C40's and SW's [RE-wrenches]

Jeffery Wolfe, Global Resource Options jeff at globalresourceoptions.com
Tue Feb 4 11:29:12 PST 2003


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.


-----Original Message-----
From: Graham Owen [mailto:graham at solarexpert.com]
Sent: Monday, February 03, 2003 9:01 PM
To: RE-wrenches at topica.com
Subject: C40's and SW's [RE-wrenches]


Dear Colleagues,

I need some Wrench assistance with a PV service call, for the Xantrex
Power Module system (or power panel for that matter), you have to set
the C40 charge controllers and the SW inverter so that the inverter can
sell PV power to the grid without the controllers restricting current
into the inverter in the first place. I am trying to determine which set
of values works the best.

Are these good figures?

C40  bulk   56.0
     float   55.0

inverter bulk  54.0
       float   53.2

Thanks in advance!

Graham

P.S. Joel, I am working on finding your CPUC answers, I was working out
of town today.

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