[OLPC-devel] Battery Charge End Sensing

[Frieder Ferlemann]

Hi,

Jim Gettys wrote:
> Frieder Ferlemann wrote:
>> 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".

Yes, in many cases neither Delta-U nor Delta-T can be used.


>> 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.

For the price of an additional battery with its additional loading
circuit one could probably switch to a >=4 Watt solar panel
thus doubling the input power.
In cases where the OLPC can be connected to the panel for prolonged
time periods this would probably be preferred.
Also the investement into a (longer lived) solar panel might
be prefered over the investment into a (Pb?) buffer battery.
(Note, administering the energy with a buffer battery is not
the perfect cure, if the residual capacity of the buffer battery
is lower than the need of the OLPC (maximum: 22.8Wh*1/loading efficiency)
the same bistable situation as without the battery can occur).



I'd consider using a comparatively low power solar
panel to a single OLPC a valid configuration and would
like to see this configuration as a design goal:


    /------------------------\
    |                        |
    -                        |
    ^ Diode                  |
    |                        |
    |                        |
   /+\                    .-----.
  (   ) xW                |(   )|
   \-/Solar               '-----'
    | Panel                  | OLPC
    |                        |
    |                        |
    |                        |
    \------------------------/


Without having this as a design goal this configuration might
exhibit a bistable behaviour:
If the switching regulator start at 5 Volt and the illumination
of the panel would be good to deliver a current of about 100 mA
then either the 100mA could be delivered at 5 V (giving 0.5 W)
or at 15 V (giving 1.5 W).



> way to solve this, rather than complicating the power supply in each
> OLPC.

Unfortunately we might not get away with an uncomplicated switching supply.
Considering only the "load" side of the OLPC (neglecting the batteries)
the ingredients of the schematics below are:

- 1/Voltage characteristics of the load current,

- various control loops (with different time constants)
  controling the charge current and the current to the PCB of the OLPC.

- current limited supply (generator, solar panel, fuse)

- resistive and inductive connection between (several) OLPC
  and power supply

- sudden changes in needed current (sleeping OLPC reacting on a WLAN packet)



Solar Panel, 1/2 classroom, no battery, daisy chain, "minimalistic classroom situation":


       _/   ___     ___          ___     ___       ___     ___
    /o/  o-|_R_|---|_L_|-----o--|_R_|---|_L_|--o--|_R_| - |_L_|--\
    |                        |                 |                 |
    -                        |                 |                 |
    ^ Diode                  |                 |                 |
    |                        |                 |                 |
    |                        |                 |                 |
   /+\                    .-----.           .-----.           .-----.
  (   )                   |(   )|           |(   )|   o o o   |(   )|
   \-/Solar               '-----'           '-----'           '-----'
    | Panel                  | OLPC            | OLPC            | OLPC
    |     1                  |    1            |    2            |   20
    |                        |                 |                 |
    |       ___     ___      |   ___     ___   |   ___     ___   |
    \------|_R_|---|_L_|-----o--|_R_|---|_L_|--o--|_R_| - |_L_|--/



If the OLPC do not have a "friendly load characteristic" this system
might show *various* modes of oscillation and bistability. (Also if
a battery is added)

Please let the community have at least *one* of the documents
for the charging circuit mentioned in this thread:
schematic, block diagram, software interface, product requirements document?

Greetings,

Frieder


PS: if the schematics show up garbled please use a monospace font