I have a few question about power supply to my electronics.
1]I have a 2 servo motors I want to power. I have hooked up 4 AA batteries to provide 6V. However, the multimeter shows about a voltage of 6.9-7V. Should I be worried about my servos because their maximum rated voltage is 6V?
2]Also, I am using two ultrasonic sensors that I want to power from the same power supply. However, I couldn't find any information online about this except using a DC-DC converter. Is there any way I could do so without doing so and instead using a resistor?
3]Along with this, can i power an arduino with a 9V battery? I have heard 9V batteries are terrible for constant current, so would it last long?
I think your meter may be a little off. A brandnew AA will be around 1.65V at most, so 6.6 V for all 4. That's within a +/-10% tolerance, so it's OK. Besides, once you start using the batteries, they will quickly drop to 1.5V.
Along with this, can i power an arduino with a 9V battery?
Yes but depending on the arduino and how you use it it could last anywhere from 1 hour to maybe 8 hours.
Also, does connecting the 9V battery in parallel with another one work for more time,and can I put the arduino in the same power supply as the source for the servo and the sensor?
You can supply the arduino with 7V to 12V on either Vin or the barrel jack.
You can connect the SR04s to the Arduino 5V output.
If you want rechargable batteries, then use two 3.7V Li-ion batteries is series. That will give you 7.4V that you can connect to Arduino Vin. Then use a UBEC or buck to drop that to 5V for the servos. The ultrasonic sensors can be connected the the Arduino 5V output.
Should last many hours depending on the battery mAh rating and the servo current.
No two batteries will have exactly the same voltage or impedance. So if you put them in parallel you will have one battery discharging into the other. For primary batteries (non-rechargable) you should never force current into the battery, it could overheat and leak toxic gas, maybe even explode.