Voltage drop musings [RE-wrenches]

[Allan Sindelar, Positive Energy, Inc.]

Wrenches,
I forwarded some of the recent string on MPPT controllers to Rick at RV
Power Products. Below is his response.
Allan Sindelar, Positive Energy, Inc.

Hi Allan,
Thanks for the opportunity to respond. I apparently don't have the
authorization to respond to the group directly, so please pass it on.

The Solar Boost (SB) products are both a charge current booster and a
Maximum Power Point Tracker. In this application the inverter is essentially
acting like a shunt controller, sucking as much current from the battery and
PV array as possible without allowing battery voltage to drop below the
inverters charge voltage setpoint. The way an SB is typically used in a grid
tied system is to set the SB charge voltage to be somewhat higher than the
voltage that the inverter is regulating to.

With an SB added between the PV and battery/inverter, and the SB charge
voltage setpoint set higher than the inverter's, the SB will work it's heart
out MPPT'ing trying to get the batteries to the SB's charge voltage
setpoint. But, the inverter won't let the battery voltage get that high so
the SB stays in "bulk" charge mode. The SB then continuously Max Power Point
Tracks the PV array to pull as much power as the array can deliver,
typically delivering more charge current and therefore more power to the
grid via the inverter.

When the SB is in the bulk charge mode, the output could be described as
being "constant power". With everything else being held constant, a lower
"inverter charge voltage setpoint" would produce greater SB output current.
The sticky part is estimating how much current this will be. Taking PV
rating divided by output voltage usually gives a current number that is
higher than you will typically see. This because of two primary factors;

1) The PV's power is rated at 1000w/meter-squared of insolation and a cell
temperature of 25C. PV's produce less power as cell temperature increases,
and since the cells heat in the sun, it may take an ambient temperature of
0C or less to produce a cell temperature as low as 25C.

2) Nothing is 100% efficient at converting or delivering power. The Solar
Boost products are highly efficient power converters at ~97% efficient, but
they do waste that ~3% of the available power. Wiring, combiner boxes,
fuses, circuit breakers, etc., produce voltage drop and therefore consume
some available power as well. So, even if a 75W panel did produce 75W, some
of it is lost on its way to the battery/inverter.

I hope this discussion helps. I will leave it to you wrenches with more
direct contact with real world experience to estimate current boost for your
peers in particular geographic and climate conditions. One final point; when
more power is not available from the array, a Solar Boost controller stops
"power converting" and transitions to a straight through connection in the
same manner as conventional controller.

Thanks for your time...

Rick Cullen
RV Power Products ~ Providing quality ( MPPT)
Maximum Power Point Tracking
solar charge controllers

Phone: 760.944.8882
Fax: 760.634.8193
http://www.rvpowerproducts.com
mailto:rick at rvpowerproducts.com

Providing Electrical Independence
---------------------------------




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