Lithium Ion for 3v device

A

awdark

[H]ard|Gawd
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Mar 22, 2003
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I want to power a 3v device with a lithium-ion/li-poly battery so the voltages are 4.1-3.7vdc

I know it would be easier to stick with 2xAA batteries but lets say I wanted to do this and I happened to have some spares with an external charger I can use.

Off the top of my head I can think of...
voltage divider - will this work as the battery drains?
lm317 - probably not enough voltage difference to work
series of diodes - haven't had much luck dropping it by 1v even though each diode should drop about 0.6 each
 
Out of curiosity, what is the 3V device? There's a decent probability it will work fine with 3.6-4.2V.

Barring that, I'd say something along the lines of a buck circuit based on an MC34063. You'll need an external transistor, though, with a low Vsat.

When your supply voltage is within 1.5V or so of your target voltage, your options are limited. The diode approach is common, but wastes a bunch of energy. A voltage divider is almost never a good idea for powering something--not only will it drop as the battery runs down, but the voltage will vary depending on how much current your device draws, and it'll be just as inefficient as anything else mentioned here. That leaves you pretty much with two choices: get a low-dropout linear regulator (which the LM317 is not), or create a potentially more-efficient but also more costly and complex buck converter based on an mc34063.
 
Not sure if it will fit, but I was thinking about replacing the batteries in my mouse with it. There are other things I might use it in but if this proves too complex might as well forget it.

Only reason why I would consider it is the fact that it has a little charging dock otherwise I would say lithium rechargeable, too hard/dangerous to charge forget it..
 
So, if I'm understanding you correctly, you have a LiI or LiPo cell and are looking for a place to use it? For something like a mouse, my inclination is to say "it's not worth it." Because your supply voltage (4.2V) is fairly close to the required voltage (3V), you'd have to use a low-dropout regulator. Low-dropout regulators also draw more current even when not driving a load, whereas the existing electronics drop the current usage to near-zero when the mouse is idle. So you would likely find your battery running out of juice far more often than what you currently experience.
 
Ah I see, thanks for all the info :)
I guess it wouldn't be worth it then, but at least I have some reference information if I ever find something where it might work better in.

Would you possibly know how well a lithium ion battery would do in a high drain situation such as a camera flash? I have seen mods using sla batteries but I have oddly never seen any that use lithium ion.
 
I'm no expert on battery tech, but IIRC SLA, Lithium-Ion and NiMH are good for high-drain applications, while alkaline is bad. SLA are probably seen more often because they're cheap and a lot less finicky about charging/discharging than lithium.
 
You don't want to use a voltage divider, they are about the least efficiant way to reduce voltage (since to get good behaviour the current going straight down the divider must be much greater than the current to the load) and really only suitable for creating reference voltages.

You want a regulator with sufficiantly low dropout and with low quitscient current so it doesn't drain the batteries.

If it's a low current device then i'd consider the LM2936Z-3.0 . It has dropout of only 0.2V typical and 0.4V max and quitscient current in the microamps should do nicely but the max output current is only 50ma which may be a bit low.

Also do your packs have built in overdischarge protection? if not you need to consider that.
 
If you can afford to smoke the mouse, I'd just do it without a voltage dropper of any sort. Many DC devices will work 'just fine' with out-of-range voltage (within reason!).

blah blah explosion death doom blah.
 
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