DC load on PV side [RE-wrenches]
Todd Cory, Mt. Shasta Energy Services
toddcory at finestplanet.com
Wed Jun 18 06:21:18 PDT 2003
Kurt,
I tried doing just this. I was running an Array Technology tracker directly
off the PV's on a system using a 3048 MPPT controller. (PV's wired for 48
volts batteries at 24 volts.) The tracker had the required 48 to 24 volt DC
to DC converter.
The problem with this was that in the morning when the tracker was facing
West, there was insufficient current to get the MPPT to bring the voltage up
high enough to run the tracker. The MPPT was dragging the array down to ~24
volts trying to get any tiny bit of charge current into the batteries and
the 48 to 24 volt converter did not have enough voltage to operate.
The fix? Fortunately there was sufficient room in the PV input conduit to
feed the array with +24 volts from the batteries to operate the tracker.
(Yes I mixed these DC voltages in one conduit.) This was something no one
anticipated and I believe it is now part of the tracker manual's
instructions about powering a tracker, array direct when used with MPPT
voltage shifting charge controllers.
So when running a MPPT charge controller in a voltage shifting scenario, the
PV voltage will vary from normal, operating series PV voltage, way down to
battery voltage during low light conditions.
Hope this helps you design your system.
Todd
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