From: jfmezei.spamnot@vaxination.ca   
      
   I am building a roughly 12vdc system to be powered by solar.   
      
   The basic design:   
      
   [solar panel]---------[solar charger]---+-[lithium polymer battery]   
                                           +------[load]   
      
      
   Solar panel gives anything between 15vdc and below depending on load and   
   amount of sun hitting it.   
      
   Now, I add a GPS to the mix. The GPS has external power input with a   
   range from 9 to 40vdc. So I can plug the solar panel in directly. This   
   bypasses any inefficiencies of the solar charger, and leaves the solar   
   charger with whatever is leftover after the GPS has consumed its power   
   (in low light condition, the GPS will draw enough that the solar panel   
   voltage will drop below voltage needed by solar chareger which then   
   shuts down.   
      
   (graphs assume monospaced fonts)   
      
   so, ideally:   
      
   [solar panel]----+----[solar charger]---+-[lithium polymer battery]   
                    +----[GPS]             +------[load]   
      
      
   However, in cases where I go through a tunnel/shade/heavy rain, I want   
   the GPS to draw from the battery. The above setup won't allow this and   
   the GPS then reverts to its own 2*AA batteries which are meant to be   
   used only as last resort, and when consulting GPS at night in tent.   
      
   So I am thinking of connecting the GPS to both the solar panel line and   
   to the lithium battery line, and put diodes to prevent any current from   
   flowing from solar panel to battery (aka prevent bypassing of   
   charger), while allowing the GPS to draw its power from either source.   
      
   [solar panel]----+----[solar charger]---+-[lithium polymer battery]   
                    +                      +------[load]   
                    +----[>]---+----[<]----+   
                               +   
                             [GPS]   
      
      
   QUESTION: how do I choose what type/size  of diode I need ?  How are   
   these measured ?   
      
      
   (load will generally be an ipod and speakers via a DC-DC converter to   
   USB voltage, but will also be used to charge camera battery (from   
   battery voltage) or iphone (via 5vdc interface)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)