On Mon, Feb 03, 2014 at 08:04:36PM +1300, RussellMc wrote:
> OLPC report 3,000 cycles of use for both NimH and LiFePO4. They take
> about 10% of capacity off both ends of NimH usual energy
> availability curve to achieve this. (ie battery is rated at 80% of
> nominal capacity) (ie terminate charge at lower than usual voltage,
> terminate discharge at higher than usual voltage.) OLPC members on
> this list may wish to comment on this.

Heh.  Thanks for the invitation.  I suspect I'm the only one here at
the moment, but if not someone else can correct me.

I was not the principle engineer involved, but can comment.

Yes, you're right about the capacity trimming, and that it is for
product life.

We switched from NiMH to LiFePO4 in early prototype of the first model
when the nickel price went up quickly in 2006 or 2007.  LiFePO4 was
quite new back then.  Also, NiMH won't charge at the high ambient
temperatures that some of our target users would experience.

I was one of the first accidental over-temperature testers in the
outback; the NiMH test batteries were never the same afterwards.

The LiFePO4 battery package contains a protection module for
over-voltage, under-voltage, and over-current.  The module drives low
side FETs.  Both LiFePO4 and NiMH packages have a DS2756 fuel gauge,
with NTC thermistor, which is exposed over one-wire bus for use by the
embedded controller (EC) in the laptop, an 8051 clone.

Over-temperature control loop is with the EC, which controls charge
current.

The NiMH support in the EC was dropped from the second and third
models.

Last year we reimplemented NiMH for the XO-4 model, again because of
cost of the LiFePO4 packs, which are custom.

Source code for the 8051 EC for the XO-4 can be found here:
http://dev.laptop.org/git/users/rsmith/ec-cl4-mmp3/

Unlike traditional manufacturers, our design goals, in order, are:

- Safe -- no children should be harmed

- Lowest Power -- low power means longer run-time

- Lowest Cost -- a lower cost means more children can have one

- Robust and Maintainable -- children drop things

- Performance (speed)

Hope that helps.

--=20
James Cameron
http://quozl.linux.org.au/
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