Fwd: side/side batt. life in suspend

Tiago Marques tiagomnm at gmail.com
Mon Dec 7 10:20:57 EST 2009

---------- Forwarded message ----------
From: Tiago Marques <tiagomnm at gmail.com>
Date: Mon, Dec 7, 2009 at 3:20 PM
Subject: Re: side/side batt. life in suspend
To: "Richard A. Smith" <richard at laptop.org>

On Mon, Dec 7, 2009 at 5:16 AM, Richard A. Smith <richard at laptop.org> wrote:
> 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?
> No.
>> 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.

I see. So when OEMs quote a number, it's not something exactly accurate also?

Best regards.

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

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