On grid w/battery (was non wrenches) [RE-wrenches]

Tue May 28 14:05:57 PDT 2002

Wrenches,

I would like to respond to the MPPT / Float issue below. It is correct that
MPPT becomes inactive when the charge controller is in Float charge mode.
This is because the controller controls battery voltage by reducing charge
current. Since it is intentionally reducing charge current during float,
MPPT operation is no longer needed. MPPT only operates when a non grid tied
system is in Bulk, which is when it is needed.

But, the situation is very different when the system is grid tied. In a grid
tied system it is the inverter that is really controlling battery charge
voltage in a "diversion controller" type strategy. The inverter "controls"
battery voltage by adjusting how much power it pulls away from the batteries
and delivers to the grid. The inverter pulls away and delivers to the grid
as much power as it can without allowing battery voltage to drop below the
inverter's charge voltage setpoint. When a Solar Boost MPPT charge is used,
it's charge voltage setpoint should be set to be slightly higher than the
voltage the inverter is regulating to. When this is the case, the Solar
Boost controller will remain in bulk, working it's heart out trying to get
to it's charge voltage setpoint which the inverter won't let happen. The net
effect of this is that more current and power is available for the inverter
to pull away and send to the grid.

The comparatively low Float voltage battery backed grid tied systems
normally reside at tends to increase output current from the MPPT
controller. If everything was 100% efficient (which of course they aren't
and some power is lost) output current = input current x Vpv/Vbat. To pick
some numbers, suppose Vfloat=26.6V and the PV's are at their standard test
conditions of Vmp=34V, then output current will increase by 26.6V/34V=1.28
or 28% relative to input current so more current is made available for the
inverter to extract. The key here is that larger the difference between Vbat
and PV Vmp, the larger the current increase will be. A system which remains
in Float holds battery voltage on the low side which is good for MPPT
performance.

Rick Cullen
RV Power Products, Inc.
Providing quality MPPT solar charge controllers
Phone 760-597-1642 x102
Fax 760-597-1731

mail to: rick at rvpowerproducts.com
www.rvpowerproducts.com

----- Original Message -----
From: "Bill Brooks" <billb at endecon.com>
To: <RE-wrenches at topica.com>
Sent: Monday, May 27, 2002 5:39 PM
Subject: RE: On grid w/battery (was non wrenches) [RE-wrenches]

> Joel and Marco,
>
> With all do respect to my friends at Advanced Energy, the old SW is our
most
> stable inverter on the market. Joel, I think your unit could handle a much
> larger array no problem. It seems that you have fallen into the trap of
> believing that max power tracking has a lot to offer you. While I believe
> there are benefits to MPPT, a properly setup battery-based system will
> harvest 95% or more of what a MPPT controller will do. In fact, the
Advanced
> Energy MPPT or RV Products controllers will save little or nothing when
> dealing with a floated battery (even so stated in RV Products' manual).
Play
> with your system some time and run the inverter up 4 or 5 volts (with the
> C-40 set to max voltage) and you will find your power begins to drop, not
> increase. It does depend on the array, but I would not be so quick to
write
> off the use of your unit. From what I remember being at your house, you
did
> not have a huge array. Stick with what works. The biggest issue with the
old
> SW was bad power factor when connected to the grid. I think you already
> adjusted your machine.
>
> Bill.
>
>
> -----Original Message-----
> From: Mangelsdorf, Marco [mailto:mmangelsdorf at hei.com]
> Sent: Monday, May 27, 2002 5:13 PM
> To: RE-wrenches at topica.com
> Subject: RE: On grid w/battery (was non wrenches) [RE-wrenches]
>
>
> Joel and others,
>
> At the risk of sounding like a syncophant salesman for our friends at
> Advanced Energy, Inc. in NH, their Multi-Mode battery-based 5 kW
> inverter was designed to do exactly what the SW can't do: max power
> point track.  Their inverter also uses no grid power at night to keep
> the batteries floated.  I've been closely monitoring a number of
> Multi-Modes in the field here in the tropics and the reports have been
> very positive in terms of performance.
>
> On another point: what's the latest with Xantrex getting UL listing for
> the SW 4.2 with the GTI?   It's my understanding that the SW/GTI has
> been approved by CTL in oh Canada but has yet to get official UL
> approval.  Does anybody know anything otherwise?  Has anyone had any
> difficulty getting their new SW/GTI assembly approved by the AHJ or
> local utility?
>
> aloha,
> marco
> ProVision Technologies, Inc.
> Hilo, Hawai'i
> www.provisiontechnologies.com
>
> >From Joel Davidson:
>
> The point is our battery bank has just sat there 35,000 hours keeping us
> off the
> PV array max power point and buying a float charge at night from the
> grid for
> less than 24 hours of relatively non-essential backup power.
>
> Of course, I'd have to change inverters unless Xantrex figures out how
> to get the SW4048 to max power point track when on the grid and function
> as a
> UPS in stand-alone mode during grid outages.
>
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