across its terminals without any load connected. This is known as the open-circuit voltage (V OC). Note that because no current is flowing across the internal resistor, the voltage drop across it is 0 V. Therefore, we can assume that V OC is equal to the voltage of the ideal voltage source in the battery.
Safety: High internal resistance can lead to overheating, making batteries, especially lithium-ion ones, susceptible to failures or even explosions. There are several methods to measure a battery's internal resistance. Here, we'll discuss two common techniques: The DC Load method and the AC Impedance method.
The most common method for determining a battery’s internal resistance is to connect it to a circuit with a resistor, measure voltage through the battery, calculate current, measure voltage through the resistor, find the voltage drop, and use Kirchhoff laws to determine the remaining resistance, which is internal resistance.
Determine the voltage drop (ΔV) due to internal resistance: ΔV = V_open - V_load. This method uses an AC signal, typically at a frequency of 1kHz, and measures the impedance (combination of resistance and reactance) of the battery. Specialized equipment like an impedance meter or LCR meter is required.
Using the voltage readings from the "10k Ω Load" and the "No Load" (open circuit), calculate the internal resistance of the lemon battery. Hint: Refer to the Internal Resistance section to see how to calculate this value.
Then, by simply connecting the test lead to the battery terminals, you may measure the battery’s no-load voltage. Because the input resistance of the multimeter during voltage measurement is relatively high, frequently higher than 1M ohm, the impact of current draw may be neglected.