Converting 2-AA battery Powered Devices to Lithium Power

[Metallover]

My $70 camera runs on AA batteries, and it sucks. I get about 5 minutes out of alkalines and 2 minutes off of rechargables.... I want to convert it to lithium power. What is the best way to go about this?

I'm open to life and lipo. I could use a 1p or 2p config of 1100mah LiFe cells, then I would only have to step the voltage down from 3.6-3.3v, if I even need to step it down at all.

If I went with lipos, I would have more capacity, size, shape, and mounting choices. The thing with that is I need to step the voltage all the way from 4.2-3.7v to 3v.

I plan on ordering the batteries from hobbycity with my next order. What is the best way of going about decreasing the voltage? Do I need a voltage limitor? Just a simple pot?

[BrianG]

Seeing as how LiFe peak voltage (3.6v) is only 0.6v higher than 2 AA's, you might be able to get away with no step down device at all.

If you use lipo, things get a little trickier. Consider the following lipo voltages:

- 4.2v off the charger. Any regulator should work here.
- 3.7v nominal. You would need a low drop out regulator to make sure you get around 3v at the output. Regular linear regulators (like the LM317T) need at least 1v input than the output. So, if you want 3v out, you need at least 4v in.
- 3.0v-3.2v cutoff. Even a low drop out regulator wouldn't be enough here.

You could set whatever regulator you use for ~2.5v (which is around the nominal voltage for NiMH) and that would give you a little more wiggle room at lipo cutoff, but would still need a low dropout type for stable output And the use of any regulator "hides" the actual battery voltage, so even if the camera has a battery guage of sorts, it wouldn't be accurate.

The next obstacle is using some type of cutoff device. Unless your camera has a voltage display/guage of some kind, you could run the risk of discharging lipo too far. Even LiFe might not like that.

The final obstacle is mounting, Most cameras I've seen are quite packed with circuitry and there is usually circuits inside the rounded area where the AA's go. So modding the holder to fit a lipo/Life cell may be difficult.

You could connect wires to the camera battery terminals and use an external battery pack consisting of whatever battery you want, but then the total package gets cumbersome.

In the end, you might be better served to get a newer camera with a better battery life.

[J57ltr]

What Brian said but if you are still looking for cells try dead laptop batteries. I have taken apart lots of packs that were dead and it's always the same thing one group of parallel cells are dead and the others are ok and usually measure about 3.8V or so. Most are a little larger in diameter .71" and a little longer at 2.5". They come in a a few different styles some are 3.6V others are 3.7V and the capacities are 1 ah to about 3ah. I have seen packs that are 6s2p,4s3p,3s3p,3s2p. They aren't really good for RC but some have found their way into my Tx and some flashlights and a few other devices.

Jeff

[Metallover]

Thanks for the input guys. Do you think a LED would put enough load on a LiFe battery to drop the voltage a little? I think LiFe or LiIon is the best choice. Lipo seems to be more trouble then it's worth; never thought about the complications of the cutoff.

I'll keep an eye out for some old lithium cells and see what I can come up with.

Also, do you think a lithium battery could replace the 3-cell nicd packs in cordless phones? Those are going to crap too..

You could also use something like this or this to regulate the voltage at ~3v just to be sure and not damage the camera.

[BrianG]

That second link would be a better choice as the first one does not specify a dropout voltage. At any rate, care must be taken to not discharge the lipo/liFe too low. You would need some kind of voltage display hooked up between the battery and regulator so you know what the true battery voltage is, not the voltage at the output.

[J57ltr]

Not unless you want to use the LED for supplemental lightning. LED's don't draw but about 20 ma sometimes less so it would be less of a load than a Blinky. You could make a small circuit fo monitor the battery and use an led to give a visual indication that you need to charge or change the battery. something like this would be easy.

http://www.edn.com/ednmag/archives/1...1395/08di4.htm

I started thinking about the voltage difference and I don't think it would be a big enough deal to even need a regulator using a single cell. If you have ever used the the Energizer Li batteries (the silver ones) they measure about 1.9V per cell brand new. I used to use them in my camera that uses 4 AA batteries and didn't have an issue. I never checked them when I started using them and it wasn't until I was going through a pile of batteries sorting them into good and bad piles that I discovered they were that high. I also used them in my older Kodak that used 2AA's. At 4.2V right off the charger for the 3.7V cells you are only dealing with a .4V difference. If it would make you feel more comfortable you could discharge your battery to 3.8V on your charger.

Jeff

[BrianG]

There you go. Sounds like a LiFe battery would be the best/easiest solution because the voltage even peaked is pretty close to 2 AA's. And like J57ltr said; if you feel the difference is still too much, either discharge them down a little, or better yet, set the CC/CV threshold on your charger to whatever value you want (if it supports it).

[J57ltr]

On your cordless phone problem you still have to deal with the LVC issue, but 1 could easily replace them although you might want to try a different style something like the one in this post:

http://www.rc-monster.com/forum/show...7&postcount=10

They are about 1/4" longer and about 3/16" wider than 3 AAA cells. The same length as a AA and the thickness of a AAA.

Jeff