I have realize that I always get differente voltaje measure from differents devices
As you can see in the next picture, no voltage is the same. The one on the FC is 22.13, using the power module it came with the black cube. (power brick)
Use a discharged battery or adjustable power supply and calibrate the Flight Controllers voltage sensor at a low voltage, using you digital multimeter as the reference. It will be closer than the cheap battery monitors.
Most standard “power bricks” that come with the flight controllers are a little non-linear, so best to have the voltage being accurate around the level where you’d expect a battery failsafe.
Set the sensor type to “Other” and follow the instructions to set the voltage shown on your DMM. https://ardupilot.org/copter/docs/common-powermodule-landingpage.html
Multimeters can run from free (Harbor Freight gives them away with a purchase) to a lot of money for a branded meter like a Fluke. I have several from cheap to expensive. A $25 one from the same Harbor Freight I picked up while in the field has been working as well as my $150 Fluke for Hobby purposes. But what’s available would depend on where you are I imagine.