Inverter/Battery Question [RE-wrenches]

Thu Jan 19 17:48:46 PST 2006

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-----Original Message-----
From: Allan Sindelar [mailto:allan at positiveenergysolar.com] 
Sent: Wednesday, January 18, 2006 8:24 PM
To: New wrenches posting
Subject: Inverter/Battery Question [RE-wrenches]

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Esteemed Wrenches,
Here's a situation that I am approaching gingerly and for which I need
advice.

Background:
New off-grid client, originally contacted us for an inverter to use with
his new Kohler RES12 generator, for the two years or so while he built
his house. He also was about to have a well drilled and an AC pump
installed. I recommended and installed a PS2 Outback system, with DC and
AC boxes, an X-240 for generator balancing for battery charging, no
charge controller or PV array, a Tri-Metric in his 5th-wheel, and a
single string of 8 golf carts. The idea is that the inexpensive
batteries would be expendable, but everything else would be as versatile
as possible as the basis for a future home system. The customer also
wanted to add the PV array later, although a
1 kW array is being added soon, as I expected it would. The client had
no load analysis or understanding of off-grid design when we started
out.
Depending on the future load profile, we would use one or two inverters,
but start out with one now, and wait and see whether a second inverter
would be needed.

I also recommended a cistern, well pit, and DC pressure pump, gave him
specs on the Booster Pump and Solar Force, and recommended that a
smaller AC well pump than the default 1HP be used with open discharge
into the cistern, to reduce the likelihood of needing the second
inverter. The system would be set up now to use the generator to both
fill the cistern and charge the batteries; once the cistern is full most
gennie runtime would be just for battery charging. The DC pressure pump
would operate normally with a pressure switch. Eventually, PV would be
added, and at that time the well pump would run off the inverter(s),
with the generator only for occasional backup charging.

He called around Christmas to tell me that his pump was in, and he was
ready for hookup, as the driller wasn't willing to hook up to the wires
we left in place. Somehow, the advice I had given had gotten lost. The
well driller had put in a conventional 1 HP 240VAC pump, with well pit
but no cistern, on a pump-and-pressurize approach at about 400' depth.
The driller had told our customer that because the well produced a good
flow, he could save the cost of the cistern and second pump by using a
larger well pump; exactly the wrong approach for offgrid.

So he has to start the generator to take a shower, which wasn't how it
was supposed to be. The well/pressure pump has to be set up now to run
off of the inverter and batteries.

We gave it a try. I put in a PSX-240 for step-up for the pump, but I
should have tested it first. Starting under load, our Fluke 337 (which
can measure inrush current) recorded 57-63 amps AC when trying to start
the pump under load. Neglecting power factor, that's 6,840-7,560 VA. The
VFX3648 is rated maximum 6,000 VA surge. We will credit the customer for
the PSX-240 and partial labor, but don't want to make an even more
expensive trial that might not work.

Needless to say, the customer's bummed. The obvious solution is to add a
second inverter and related peripherals. But finally we get to my
question:
Can a single string of golf cart batteries provide the necessary current
to run two inverters with this load? 7,560 W at an estimated 44 volts
during heavy draw is 172A on a single long string of 2/0 interconnects
and cables.
That's a C/1.26 discharge rate for a second or two, followed by a 110A
load for the duration of the pump's runtime, or C/2 discharge rate for
probably
10-15 minutes. I've never tried to do this. Before we invest in another
inverter, I am asking for some guidance as to whether we can expect it
to work. Do we have to add a second string of batteries? Is there no
choice but to put in a smaller well pump and DC pressure pump, as I had
originally recommended? Any other good ideas?

As usual, thank you in advance for the collected wisdom.
Allan at Positive Energy

Situation

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