Silicon magic: power harvesting from solar cells doubles power out

A power harvesting chip doubles the power of solar cells, when the solar cells are in parallel, compared to when the solar cells are in series.

Of course the power harvesting chip is doubling the voltage.  A power harvester is a voltage booster.  It is not so obvious that it can double the power.

I ran an experiment with a BQ25504 power harvesting chip, two PowerFilm SP3-37 3 volt solar cells (panels actually, comprised of multiple cells), and a motor load.  When the BQ25504 signaled battery OK, I dumped the stored power into the load until battery OK went low.  I counted the frequency at which the load was dumped, as a measure of power out.  I tested two configurations:

  • with the solar cells in series, feeding a nominal 6 volts into the power harvesting chip
  • with the solar cells in parallel, feeding a nominal 3 volts into the power harvesting chip

The result is that the series configuration dumped the load roughly twice as often, i.e. harvested twice as much power from the solar cells.

(The light condition was bright indoor light, so the actual voltages from the solar cells was slightly less, but that is immaterial.)

(I set the battery OK threshold at 2.8 volts, and the hysteresis at 2.4 volts ( started dumping to load at 2.8 volts, and quit dumping when the storage reached 2.4 volts.)  But the results would be the same even if the threshold was 5.5 volts, the limit of the chip.)

Much of semiconductor technology seems like magic.  The solar cells and the chip are arrangements of doped silicon.  Certain arrangements (solar cells in parallel feeding a power harvester chip) are even more magical than other arrangements.

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