>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of Rolf
>Sent: 16 January 2007 15:46
>To: Microcontroller discussion list - Public.
>Subject: [EE] Li-ion charging questions
>
>
>
>I have 3 questions...
>1. Is there some way I can calculate a "degree of 'chargedness'".  I 
>want to get an idea of how close the cell is to being fully 
>charged, and 
>preferably  to indicate something like "35minutes charging 
>still required". 2. Is there a way to calculate the capacity 
>of the cell during (or even 
>at the end of) the charge process? It would be nice to say 
>"The cell has 
>been charged to 1800mAh capacity". 

If you want to do this yourself, then beware that it's a complex problem. The basic strategy is to integrate the current  going into or out of the cell.  The nominal value of the integrator gives an idea of the current state of charge of the battery, BUT the rate of integration has to be modified according to cell temperature and current (e.g. cell capacity drops as current demand increases).  The integrator also has to be reset when the cell is fully discharged, and the maximum value of the integrator has to be stored when the cell is fully charged, to take into account the aging of the cell.

The good news is that there are several ready made IC's that will do all this for you, Google for "li-ion gas gauge" or "li-ion fuel gauge".  Every Li-Ion laptop battery will have one of these devices in it, accessed via an SMB or System Management Bus, which is essentialy an I2C bus.

Microchip used to have some devices for Li-Ion "smart" batteries but unfortunately a quick search shows that they are no longer recommended for new designs. http://ww1.microchip.com/downloads/en/DeviceDoc/21904b.pdf

http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=412&uiTemplateId=NODE_STRY_PGE_T
http://para.maxim-ic.com/cache/en/results/4669.html

Hope that helps

Regards

Mike


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