# Running 7V motors with 2x 3.7v batteries

We're really enthusiastic to build a homemade quad-copter with almost no knowledge about such things. We will probably use Arduino and Bluetooth to control it, but we have a question about the motor power supply. We're planning to use a phone battery to power the motors. Our motors run at 7-12V, but our batteries are 3.7V, 1450mA. The question is will the motors run smoothly if we connect them like this:

and theoretically will they be enough to power 4 motors. If they're not, can you suggest any cheap solution.

• Wow great diagram. Yeah they'll probably run OK but a quad copter requires a little bit of subtle control to stop it tipping up, crashing and burning. If the four props aint giving precisely the same lift things can go wrong in about 1 second!!! Commented Jun 1, 2013 at 23:12
• Well, I have 6 year experience of programming, I'm planning to catch a android phone to the quadcopter and use its accelerometer to control the 4 engines. I will connect arduino to phone by bluetooth, then I'm gonna stream via UDP by mobile network to my laptop for maximum range. On the question, do you think it will run smoothly? Commented Jun 1, 2013 at 23:14
• @Deepsy If the four motors are in parallel (as per your diagram) you cannot control/trim the quad-copter so NO, I have to say I think you'll crash and burn, sorry. Commented Jun 2, 2013 at 13:37
• @Andyaka they won't be connected like this of course, this is just a simple diagram to get better imagination what I'm trying to do and it will be enough. Commented Jun 2, 2013 at 19:00

Battery voltages add in series, so two 3.7V batteries should give you 7.4V, which should work.

You are also limited by the current your batteries can supply. With them in series, two doesn't change the current they can supply, so you can get 1450mA. You have four motors that will need to share this current, so each motor can have up to 362.5mA, if the batteries are powering only the motors. The control electronics will also require some current, and running your batteries at the maximum current will discharge them quickly.

Is this enough? How heavy is your quadcopter? What does the motor datasheet say? The quadcopter you are describing, with an Arduino, and an Android phone, sounds pretty heavy. Without more information it's impossible to know for sure, but I'm guessing you will need bigger batteries. There are plenty of batteries designed for remote control aeroplanes and helicopters. I'd investigate those.

• He probably means mAh not mA on the batteries, so there isn't such a hard limit on the motor current. Still sounds hopelessly underpowered to lift a phone though.
– user16324
Commented Jun 2, 2013 at 11:03

If the motors are brushless (common for quad-copters) you should look into using one ESC (electronic speed controller) to control each engine. They will provide all the circuity for using your batteries to control the engines and change the speeds more easily.

Electronic speed control is used to control the motor and then you can use PWM (Pulse width modulation) to control the speed.

There are good resources on how to build a quadcopter and this one is just the tip of an iceberg. I recommend you reading a lot before you even touch the soldering iron. Even if you collect the necessary components and assemble it together, there are still many days of work and debugging before you fly for real (and debugging a flying device means frustration and money).

First of all, carrying a phone is not a good idea. It is heavy and expensive. Better buy an IMU breakout board and connect it to Arduino. Then use the extra power to carry a bigger battery and fly longer.

I built a quadcopter with Arduino Mini clone and MPU6050 accel+gyro. That makes a good and simple Flight Controller for ~\$20. You'll need 4 ESCs, a receiver and transmitter as well.