[OLPC-devel] Battery Charge End Sensing

[Jim Gettys]

On Thu, 2006-07-13 at 09:56 +0200, Frieder Ferlemann wrote:
> Hi,
> 
> James Cameron wrote:
> > If the software could identify proximity to other mesh members that
> 
> Unfortunately this is probably too late: the problem of maximum
> allowable current (or to put it positively: friendly load
> characteristics) also exists if only *one* OLPC is connected
> to a "weak" supply...
> 
> 
> Jim Gettys wrote:
> > We'll probably end up with considerably more capable battery information
> > than most machines, as far as I gather.
> 
> Can this information then be used to control the charging current?

I have no clue if the EC (embedded controller) has such capability.

Ray Tseng might know.

> 
> Or more low level: is there a means to control the duty cycle
> of the switching regulator for the charging circuit (preferably
> with little or no load for the main CPU) ?
> 
> Note, the ability to charge from "weak" supplies will be quite
> a challenge for Delta-V detection (which is not too reliable
> anyway as James pointed out before).
> A Delta-T detection might also be problematic because of
> external temperature gradients and low charge current.
> 

Good point.  I gather the temperature change to be detected is quite
small, so slowing the charging rate sure sounds problematical on
detecting the battery being "full".

> 
> Maybe the term "weak supply" needs a comment - Would it be
> reasonable to assume a 2 Watt solar panel (about 10x15cm²
> and operating at 40% of its maximum power) to be the "weakest"
> supply the OLPC charging circuit should be designed to
> "work" with?
> Defining the word "work" here as: "reviving an OLPC with
> completely discharged batteries within 0.x hours"?
> 

I suspect that putting a conventional battery as a bridge would be the
way to solve this, rather than complicating the power supply in each
OLPC.
                           Regards,
                                 - Jim

-- 
Jim Gettys
One Laptop Per Child