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Electronics => Electronics => Topic started by: jsmoker on February 21, 2007, 08:36:39 PM

Title: Lithium Polymer Battery turnoff needed
Post by: jsmoker on February 21, 2007, 08:36:39 PM

I'd like to use a Lithium Polymer battery for a project of mine, but I've read that the battery should be allowed to discharge below a certain minimum (for my case a 2 cell 7.4V batter shouldn't discharge below around 6V) otherwise the battery's performance would drop drastically.  I know there are a few packaged devices to do just that, but I need a very small, 1-3 component solution that I can actually place on the board itself.  I was thinking of using a zener diode at first put in a reverse position so that the zener voltage would cut off the device if the batter drops below, but in initial testing, the diode doesn't seem to be working as planned.  Does anyone have any ideas?

-Jason

Title: Re: Lithium Polymer Battery turnoff needed
Post by: 555 timer chip guy on February 25, 2007, 05:23:48 PM

I have two suggestions one is you could rig up a small DC motor to drain it or you could set up some big resistors to drain the power like they used to do in the subway braking system in new York.

Title: Re: Lithium Polymer Battery turnoff needed
Post by: JonHylands on February 25, 2007, 05:48:36 PM

If you've got a micro-controller on the board, and it has a spare analog port, you can make a resistor voltage divider (between the power switch and the voltage regulator), and feed the output of the voltage divider to the A/D port...

- Jon

Title: Re: Lithium Polymer Battery turnoff needed
Post by: jsmoker on February 28, 2007, 12:32:22 PM

I ended up finding a couple things at the end of the week last week.  Here's some info for any future users.

Some battery places sell they're own very small seperate module like all battery.  This was still too big for my application but I thought I'd mention it.
http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=550

Other places actually sell their module attached to the battery itself.  This one actually ended up fitting my limited area.
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=1339

Finally, I also found a forum, that actually gave the components of the module so you could make one yourself on your own PCB
http://forums.all-battery.com/index.php?showtopic=246
I probably would have done this last one, if I didn't find the small sized version in the second above link.

-JSmoker