Voltage drop musings [RE-wrenches]

[Bill Brooks]

To all,

Lots of numbers are being thrown around about MPPT controllers without a lot
of good test data. Beware of manufacturers claims. Having lived with a
battery-based grid-tied system for over two years with lots of operating
experience, I believe that an MPPT controller would provide no benefit in
this climate.

Not necessarily so in other climates, but MPPT controller are really better
suited for stand-alone systems. Be very careful about using higher voltage
battery strings. C-40s and other controllers have a very real upper voltage
limit that will bite you. Just returned from check a 54 volt system that was
experience controller voltage issues.

It's nice to throw around a lot of numbers, but until you constrain the
application, it is way to broad of a statement to say that MPPT controllers
are the way to go. When are they going to get UL? The controller does not
exist in my book until that happens.

Bill.


> -----Original Message-----
> From: jay peltz [mailto:jay at asis.com]
> Sent: Wednesday, May 02, 2001 9:11 PM
> To: RE-wrenches at topica.com
> Subject: Re: Voltage drop musings [RE-wrenches]
>
>
> Hi Joel,
>
> One way is to increase the number of batteries in the system.
> This increases the
> operating voltage ( more watts) without the need for a MPPT
> controller.  48v systems
> are pretty easy as you add another 6v ( if you are using 6v
> batteries.  If you are
> using 2v cells, then maybe 8v would work better?) battery to the
> system, works
> pretty slick- cell voltage is lower, overall voltage is higher=
> more watts to the
> grid.
>
> And yes many people are using the MPPT controllers to get around
> this problem called
> "batteries".
>
> Jay
> Peltz Power
>
> Joel Davidson wrote:
>
> > Yes, I have 32 each 70-watt modules 4 in series for nominal 48
> volts. The C40 is
> > set at 54.8 volts so that voltage flows downhill to the
> inverter which is set at
> > 54.4 volts.
> >
> > Note that this is a grid tied net metered PV system with
> batteries - not a stand
> > alone or standby PV system.
> >
> > The problem with a net metered PV system using the SW-series
> inverter with
> > batteries is that the inverter operates at the nominal battery
> voltage, not the
> > optimum PV array max power point. Since the batteries are just
> sitting there
> > being float charged and power passes through the inverter to
> the load and grid,
> > can the SolarBoost send max power from the array to the
> inverter? Has anyone
> > used a SolarBoost to get around this SW inverter shortcoming?
> >
> > "Travis Creswell, Ozark Solar" wrote:
> >
> > > Joel,
> > >
> > > Are you trying to say you have thirty two 70 watt modules?  8
> strings x 4.?
> > > amps = 30 + amp array.  You will need two Solar Boost 3048's.
>  Better yet
> > > put 16 on the C40 and 16 on the SB3048 (you'll need a DVM
> option for it too)
> > > and compare the outputs.   Do you have a Fluke 36 ammeter?
> If you don't
> > > borrow/buy one (~$250 and you really should have one) and
> compare it's amp
> > > rating to your C40's.  I almost always find the C40 DVM's to
> read amps ~10%
> > > low.  I have found the SolarBoost products to be very
> accurate.  (Never pass
> > > a chance to beat up on Trace, just kidding, maybe)
> > >
> > > Oh yeah, your question,  you should produce up to 30% more
> amps when its
> > > colder, say less than 40F.  In the summer you will only see a
> 10% (maybe)
> > > difference when the batteries are under 50 volts in the
> cooler mornings
> > > which you won't see since you are grid tied.
> > >
> > > Travis Creswell
> > > Ozark Solar
> > >
> > > ----- Original Message -----
> > > From: "Joel Davidson" <joeldavidson at earthlink.net>
> > > To: <RE-wrenches at topica.com>
> > > Sent: Wednesday, May 02, 2001 6:51 PM
> > > Subject: Re: Voltage drop musings [RE-wrenches]
> > >
> > > > Is the SB3048 a current boosting battery charger or a PV
> array max power
> > > point
> > > > device? I have 8 each 4-module series strings of Siemens
> SP70 modules
> > > feeding
> > > > into a C40 and then into a net metered SW4048 with a small
> backup battery
> > > bank.
> > > > The batteries just sit there (pulling the inverter off max
> power point),
> > > but the
> > > > net metered load is the grid so it can take all the PV I
> produce. Right
> > > now
> > > > (4:30PM) my C40 digital meter says 16 amps at 54 volts
> equals 864 watts.
> > > Will I
> > > > produce 19.2 amps at 54 volts or 19.2 amps at 45 volts with
> the SB3048?
> > > >
> > >
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