side/side batt. life in suspend

Richard A. Smith richard at
Mon Dec 7 00:16:28 EST 2009

On 12/06/2009 09:00 PM, James Cameron wrote:
> On Sat, Dec 05, 2009 at 06:56:58PM +0000, Tiago Marques wrote:
>> I went to the wiki yesterday and found the information that the
>> LiFePO4 batteries have 2 or 4 cells, rated at 22Watt hours. Is this
>> rating the same or is it half of that with 2 cell models?
> I think that is unlikely, but I don't know.

Batteries can have n number of cells where n = (Output voltage / Cell 
voltage) * number of strings in parallel.

OLPC batteries have 2 cells in series with a nominal cell voltage of 3.25V.

>>  From my battery it seems it's 22 Wh since it's rated at 6.5v @
>> 3100mAh. Am I right?


> I'm not sure.  6.5V at 3100mA is 20.15W.

Here is where I repeat my power mantra that power numbers must include a 
_specific_ load case or they are wrong.  Mfg test specifications are 
only valid under mfg test conditions.

In our case the mAh rating is for a discharge current of .5C (aprox 
1.5A) less than that you get more mAh, more than that you get less mAh.

Plus you can't do a straight V * I = W calculation because the V of the 
battery decreases as you use it so Wh calculations have be iterative.

Richard A. Smith  <richard at>
One Laptop per Child

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