I was wondering if anyone has any real numbers on how long a battery will last on a IR pen.  Assuming that you use a standard AA or AAA battery (or rechargeable), how often would one need to replace/recharge the battery?  Can you anticipate being able to use a IR pen for a whole day of interactive work before needing to replace/recharge the battery?

I guess I could set up an experiment to keep the LED on using battery power and set up some sort of timer to log the usage, but was wondering if someone else has already done such a test.

Iderooy/freezetech
Best way is to test in your situation. Freezetech because you are using a 1.5v power source you should have a light resistor this alone will extend battery life. Iderooy if you go for the 1.2v NIMH it will have a shorter life than a 1.5v as a below optimal performance voltage will be achieved quicker. Thus, there is little use comparing your setups.

You can do a constant drain test, but it is never used like that. Most importantly voltage and current decline is exponential against time so a constant drain test doesn't accurately transfer to a linear time sum. Simply put if the battery yields 1hr in a constant drain environment you can run it for "1min with 2min breaks" 60 times and the battery won't be as drained as after the constant 1hr.

My solution:
I have two pen's and use one until drains then charge it while I use the next until it drains then the cycle continues. They last anywhere from a few days to longer than a week depending on how many times I use and most importantly much I use in the lesson sometimes I will only use for 15-20 in a lesson and very intermittently within that 15-20min other times the whole lesson.

In my first few months I setup a whole lesson then couldn't calibrate, took me a while to work out that because I stored everything in a plastic bag, the switch must have been pressed 'on' draining the battery. I now carry two pens and have outfitted a fancy tuperware container with fitted foam to fit my 2 pens, charging cable, wiimote, Presenter IR-Source & minitripod (looks pro!).

Benpaddlejones

Ben's right, it all depends on your setup, so hard to compare different pens. We chose alkaline AAA to power our pen. They drive an LED That needs a minimum of 1.2v and can (briefly) handle 1.6v. With our battery choice (alkaline), you have to be aware that the voltage decreases as the battery is drained of energy. Through lots of measuring we have found that a quality AAA alkaline starts at just under 1.6v and decreases to around 1.35v before it doesn't supply enough current to be useful.Again, through lots of testing, we have found a resistor value that protects the LED at full battery power yet still allows it enough current for as long as possible.
We are getting approximately 28 hours in  constant 'on' tests. This is at least several weeks of use in classrooms, but will obviously vary depending on how you use the pen. A complex question to give a precise answer to! One thing I will say though is that with cheap batteries you get what you pay for.