Arduino Uno + Ardumoto + BT robot works from USB but not from baterries

Question

My Arduino UNO board is part of a small robot that works when connected to the laptop by USB but it does not when plugged to the batteries. The baterries plug seems okay, it outputs 7.8 V. My UNO is connected to an ArduMoto board that manages the robot motors as well as a bluetooth chip. I have little experience with Arduino or electronics in general so I am stuck on the best course of action.

1) Can I find out what is wrong with the power supply without completely isolating UNO from the robot?

2) If I separate UNO, how can I find out with certainty if my UNO power supply is damaged or not? Is there something in particular that I could measure with my multimeter?

UPDATE

After fully charging my batteries I observed that the LEDs attached to the motor outputs are blinking fast. They blink for about one minute then nothing happens anymore. On USB everything works as expected. Those LEDs show that signal is going towards the motors, but why is it so different to USB? On USB power, the LEDs blink only when I command the robot to move through the BT board while under baterries they blink furiously for a minute and that is all that happens.

I also don't know what happened to the batteries, they seem to be fully drained in just one minute, but I don't know how to test that, they appear to still have high voltage.

I use two LC 18650 4800 mAh 3.7 V UltraFire baterries. The motor shield is Ardumoto Keyes_L298P, very similar to Sparkfun's Ardumoto shield.

I do not have the actual code that is on my Arduino. It must be similar to the code on this page, since I am using their Android app for BT communication with my robot, and it works on USB power supply.

• TheDoctor: Do you have anything in your code that says while (!Serial); inside setup()?
• imjosh: what happens when you plug in the battery plug? any LEDS on the arduino light up?
• Anonymous Penguin: Does the basic "blink" example work? If not, then it's hardware. Otherwise, software. A few other things: is battery providing enough current and charged fully? Check both of these with a multimeter. Are you positive that all connections are correct (VIN and ground) and there are no loose connections?
• grokkaine: on usb everything works, on bateries dead silence not even a led. to test the basic blink I guess I must replace the current program. I was hoping that I can diagnose the power somehow be fore it bridges with the USB source since that works. Batteries and plug are fine, I only measured the voltage, 7.8 should be ideal right?
• evan.stoddard: Stupid question but you sure the polarity is correct? The power led should at least light up... Also, are you using the VIN pin are the power jack... goes to the same place...

Answer 1

Are you powering the motors with a separate supply, and not direct from the Arduino? Motors should be powered separately and the power only switched by the Arduino. Otherwise, their current draw could be pulling down your battery voltage. And even with USB power, they could be drawing more current than spec'd for either the USB power source or the Arduino's regulator or output pins. The chip itself is spec'd for a maximum current of 40mA per pin and 200mA total.

• grokkaine: The motor shield also contains the bluetooth slot and everything is powered through Arduino UNO, either through USB or batteries. I only measured the voltage on the source, but I will try to verify the intensity details at the batteries as well as the Arduino output pins.