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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