Is 20 days the max life for a unit on 9V battery?


I am looking at using an Arduino unit for a remote wireless (battery powered) door open/closed detector. I'm concerned however that even with a 9V battery that the battery life will only be around 20 days even with putting the Arduino into STANDBY mode (to save power).

Question - Is this correct? Am I missing something here re how I could make a 9V battery on an Arduino unit last say 6 months (which would need an average of 0.12mA current)?

Here are some of the current readings I've made on my Arduino Pro in each of the modes, plus my calculations re how long the 9V battery would last.

Table (I measured this using the 9V battery when the unit was in each of the modes)


If the Arduino Pro is a 5V board tt is probably worse than you calculate.
I am not sure which battery you are using but the Energizer 522 is apx 600mA
to 4.8V. You need to check the minimum voltage required to provide 5V
output and adjust the capacity.

Going from 9V to 5V wastes 4V so you have an efficiency of 5/9 (55%). A switching
converter will get you to 80-90%. Using a 3.3V board and a Li-ion battery (3.7V) gets
you to 85-90%.

(* jcl *)


If your external circuitary isn't too voltage critical, you can get away with 3 AA alkaline cells in series fed directly to the 5V input. It has several great adavnatges, 1-Its cheap, 2 its 100% efficient,3 the capacity of AA cells is vastly superior to 9V batteries and 4, it doesn't involve the tetchy demands of LiPo/Li-Ion batteries. (Hint: don't ever let LiPos discharge below 3 volts).

It has one or two downsides as well, but thst's life..... :slight_smile:

Standard 9 volt batteries are pretty useless for most typical Arduino applications. I don't know why the Arduino site shows so many photos using them? They are very costly for the limited mah they supply and lack the kind of peak current capacity that so many projects require. I guess their size form makes them attractive for begineers, but most find out soon enought what a waste of money they are.


thanks - so with 3 x AAA I should really be able to extend the range from say 20 days that I'd thought to about 70 days

The AAA cells have a capacity of apx 1200mAh which is about twice the 9V cell.
the 1200mAh spec is to a discharge voltage of 0.8V.

During the life of the total battery voltage changes from 4.5V to 2.4V If you have
a linear regulator in series with the battery you have an additional voltage
drop that will increase as the batteries discharge. You need to adjust the
capacity numbers for these additional voltage drops.

Also for lower voltages you need to decrease the operating frequency of the uC.
The maximum frequency at 2.7V is 10MHz (328P)

(* jcl *)

door open/closed detector

What is the microcontroller doing in this application?
I can't imagine a door monitor needing to operate at much more than two or three Hertz.

What is the microcontroller doing in this application?
I can't imagine a door monitor needing to operate at much more than two or three Hertz.

Good point :wink:

if it just detects the door open and closed (which could be done
interrupts) it would be very low duty-cycle. A stand-alone XBee could
easily give a few years on 2-AA cells.

(* jcl *)

Have you used a multimeter to see just how much current the arduino is drawing?

Have you used a multimeter to see just how much current the arduino is drawing?

yes - see my original post at the top

What is the microcontroller doing in this application?

Well I'm new to this stuff. I started out looking at X10 devices. The sensors seemed OK but I couldn't find the display panel I wanted. I stumbled across Arduino and thought they be good to let me customize my display of things. Then rather than trying to get an X10 RF receiver interfaced to an Arduino I thought just to use Arduinos for the sensors too. :-/

A stand-alone XBee could easily give a few years on 2-AA cells.

Wow thats great if it's an option ;D I had come across an XBee as just a way to get wireless comms from one Arduino to another. Because they were a bit expensive I've actually been using the cheap RF transmitter & receivers that are available. I've got a basic reed switch setup from Arduino to Arduino using these, but my issue I'm seeing now is the lifetime of the batteries to power the remove sensor.

Do you know off hand which is the cheapest XBee unit I'd need to be able to use it to send door open/close messages back to a base Arduino (with a mounted XBee), noting I'm planning to have several (< 10) monitoring points transmitting back to the base. I see there are quite a few options for them at: XBee - SparkFun Electronics

A standalone XBee is more difficult than an XBee + Arduino. You
need to program the XBee directly. To get the low power consumption you need to manage the sleep modes properly. My writeup is at Loading...

On my boards (Loading...) I usually
use the XB24-ACI-001. These are Series 1 devices with a chip antenna
and cost between $19-$23. The range is 200-300ft. I like the chip antenna
because it does not protrude the board and is less likely to be broken.

(* jcl *)

Thanks again - is there something easier than the xbee by the way? Or is this the only real obvious choice for what I'm doing?

I have only used the XBees. The XBee is a system-on-a-chip (SOC) and has FCC certification. Very quick to prototype with. Standalone is more work. Optimizing
for the absolute minimum power consumption will require external

I am starting to look at Ember modules. Same price point. Not sure
about usability.

(* jcl *)

for the remote XBee do I need the "XBee Explorer Regulated"? X Bee Explorers Australia | Little Bird Australia. Or can I just plug say 3 x AA batteries directly into the XBee itself?

The XBee requires a power supply voltage between 3V and 3.3V so you
need a voltage regulator. If you are interfacing the XBee to 5V circuitry
you need to level shift too.

Your system sounds like you may just need a voltage regulator but it depends
on what types of other devices you need to interface.

(* jcl *)

For some reason I've needed to learn this lesson the hard way on about 5 different little projects :wink:

Toss all your 9V adapters out the window - they're virtually useless for most applications.

I don't know why so many tutorials and such online talk about using a 9V battery but for anything that is going to use anything more than the ATMega's (~30ma?), they just won't do.