Can I use two 9V batteries in series to power an Arduino Uno?

I have 9V to jack (photo above) and I want to splice in a second 9V battery so I the Arduino will be running on 18V. Is there any potential issues with this?

I also want to splice in a switch too, so I can turn the whole device off to save on battery life but that's an afterthought.

It would be over the recommended 12V and just under the max of 20V, and anyway it works on 7V…

The board can operate on an external supply of 6 to 20 volts… If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.

Why would you want to do that? The regulator on the Arduino Uno is a linear regulator. All the power, and I mean every bit of it, from the extra 9V regulator will be wasted as heat.

I am going to be running a lot on the Arduino UNO including a Digital-to-analog converter, an LM3914N-1 bar/dot controler, a micro SD card reader, a transistor that is pulling 5V to power a speaker and several LEDs.

I want to make sure i have enough power for everything.

But putting two batteries in seriese will not give you any more power than from one battery. It all gets wasted in the regulator.

It was recommended by a friend of mine and I thought it sounded odd, so I thought I would ask. So wound one 9V handle all my equipment?

Well those 9V things are notorious for not holding much energy. It may have enough power, but it probably won't last long, since energy is power x time.

It's for a prop so it's not going to be running 100% of the time and I can make the battery easy access so I can carry spares.

You can put identical batteries in parallel, then the combination will last about twice as long.

jremington: identical batteries

.... but that's the tricky part.

jremington: You can put identical batteries in parallel, then the combination will last about twice as long.

No you can't. They will cross charge and heat up.

Grumpy_Mike:

jremington: You can put identical batteries in parallel, then the combination will last about twice as long.

No you can't. They will cross charge and heat up.

Surely not if they were truly identical? (Not that they will be......)

When someone asks for help I assume it is practical help they want, not theoretical help that postulates impossible conditions.

No batteries are ever truly identical even when brand new from the same pack.

Exactly, so stick to just one, assuming it can provide enough power in the first place.

The worst case scenario is I can run some of the devices from a secondary battery that is not connected to the Arduino. I can run the DAC on a secondary battery without issue and I might also run the SD card reader off that as well.

I can run the DAC on a secondary battery without issue and I might also run the SD card reader off that as well.

You can but I would assume that your DAC requires 5V and your SD card requires 3V3 so you need some regulators as well.

Instead of all this fannying about why don't you just calculate the current you will need?

Grumpy_Mike:
Instead of all this fannying about

Will the real DI Grim please stand up…

Do you have more room? Can you fit something slightly larger in, maybe something longer? A 6 cell AA pack will last a lot longer than a 9V radio battery.

9V alkalines don't pack much punch, note that the best on this list is 490mAh at 100mA draw but that is for a brand I've never heard of. The other alkalines are more in the range of 300mAh, with capacity going down a lot at 500mA current: http://www.powerstream.com/9V-Alkaline-tests.htm

Whereas a 1.5V AA alkaline is over 2Ah at 100mA, and still over 1Ah at 500mA: http://www.powerstream.com/9V-Alkaline-tests.htm

6 cell AA holders: http://www.amazon.com/Philmore-Battery-Holder-Standard-Connector/dp/B000LFVFU8/ref=sr_1_1?ie=UTF8&qid=1412789999&sr=8-1&keywords=six+aa+battery+holder

http://www.amazon.com/Battery-Holder-6-AA-Long/dp/B00LPXLDF2/ref=sr_1_2?ie=UTF8&qid=1412789999&sr=8-2&keywords=six+aa+battery+holder

http://www.amazon.com/TOOGOO-Battery-Batteries-Holder-Storage/dp/B00H8KVHT8/ref=sr_1_5?ie=UTF8&qid=1412789999&sr=8-5&keywords=six+aa+battery+holder

Enclosed with a power switch: http://www.amazon.com/Spring-6x1-5V-Battery-Holder-Storage/dp/B00HG8EMM6/ref=sr_1_4?ie=UTF8&qid=1412789999&sr=8-4&keywords=six+aa+battery+holder

Powerjack for Arduino already installed: http://www.amazon.com/Beautyforall-Battery-Storage-Holder-Arduino/dp/B00L0642SE/ref=sr_1_6?ie=UTF8&qid=1412789999&sr=8-6&keywords=six+aa+battery+holder

Or use an Ultralife Lithium 9V battery. http://www.batterymart.com/p-ultralife-9v-lithium-battery.html

Grumpy_Mike:

jremington: You can put identical batteries in parallel, then the combination will last about twice as long.

No you can't. They will cross charge and heat up.

That would not be putting two batteries in parallel, but in series and then shorting them. Two batteries in parallel is both positive terminals together and both negative terminals together. They can then supply 9V to the circuit for twice as long as a single battery.

And they only need to be the same capacity (Ah) and voltage; they don't need to be "identical". The same brand bought at the same time would be enough.

Wade.