Check engine light

I got my 2001 Toyota rav4 fixed a few weeks ago
since the check engine light was on. Two days
after repairs the light came back on and was
affecting the gear changes again. I waited a day
and it went away and the car drove normally. It's
been approximately 2 weeks and the light came
back on.
I told my parents and they told me that the
mechanic told them that it was due to the fact that
I had plugged in my phone chargers and the
battery was affecting my engine.
I want to know if this is a reason for my engine
light to be on or is my mechanic scanning us.