UI w/SB50 controllers , was C40's and SW's [RE-wrenches]

Kirk Herander, VSE kirk at vtsolar.com
Tue Feb 4 12:27:49 PST 2003


I have a dual SW5548 system with 4.8 kw array, divided into 4 sub-arrays,
each controlled by a Solar Boost 50 MPPT controller, utility-intertied  in
combo w/ a sealed AGM battery bank. I have set both bulk & charge voltages
on the inverters to the batteries recommended float voltage of roughly 52.5
vdc. I trust that this voltage is acceptable to keep the batteries well
maintained, and I want to maximize the delta between panel MPPT voltage and
battery voltage, so there is more voltage to convert to current and
therefore more power to send to the grid.

In the middle of a cool and sunny Vermont day last fall, I measured an
efficiency of 91% when comparing DC PV power into the Solar Boosts vs. AC
power sent to the grid. The gains to the grid using MPPT vs. the C40 are
impressive, so I would suggest getting rid of the C40 altogether. It is not
appropriate for a battery-based UI system.

Kirk



----- Original Message -----
From: "Jeffery Wolfe, Global Resource Options"
<jeff at globalresourceoptions.com>
To: <RE-wrenches at topica.com>
Sent: Tuesday, February 04, 2003 2:29 PM
Subject: RE: C40's and SW's [RE-wrenches]


> Bill,
>
> Although you're right that the C40 should never control, the point of the
> higher voltage settings is to make more power. We all know that the PV's
> will put out more power at higher voltages, so setting the inverter
higher,
> for float, is better. We also need to balance this with battery high
limits,
> and the fact that you need a pretty significant "dead band" in the control
> setpoints between the C40 and the Trace, due to inaccuracies in the
voltage
> meters in both units. (Trace told me 1.5 V, I'm typically using about 1.0
> V).
>
> The 53.6VDC default on the inverters is also on the very low side of where
> we're floating batteries, even on-grid, today. We're moving hotter, more
> like 54.8 to 55.2. Batteries that float all year long tend to have
sulfation
> problems, and it seems that the hotter setpoints help reduce this. (Along
> with the little black boxes.) Couple this with the fact that on-grid
higher
> float voltages are going to produce more power, the case is made for
setting
> the floats about as high as the manufacturer's will let you. We do use
bulk
> mode, as it's good (IMHO) to bulk the batteries occasionally. So we set
the
> bulk up at about 58 VDC. This then pushes the bulks on the C40's up to 59
to
> 59.5, in order to provide as much dead band as possible.
>
> We've got three years on our double C40 / SW system, as well as several
> others around the area. The low set points "work", but are not the best
for
> power production (remember, we all complain about power production on PV
> systems), and are not necessarily the best for battery life.
>
> I do agree that the C40 float and bulk can be set the same, but it's more
> like setting the float to the same as a more typical bulk, rather than the
> other way around.
>
> I also always use a Fluke to set up the C40 (and any other controller). I
> never trust the little dials or displays. They can be wrong (on any
> controller).
>
> Jeff
>
> -----Original Message-----
> From: Bill Brooks [mailto:billb at endecon.com]
> Sent: Tuesday, February 04, 2003 1:10 PM
> To: RE-wrenches at topica.com
> Subject: RE: C40's and SW's [RE-wrenches]
>
>
>
> Graham,
>
> I believe that bulk charging is meaningless in a grid-connected
application.
> Since the system is floated all the time, except during a power outage,
the
> focus is on the float settings. I would set the bulk timer on the inverter
> to 0:00 so it automatically goes to float. The default for the inverter is
> 53.6 Volts for float and I think that is fine for most lead-acid
batteries.
> The key is making sure the C-40 is set at least one volt higher. It is
also
> critical that all controllers and inverters have temperature compensation
> probes installed and sensing basically the same temperature.
>
> Finally, check the C-40 while operating to make sure it is not
controlling.
> By testing the voltage drop across the C-40 positive battery and PV array
> inputs, you can tell whether it is controlling. The voltage should always
be
> below 0.75 Volts, otherwise it is controlling. It should only control in
an
> outage. Set the C-40 bulk at the same as the float, since the array should
> not be bulk charging the battery anyway. Bulk charging doesn't help a
> battery that stays at float all the time. It can actually hurt a VRLA
> battery by overcharging it if you allow it to bulk for more than a
half-hour
> or so. Hope that helps. Others may see it differently but I have over four
> years of grid-connected experience with my C-40s and SW.
>
> Bill.

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