Li-po battery for Duemilanove and two servos

Hi all!

I've made a simple circuit with an Arduino Duemilanove and two servo motors (connected directly to the board, i.e., no shields). I've been powering it via power jack with 9V alkaline batteries, without problems. They usually last about 2h (which is OK for me), but I'm looking for a rechargeable battery to save costs and, possibly, extend this time a little bit.

My application requires a lightweight battery, so I'm thinking about a Li-po (even though I know about its overcharge / over discharge problems), and I know the recommended input voltage is from 7V to 12V.

So, I'd like to read your suggestions:

1) From what I read in other posts, a 1S Li-po (3.7V) with a step-up regulator seems simple enough and I could monitor the battery voltage with the Arduino itself (via analog input). Do you think this is the best way to go? Are there better alternatives? What step-up can you suggest? I have never used one before, so any pointers about this are also appreciated.

2) A 2S Li-po (7.4V) might (or might not) require a step-up converter, while a 3S Li-po (11.1V) seems like a safer bet, but OTOH, I can't measure their voltages directly. I read some messages about voltage dividers, but it seemed they were being used with power going to the VIN pin instead of the power jack... and most links for more information were stale (not found / 404 error). :(

3) Do you have any recommendation of how to make the Arduino shutdown itself when a low voltage is detected?

4) Extra question: do you know if/how I can cut off the power only for the servos (or for a specific servo), to save some power?

Thank you!

Ah, so many options! The motors will run on 3 LiPos? Why not do that then. Then get one of the stepdown regulators from pololu.com (on sale this weekend?) to make ~3.9-5V for the arduino. http://www.pololu.com/catalog/product/2120

Get some resistors & a couple of relays or analog switches (like DG406, search around at maxim-ic.com) to make voltage dividers so you can connect the 3.7V, the 7.4v (ex two 5Ks, measure the middle), and the 11.1V (ex 3 5Ks, measure across the bottom one) to the ADC to make measurements and deduce what ever you can. Put in some MOSFET switches so you can disconnect the motors from power via under s/w control.

Autoshutoff - software issue to deal with after you get the other stuff going. Have another voltage divider driving an input when, monitor the pin, when it gets dragged low then call it quits.

Thanks, CrossRoads!

A few comments: - I'm sorry if I was not clear in my first post. My plan is to power the Arduino with just one Li-Po battery, and power the two servos from the Arduino 5V pin - which seems to provide up to 800mA (even though I could not find this information in the product page). - It seems I can use a Pololu Pushbutton Power Switch SV (or LV) for a nice auto shutoff solution! :) - If I use a 2S (7.4V) OR a 3S (11.1V) Li-po battery, I won't need a step-down converter, and to measure the voltage I could use a 10K ohm, 1/8 watt potentiometer for a voltage divider and scale the value read from the ADC by software (so I can monitor it).

Does this sound right?

I would power the servos separately from the Arduino ouput. Why put all that stress on the board if you don't have to? The regulator will already be warm from dropping 7.4V or 11.1V down to 5V.

I wouldn't count on the 800 without looking up the regulator on your board. For instance, on my Duemilanove the regulator is marked 1117M3 500952L C249694 and the body is visibly different than what can be seen on the current Uno's.

I have been trying to figure out what part of that is the part # so I could look it up. Nothing looks like MC33269D-5.0 that is on the Deumilanove schematic. I am thinking it is this http://pdf1.alldatasheet.com/datasheet-pdf/view/46023/SIPEX/SPX1117M3-5.0.html in the smaller SOT-223 package.

For servo batterys, check the online RC shops. Use a UBEC similar to below with LiPo batterys.

http://www.hobbypartz.com/ub2liin.html

Thank you for your replies, I appreciate it!

I have a few concerns about powering the servos separately: - The servos deal with just a light load, so I don't think they will stress much the board; - I've been powering the Arduino Duemilanove and servos from the same battery (9V alkaline) for almost a year without any apparent problems; - Adding more components means adding cost, complexity and weight, so I'd like to avoid it if possible (even if it's just 50g, I must be sure they are worth it).

The servos deal with just a light load, so I don't think they will stress much the board; I've been powering the Arduino Duemilanove and servos from the same battery (9V alkaline) for almost a year without any apparent problems; Adding more components means adding cost, complexity and weight, so I'd like to avoid it if possible (even if it's just 50g, I must be sure they are worth it).

If you are happy with your current setup, then just get the 11.1v LiPo, as the arduino regulator can handle that voltage.

That UBEC part looks pretty neat zoomkat, will have to keep that in mind for a future project.

funking, good luck - seems you are a lot farther along than you let on, just needing a battery change and a voltae divider to self-monitor it.