Detecting Battery Level

hiskeyd

Hi,

I'm building a small little robot that uses a 9V rechargeable lithium battery to power itself with a basic stamp as its brain.

I'm wanting to add on to this a small solar panel i'm making out of a bunch of solar cells. Depending on how many cells i include and what not i can get a peak of about 18 watts out of the cells; depending on how i wire them up and how many i include i can get 9 volts and 2 amps out of them more or less when there is good light available.

What i'd like to do is hook this solar panel up to my 9 volt battery to charge. My understanding on charging the 9 volt lithium cell is that i will need to supply at least the 9 volts to charge it.

So then i get more or less how to wire it up to charge. The part i need help with is how to monitor the charge level to stop charging when the battery is charged up.

What i was thinking is that i could use an ADC to read the voltage levels coming from the battery and if it dropped below some threshold, to then switch on the charging.

Alternatively, maybe i could just monitor the 5 volt regulated power powering the stamp. Then, if it dropped below some threshold, i could switch on the battery charging.

I also know there are chips out there that will handle all this for me, with no need to involve the microcontroller. I'm a bit confused as to which one would be right for my job though (i'm a bit of a noob with all this). I need it to be breadboard mountable and be able to handle charging a 9 volt rechargeable lithium battery (energizer). And preferably be easy to wire up.

Any help / ideas here would be much appreciated. Thanks!

Post Edited (hiskeyd) : 2/9/2009 3:42:02 PM GMT

Humanoido

Take a look at the Bat Penguin robot here.
http://forums.parallax.com/showthread.php?p=760066
It monitors the battery voltage by using a small power LED and a
simple CaS photocell.


The Bat Penguin has a
battery monitor using
the existing board.

humanoido

Post Edited (humanoido) : 2/9/2009 3:56:26 PM GMT

J. A. Streich

S quick google search turns up:
http://psn.quake.net/eco8/battery_monitor.html
http://www.evconvert.com/article/led-bargraph-battery-monitor-part-2

This is a very neat idea. Please, let us know what you come up with and what parts (and where from). I was thinking to switch to rechargable 9 volts for my protoboard bot, if it could charge itself, all the better.

Franklin

Also you need to control the charge for lithium batteries and this is not an easy task. I would not recommend this without doing some research on battery chemistry and charging schemes.

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- Stephen

Ugha

Be careful of lithium batteries... they have a nasty habit of "Venting with Flame"... in other words, exploding... when not charged or used properly.

hiskeyd

so does anyone recommend any chips that will handle charging lithium batteries like the 9 volt energizer rechargeable and would be simple to wire up? I was looking at some TI chips that supposedly do that, but i'm not exactly sure what i need here.

I'd like to avoid the lithium "battery flaming" problem, if possible I suppose it could be a "feature". Sort of a small little "suicide bomber" robot. I could make several and have little robot wars with overcharging flaming batteries.

Anyways, I found one circuit that did it with one of these chips and was pretty simple to wire up, but was for a 1 cell AA style battery and peaked at 4.5 volts, so it wouldn't work for charging the 9 volt. I looked around for one that would work for higher voltages on digi-key, but was a bit confused on their descriptions and what not.

GWJax

This circuit should work for you just fine. [noparse][[/noparse]url]http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1004,C1012,P1220,D4312

Also you can try these links as well. [noparse][[/noparse]url]http://www.discovercircuits.com/B/batt-chrg3a.htm

Hope this helps you.

Jax

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If a robot has a screw then it must be romoved and hacked into..

icepuck

First of all there different "variations" of lithium batteries, LiIon and LiFeP04(lithium iron phosphate) and some others I can't name at the moment. These different "variations" also have different voltages, such as LiIon of 3.7 volts and LiFeP04 at 3.2 volts, then there's the cut off voltage that you have to watch out for. Basicly what this means is you are NEVER to discharge a cell below an allowed minimum voltage before it is damaged. That is why some cells require a BCM(battery control module) to cut the power to prevent damage. You need to find a "data sheet" usually a pdf file from the manufacture of the battery you plan to use. You will need it to find the max charge current and voltage it can handle during a charge then minimum voltage it can be discharged to without damaging it.

Here's some links that may help out-some even have links to the data sheets...

http://www.batteryspace.com
http://e-lectronics.net/liion-rechargeable-batteries-c-51_36.html?osCsid=f6e03adf879088cf880e3c9b582d390d

or just use google to help find more info...hope this helps...
-dan

Ugha

Have you measured the voltage coming out of your 9volt?

If its shaped like a regular nine volt battery then it is likely 7.something... 2 i think.

GWJax

hiskeyd said...
so does anyone recommend any chips that will handle charging lithium batteries like the 9 volt energizer rechargeable and would be simple to wire up? I was looking at some TI chips that supposedly do that, but i'm not exactly sure what i need here.

I'd like to avoid the lithium "battery flaming" problem, if possible I suppose it could be a "feature". Sort of a small little "suicide bomber" robot. I could make several and have little robot wars with overcharging flaming batteries.

Anyways, I found one circuit that did it with one of these chips and was pretty simple to wire up, but was for a 1 cell AA style battery and peaked at 4.5 volts, so it wouldn't work for charging the 9 volt. I looked around for one that would work for higher voltages on digi-key, but was a bit confused on their descriptions and what not.

Here's a nice AC recharging station for your batteries. I will find another good circuit for you for low level detect on your robot.

Jax

Add-on here is a curcit that I made for another robot to monitor it's batteries, change it at will if you like..

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If a robot has a screw then it must be romoved and hacked into..

Post Edited (GWJax) : 4/4/2009 7:28:13 AM GMT