[Elecraft] K1 -- VW Solar Panel -- Controller Help

[G3VVT at aol.com]

In a message dated 06/11/06 09:06:10 GMT Standard Time,  
elecraft-request at mailman.qth.net writes:

The VW panel reads about 20V in full sun. When connected to Don Brown's  low 
power controller, it drops to 12.75V (full sun). I have a few panels and  
tried them all....same results. Even paralleled two.




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Hi Ken,
 
I would think the VW panels are too puny to provide sufficeint power to  
fully charge a battery as they were designed only to trickle charge an auto  
battery in transit.
 
The 20V you describe out of the solar panel is the open circuit voltage.  
Most 12V solar panels depending on type will give 17V to 21V under these  
circumstances. However, when you connect it to a battery, the voltage will fall  to 
the charge voltage of the battery depending on it's current state of charge.  
Disconnect the controller and connect the panel direct to the battery if you  
want to check for sure. With most types of controllers the effect will be the  
same in that the readings will be identical less the current needed to run the 
 controller.
 
Unless you get hold of some solar panels with the necessary capacity the  
result is going to be the same in that you can only trickle charge the  
batteries. Apart from long term float charging where the controllers would only  come 
into use after a long period of charge. For most cases with low power solar  
panels a controller is not needed unless the panel does not have a series diode  
fitted. Many more of the VW solar panels in parallel or a larger solar 
panel(s)  to increase the power is the only answer if you need to use this facility 
in  real terms. The VW solar panels will most probably have series isolation 
diodes  fitted to prevent discharge of the auto battery back through the solar 
panel at  night or when not insolated sufficiently
 
As a quick test to see what your panel is capable of, remove the solar  panel 
from circuit and place a current meter directly across the solar panel  
terminals. The current reading will be limited to the available capacity of the  
panel. This is how we checked the 15kW solar arrays at the workplace when they  
were originally commissioned. These were built up from about 200 of the 75W  
solar panels arranged in series/banks to provide the necessary voltage/current  
capacity and each individual one had to be tested to ensured that it met  
specification. Was not unusual to see 120A going into the 7500Ah 48V battery  
plant in the morning sun. For that we really did require multiple solar  
controllers of sturdy proportions!
 
Regards,
Bob, G3VVT
(ex civilian desert rat)