[Rhodes22-list] Solar Panels

[DCLewis1 at aol.com]

Slim, 

A little knowledge is a dangerous thing, and I’m proof of that.   I’m 
confident the guys on the board will have a good answer for you, but until  they 
respond let me take a wild crack at some of your questions.

If your boat is like mine, it’s plumbed for 12 volts DC. You’ve got 2  batts 
aboard. The main power switch adjacent to the V-berth lets you chose which  
battery to use to power your boat, or you can set the switch to BOTH and they  
will both power the boat together. If the batts were in series your electrical 
 systems would be running of 12 volts when only 1 batt was being used, but 24 
 volts when they both were being used - not likely.  Conclusion: when run  
together your batts are in parallel.

Your solar panels are supposed to trickle charge your batts. One may go  to 
one batt, the other to the other, so that when your main power switch is OFF  
each batt still gets a trickle. You can check this out on a sunny day.  
Disconnect the leads to your batts and using a voltmeter measure the voltage  across 
the loose disconnected leads, you should measure 12 volts, coming down  from 
the solar panels. Cover one panel, if the voltage goes away on one set of  
loose terminal leads, that’s the panel that’s supporting that batt, but also  
check the terminals from the other batt. If you cover one solar cell and both  
batts still show 12 volts (or if the voltage disappears on both sets of leads)  
they are wired in parallel from the solar panel’s perspective. I haven’t done 
 this, but it should work.

Bigger (area) solar panels generally mean more power, or amps, but the  
output voltage is fixed. Generally, in my experience, they are sized for 12  volts, 
because they are usually used to trickle charge a lead acid auto batt.  But 
you should probably check, because the minute you don’t you will find that  
what you’ve bought is designed for some other voltage. 

If the panel is designed to work with a lead acid auto batt, it’s  output 
voltage is about 12 volts. The other 2 characteristics are power (watts)  and 
current (amps). This is DC (Direct Current), so Power = Volts x Amps, you’ve  set 
the volts, if you set the power you’ve automatically set the amps and vice  
versa.  So if you know the Watts you know the Amps and vice versa, if you  know 
the design working voltage (12V).

How big a panel is “enough”?  Solar panels are generally  trickle chargers, 
not prime power sources. Your batts handle the big surges -  and they also 
carry all the load at night. If you’ve got a generator on your OB,  you may not 
even need the panels, unless you go a long time without running your  OB. If 
you don’t have a generator, you may need the trickle charge the solar  panel 
provides. How big a trickle charge? You can assess your typical usage and  the 
size of batts and come up with an estimate, that should then be rounded up  
significantly, but for your purposes a good start might be the realization that  
what you had worked, and the holes are pre-drilled, i.e. you might consider  
getting what you had because it worked. 

If you want to ignore the pre-drilled aspect of your project and get a  new 
and different solar panel, you might measure the area of the panel and get a  
panel that is about that size. We all work with the same sun, the same silicon  
solar cells (approximately), and the systems are designed to work into 12 
volts  (be sure that's the case). Getting the same area solar panel should give 
you  roughly the same current generating capacity as your old solar panel - 
within  bounds - unless the cells are markedly more or less efficient. 
 
Or you might consider scaling down and not replacing the panel.  Particularly 
if you've got a generator on your OB, that might suffice to charge  your 
batts.  I assume (but don't know) that a lot of the people on the  board do very 
well with 1 panel.  Your usage profile may not be much  different than theirs.
 
I don’t know if this helps. 

Dave