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I have a few Arduino's along with XBee Shields communicating with one another.

The problem I'm having is that, when all the Arduinos+XBee Shields are powered via USB(PC) they seem to work fine and all transmissions get through to one another. But, when I use them with 9Volt batteries(fairly new batteries) the whole thing crashes and I never receive responses back from the battery powered ones.

I'm wondering if this is really a power issue? wouldn't a 9volt be enough to power a single arduino(uno)+xbee shield?

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Using a multimeter you can confirm your theory by looking for a voltage drop on the power connections to the XBee and into the UNO.

9V batteries don't store much charge, nominally 563mAh according to wikipedia. It is possible that the fairly new batteries are not that new after all.

Depending on the XBee, power consumption varies. 40mA for a normal XBee (ZigBee), 200mA for pro, and 309mA when transmitting for the new XBee WiFi. If the battery is low, the high current draw plus internal battery resistance may drop the 9V output to below the 7V needed for the UNO's voltage regulator to function correctly.

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The usb port on your computer can supply about 500mA of current. If geometrikal is correct about the 563mAh and you are using an XBee pro or a higher current draw radio, you could easily be hitting the current limit of the battery. Another aspect of this is the internal resistance of the battery. I'll let you read about it, but it is another current limiting factor. All of this boils down to once you are drawing more current than the battery can provide the voltage drops. Once it gets down to below what the Arduino board can regulate the microcontroller will reset.

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Is space an issue? Have you considered using 6 AA batteries? From what I've read basically the current provided by a battery is determined by it's size so C is bigger than AA which is bigger than AAA.

The 9volt battery is from what I understand made up of 6 AAAA cells and therefore has a much lower maximum current. about 563mAh rather than about 1500mAh depending upon the type you choose.

  • Current is measured in A (amps) or mA (milli-amps). mAh stands for milli-amp hours. This refers to how much current can be drawn for a single hour. For example with a 500mAh rated battery you could draw 500mA for 1 hour or 250mA for 2 hours. mAh says nothing about maximum current draw, except for how fast the battery will be drained (if you know the current draw). – Hair_of_the_Dog Jul 24 '14 at 16:17
  • I know i was over simplifying things but does not the fact of a higher mAh not indicate that the potential current is higher? I guess i need to do more reading – Nick J Adams Jul 24 '14 at 17:27
  • It is solely an indicator of capacity. For example if the ESR (en.wikipedia.org/wiki/Equivalent_series_resistance) is low enough you could draw 1000mA from a 500mAh battery for 30 minutes. – Hair_of_the_Dog Jul 24 '14 at 22:28

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