Powering circuit and Arduino with single power source

I am trying to power an Arduino Uno as well as my circuit using Li-Ion batteries. They are 2 18650 Lithium Ion batteries (rated at 3.4 V each = 7.4 V total).

I have this battery plugged into the Vin pin of the Arduino and in parallel is the circuit I am powering. The circuit requires approximately 450 mA.

The issue is, when I have everything actually hooked up I only get power going to the Arduino. The remaining circuit can pull a max of maybe 15 mA. When I use a regulated power supply (such as a programmable power supply) to power the Arduino, and the batteries to power the circuit, everything works just as intended. The issue arises when I try to power both Arduino and circuit with just the Li-Ion batteries.

Also worth noting is that the input of the circuit I'm trying to power is a 5 V voltage regulator. But it is rated up to 1.5 A; much more than my circuit needs.

Where am I going wrong?

• Can we look at a schematic of your circuit? That would help a lot. – Ricardo Aug 13 '14 at 22:56
• What result do you get if you use only the bench power supply instead of the batteries (rather than in addition to them)? – JYelton Aug 13 '14 at 22:57

Depending on who's 7805 you have, the minimum voltage you'll need on the 7805's input to get a reliable 5V out could be 7.5V, so the 7.4 volts from the battery may not be enough, no matter what.

To find out what's going on, you could do the following:

1) Connect the battery to the Arduino, the programmable supply to the 7805's input, and an ammeter between the 7805's output and the load.

2) Adjust the supply's output to about 10 volts and turn everything on.

3) Reading the 7805's output current, adjust the supply until it falls to 15 milliamperes, and make a note of that voltage.

4) Increase the supply voltage until the 5 volt load draws its proper current, and make a note of that voltage.

5) Turn everything off, disconnect the supply from the 7805's input, and connect the battery + to the 7805's input.

6) turn everything back on and measure the 7805's input voltage.

If it's less than the supply voltage you noted earlier with full current into the load, then the 7805 is the problem.