PV Cell Basics for Hobbyists
Photovoltaic (PV) cells convert sunlight directly into electricity. They
are a convenient source of power for certain hobby projects. Effective
use of PV cells requires proper matching of cells to the electrical load.
Here is a calculator that will help you estimate the electrical current and
voltage that can be supplied by various PV cell configurations.
The first part of the spreadsheet deals with characteristics of an
individual cell, allowing you to relate the size of the cell to the
amount of power it can deliver. Use the "?" buttons to get more
information on individual items.
Any of the values in the following table can be changed. Everything
is recalculated when you hit the "tab" key or click outside of the box
PV cells are sometimes used alone, but more often connected together
to give more capacity. Connecting cells in series (positive of one to
negative of the next) gives higher voltage. Connecting them in parallel
(positive to positive, negative to negative) increases the current capacity.
Filling in the "series" or "parallel" boxes below will allow you to see the
results of connecting multiple cells in series and/or parallel.
Remember that PV cells are not like batteries. Once their current capacity
is exceeded, their output voltage drops rapidly (they act as "constant current"
sources). If you draw less than this current, the cell supplies a relatively
constant voltage. But, there is only one operational point at which the
cell is operating at maximum efficiency. This is the boundary between the
constant current and constant voltage regions, and is known as the "maximum
power point" (MPP).
In practical terms, if your cells cannot supply enough current, your
project may cease to work. If you size it to supply too much current, your
cells will operate inefficiently. An excellent solution to this problem is
often to use the PV cell to charge a battery.
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