>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of alan smith
>Sent: 26 October 2007 15:17
>To: Microcontroller discussion list - Public.
>Subject: [EE] LiOn Batteries - damaged if voltage goes too low?
>
>
>I'm acting as a "consultant" on a project..ie....I just review 
>whats being done, and give my opinions.  So if you may 
>recall....when I posted about the battery issue...Great 
>Power..etc.  Well...the power supply design was based on just 
>using a regulator for the 3.3V rail, and when the battery 
>reached 3.3V they shut down due to low battery.  I 
>said...ought to use a buck-boost circuit so you can run this 
>thing when the primary cells run below 3.3V.  The battery 
>itself is a two cell in parallel to give the required ampHr 
>capacity.   The engineer working on this came back and said 
>the battery works from 3V to 4.2V, typical output is 3.7V, and 
>the spec on the battery said if the cell drops below 3V, it 
>may damage the battery.  So is this a characteristic of the 
>LiOn chemistry?  I would think you could run them down pretty 
>low before they give up...ie...once you drop into the boost 
>mode, the capacity will drop off quicker but still should be 
>able to pull power from it?
>   
>  Thoughts?

2.7 volts is a common cut off voltage, but it varies between manufacturers.  Deep discharging these cells damages them, to the point where trying to recover them from very low voltage is actualy somewhat risky.  This is why laptop batteries will blow an internal fuse once the cell voltages have dropped too far, to prevent it being re-charged.

Regards

Mike

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