I have a very simple PCB that just connects an arduino nano, an MPU6050 accelerometer, an NRF24L01 transceiver, 4 resistors, a speedybee 4in1 esc and a 4s lipo. This is the schematic:
When I plug it in via USB, the 3.3V pin is perfect at 3.3V and the 5V pin is ok at 4.8V, but when I plug it in to a 4s Lipo charged to about 16.5V, the 5V pin sits at 2.9V and the 3.3V pin sits at 2.55V. One strange thing to note is that right after it's plugged in, both pins are at the correct voltage, then over the course of about a minute they drop to the values I mentioned above. Why would this be happening?
P.S. after further measurement, it seems like the voltage oscillates slightly before it settles. the 5V pin might go as low as 2.7V before going back up to 3V then back down to 2.9V. I'm not sure how many times it oscillates like this. If there are more measurements i can do to provide more info please let me know.
Vin=16.5V? Way over voltage for the Nano's regulator. Your regulator is struggling, as evidenced by 2.9 V at the 5V pin. When the 5v droops that far, the 3.3V regulator (which has as it's input, the 5V output) cannot regulate.
You'll have to find some way to drop that input, I'd suggest to around 7 VDC. Use a buck regulator.
Also, what @Delta_G said. You've got two problems here.
12V.
Clones generally have similar or poorer/cheaper regulators. The regulator in common use has, I believe, a 15V max, so 12V is a nice 80% of design max rule of thumb.
Two limits to observe, maximum voltage, and maximum power dissipation; it's all about heat dissipation: (VIN-5V)*total current. That heat has to go somewhere. Bare Arduino, 12V is fine. Arduino with loads on 5V(LEDs, etc. count), you soon run into heat dissipation limits.
Other Arduinos, clone or real, have different regulating arrangements and hence different voltage input ranges.
Can I suggest a 10uF capacitor on the 3V3 supply to gnd, this is to help with the peak currents caused by the NRF going into transmit, thus stabilising the supply for the MPU.
Also a 0.1uF capacitor in parallel with the 10uF for bypassing.
A 0.1uF between A0 and gnd would also help minimise noise on the ADC measurement.
They are somewhat standard values to start with, 10uF on the 3V3 is about the maximum as the initial turn on current may exceed the limit of the on board 3V3 regulator.
Also you can get adapter boards for the NRF that let you run off 5V, from what I have seen they also use this combination on the 3V3 output.
