Re: [Gumstix-users] Li-ion Battery Supply

[Ned F. <nfo...@wh...>]

On 05/04/2010 12:19 PM, Ben McCarthy wrote:
> Hi,
> 
> I have seen the recent battery discussions on the list and wondered whether
> this Li-Ion pack would be suitable to power an Overo Air:
> 
> http://www.sparkfun.com/commerce/product_info.php?products_id=8483
> 
> I saw reference to the Overo requiring 3.7v and this pack provides 3.7v
> exactly. Is using something that supplies exactly 3.7v unadvisable, does the
> Overo ever temporarily require slightly more than 3.7v (say during boot)?

Note that the quoted voltage is the NOMINAL value of 3.7V.  Li-ion cells
exhibit a decline in voltage during discharge (monotonic, but not
linear; there is usually a plateau near 3.7V).  At full charge the cell
will start near its charge voltage of 4.2V and then decline, having
yielded its specified capacity when it reaches 2.75V.  These details are
listed in the spec sheet that is linked to the above page:
UnionBattery-2000mAh.pdf.

Because a Gumstix computer makes 3.3V from the supplied voltage using
low dropout regulators, it will likely keep running until the battery
reaches 3.4-3.5V (someone may have accurate data).  Therefore, you will
probably get about 60%-80% of the advertised capacity of the battery
before it drops below that voltage (between 3.4 and 2.75V the voltage
will drop steeply, and not much more energy will be provided by the cell).

Be aware that this cell/battery may not have under-voltage protection (I
can't tell from the data provided). Such protection will cut off the
load (the Gumstix, in this case) when the voltage drops too low. Without
such protection, the cell can be discharged to a low voltage that will
ruin the cell.  Generally, a Li-ion cell will be damaged if it is
allowed to stay below 2V/cell for any significant time (I'm not sure if
significant is minutes or weeks, but, based on experience, I think it is
at least hours or days).

The best storage condition for a Li-ion cell is at about 50% of full
charge (about 3.7V), and at room temperature or below.  At high charge,
or very low charge the capacity of the cell degrades (by different
mechanisms).

Be sure to use only recommended chargers or charging parameters.
Accidental overcharge generally results in an exciting fire.

-- 
Ned Forrester                                       nfo...@wh...
Oceanographic Systems Lab                                  508-289-2226
Applied Ocean Physics and Engineering Dept.
Woods Hole Oceanographic Institution          Woods Hole, MA 02543, USA
http://www.whoi.edu/
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