I have a 24V battery powering a brushless DC motor.
When the motor runs at low RPM and draws, say 10A, the battery reads 24V.
If I go full throttle for a second, motor will draw 60A and battery will read like 22V.
If I cut throttle and go back to low RPM, the battery will now read almost 24V again.
According to Ohm's law, V=RI. So I do not understand why/how the battery voltage temporarily reads 22V instead of 24V when I very briefly draw more current from it.
Most energy sources will "sag" when you increase the load. (The exceptions will be ones that use some sort of regulator and feedback.) Your battery is one of the ones that sag.
Two points:
Figure 1. A very simple example - an LED and a button cell. Image source: Battery and LED without resistor (mine).
LEDs are supposed to have their current limited and a series resistor is commonly employed. Many of the cheap key-fob LED lights don't have one. Why not?
The answer is that the cell's internal resistance is just about right to limit the current to a safe value for both the cell and the LED.
If you can measure the load current it is easy to work out the internal resistance, \$R_i\$ (using Ohm's law).
$$ R_i = \frac {V_{OC} - V_{LED}} {I_{LED}} $$
where \$ V_{OC} \$ = the cell open-circuit voltage, \$ V_{LED} \$ = the voltage when the LED is connected and \$ I_{LED}\$ is the current through the LED.
You can read a little more in the linked article.
To a first approximation, battery voltage is:
$$V_{bat} = V_{oc} - I_b \times R_s$$
Where:
The instant you put a load on the battery, its voltage will drop a bit. And the instant you disconnect the load, the voltage will increase by a bit. This instantaneous change is due to current flowing through the series resistance of the battery.
In addition to this, there is also a phenomenon of voltage sag in batteries where if they are under heavy load for a longer time, the voltage gets depleted above and beyond \$R_s \times I_b\$. This sag will eventually recover if the battery is allowed to rest with no load.
Only when the battery has been at rest for some time can \$V_{oc}\$ be directly measured at the battery terminal. In other words, if a battery is allowed to rest, then \$V_{bat} = V_{oc}\$.
