New XO 1.5 firmware with MPPT. Please test!
Richard A. Smith
richard at laptop.org
Fri Apr 8 13:52:58 EDT 2011
On 04/07/2011 03:43 PM, Samuel Greenfeld wrote:
> Is there any expected impact to battery health or life from using a MPPT
> (maximum power point tracking) charging technique?
The mppt does not affect charging. It affects power input into the
system. So no there is zero impact on battery health.
> How does this behave when it encounters an external charger which may be
> inflexible in terms of voltage or current generated?
It doesn't quite work that way. You draw power from a power source.
It's not pushed.
The exact behavior depends on the adapter and the state of the laptop.
When external power is detected the new code attempts to figure out if
its a PV type device or a power adapter. It does this by turning on the
and sampling the input voltage with the input power limit set to 0 and
then again with it set to max. If the deltaV between these 2 readings
is greater than a threshold then PV is assumed OR if the absolute value
of the 2nd reading under load is < a threshold.
PV or adapter influences the starting point of the MPPT algorithm. An
adapter starts with no power limit and a PV starts with almost full
power limit. The reason for this is that when the laptop is running and
the power limit is set very low its difficult for the MPPT to figure out
what to do. The dynamic power draw of the laptop means that my
adjustments to the input power may or may not be correlated with an
actual input power increase or decrease. So the algorithm can have a
bit of a slow start until it gets enough correlation and starts
tracking. The adapter check tries to jump around that since we know
there is enough power available.
Your "inflexible" adapters are actually more flexible than you think.
The white ones seem to have a loose enough voltage regulation that some
of them trip the PV check and so it starts charging slowly. This quick
check feature is a new addition and I'm still fine tuning the values.
Richard A. Smith <richard at laptop.org>
One Laptop per Child
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