spudulike-ga:
Thanks for your Question.
I actually have two very similarly-rated solar panels that I've
directly wired (in parallel) into my car's electrical system to help
keep my car battery topped up, just like you've described. While a
couple of watts is not enough to prevent the battery from being
drained if I'm forgetful enough to leave the lights on, it *is* enough
to help balance off the parasitic consumption of all the various
electronic devices that need to maintain their settings in memory
while the engine is off (ie. the audio system, the trip computer,
etc).
The answer to your question lies in a couple of simple electrical
principles. First, let's start with the diode(s) in the panel(s). If
you aren't already familiar with how a diode operates, please check
out:
http://en.wikipedia.org/wiki/Diode#Semiconductor_diodes
Essentially, in one direction, any voltage potential greater than
about 0.6 VDC will cause the diode to have essentially zero
resistance, so current can flow freely. In the opposite direction,
current basically cannot flow. So, if the solar panel is generating
greater than 0.6 VDC, technically current can flow out from it. Since
the panel is hooked up to your car's electrical system through the
cigarette lighter adapter, effectively the panel will need to generate
at least the same voltage as the battery's voltage before current will
flow. Conversely, if the panel is not generating at least the same
voltage as the battery's voltage, power from the battery is prevented
from reaching the panel by the diode(s), thus preventing electrical
damage to the panel.
The next principle is that of voltage potential. If you think of
voltage as being analogous to water pressure, and if you think of the
diode as being similar to a spring-loaded flapper valve, then imagine
a situation where you have two pumps, both trying to pump water into a
water tower where the only way in is through a pipe at the bottom of
the tank. In order for either pump to push water up into the tower, it
needs to generate enough water pressure (again, think voltage) to
exceed the pressure being exerted by the water already in the tank. As
long as either pump can do this, water will flow up into the tank. If
both pumps are pushing, they need to push with the same pressure,
otherwise the flapper valve for the weaker pump will close to prevent
water from running backwards into the pump due to the higher
pressure(ie. just like the diode protects the panel). While this
analogy is very basic, it's a good way of understanding how the two
panels would work with each other to charge the battery, and how if
one of the panels is 'stronger' than the other, it will do the bulk of
the work. This is extendable to more than two panels, too. So you can
have two, three, four, or more panels all in parallel working to
charge your battery. And, just like with the pumps and the water
tower, the more panels you have, the more water (in this case,
electrical charge) gets pumped. If one of the panels goes bad and no
longer generates usable power, the others will continue to work away,
and the diode in the bad panel will protect it from reverse current
flow that can drain power from your system.
This is actually the approach used in those solar-powered racers that
engineering students build and race across Australia's Outback every
year. Enough solar cells are connected in series to generate
sufficient voltage to power the drive motors and charge the batteries,
then enough banks of cells are connected in parallel to generate
enough current so that enough wattage (power) is delivered to the
drive motors to propel the vehicle at racing speeds. Again, an overly
simplified description, but suitable for this explanation.
http://en.wikipedia.org/wiki/World_Solar_Challenge
I hope that this answers your question; please let me know if you need
me to clarify any part of this Answer!
Regards,
aht-ga
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