Voltaje is like the amount of water you have in a tank. If at the bottom of that tank there is a hole the water will start flowing through it and the pressure the water is flowing with depend on the amount of water on the tank, the more water the more pressure you just need to set a sensor (count how much water is flowing per second/time) at the hole to measure the pressure and you will know how much water there is left.
Edit: and voltaje is the amount of energy you have on a battery
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