problem connecting two arduinos

i have a question. im connecting two arduinos. one is sending a digital signal to the other. when i connect both grounds and connect the battery to the arduino that is sending the digital value, the other arduino turns on even when it doesnt have a battery connected. it appears that both arduinos are sharing the same battery and i dont really know why this is happening. does anyone have a clue about what is going on? i really need help. ty all

Yes, it’s possible but is not a recommended way of doing things. You should always provide power to all devices that are operating.

There are diodes built in to the I/O pins of each processor and when you send data to an I/O pin, current can go through the diode and provide power to the rest of the chip. Those are very low-power diodes however and easy to destroy (they are meant only for short-duration ESD events, not constant current draw).

Connect the battery to both Arduinos.

but is it ok that when i connect both grounds and power up the arduino that is sending the signal, that the other arduino turns on? so should i just connect the battery to the other arduino even if it is already turned on?

I would connect the battery to both Arduinos, even if it appears that the second one is on.

Basically, you are "getting away with it" for the moment, and there's no guarantee how long that (and your chip) will last.

You are most likely 'powering' the second Arduino through it's I/O pin by a 'back door' route via it's clamping protection diode to it's Vcc. That is a not a safe way to proceed and you should power both boards at the same time, either from one power source or two independent voltage sources.

Lefty

allright ty for your answers. i'll do as you say. i thought that perhaps i was connecting something wrong because the other arduino turned on and i didnt know how to proced but now i know that it will work if i power up both arduinos.