Mppt

Maximum Power Point Tracking

When running the laptop from a power source with a non-linear source resistance, such as a solar panel, it is difficult to extract the maximum power available from the source. With such a source, there is an optimum input voltage for obtaining the maximum power. If the source is a solar panel, it varies with the insolation of the solar panel and must be tracked.

XO-1.5

XO-1.75/XO-3

EC MPPT algorithm

Overview

This describes the MPPT algorithm present in the XO-1.5 embedded controller. When operating from a PV solar panel the available power can be very dynamic even in seeming very sunny conditions. In many cases the maximum power available from the solar panel is less than the maximum power draw of the XO-1.5. The maximum power input to the XO-1.5 is 25W but the most common OLPC solar panel in use is only rated at 10W. This can create up to a 15W power deficit.

The voltage output of a PV panel is related to its ouptut current. The VI curve for the OLPC panel looks like this. This is typical for almost all PV panels.

As the current draw increases from the panel the voltage decreases. This creates a power curve that looks like this

So the the maximum power point of a PV panel is obtained when you maximize both the voltage and the current.

Definitions

Pin 

Power input to the XO-1.5.

Psystem 

Amount of average input power that results in an average battery current of zero or very near zero. Empirically determined on an XO-1.5 to be about 7W average if the system is doing work.

Plimit 

The charger power limit % that corresponds to Psystem. Empirically determined to be 20%.

Psystem and Plimit Data

Squeal Detect

Squeal Detect is the ability of the EC to detect when the system is attempting to draw much more current than the PV source can provide. When this happens the external power good pin can start to oscillate. It usually oscillates in the audible range and the inrush currents into the inductions produce an audible "squeal". The squeal detector watches this pin and indicates when its oscillating.

Assumptions

TODO:

Flow Chart

Wikimedia seems to hate trying to render my .svg . Here's a link to the .svg file Media:mppt.svg

Flowchart

The dia file is also available.

File:Mppt.dia

And here's the same thing as pdf:

File:Mppt.pdf