[RE-wrenches] PV testing

[frenergy]

Wrenches,

        As time passeth, we're bumping more into older PVs/arrays that have 
questionable outputs.  After minimizing the variables (dust, temps, sun 
angle, wiring, expected deterioration with age, etc) and still coming up 
with low numbers.  Testing individual PVs' Voc and Isc don't seem to tell 
the whole story as they often look good.

        Would not a definitive test be to wire a suspect PV to an 
inexpensive mppt CC (oxymoron?) and then to an orphaned battery.  The 
battery would have to have a mondo (surplus) rheostat paralleled to it to 
maintain a steady voltage during the test.  With appropriate metering in 
place, you would then have Vmmp and Immp as the rheostat is tweaked to hold 
a steady batt voltage and the CC finds the knee.

        Help me here.  I know its a little spendy to cobble up but the value 
of nailing an offender might be worth it.  Am I missing something?  Is it 
common for inverters to fall off in output gradually?

Bill
Feather River Solar Electric