Inverter/Battery Question [RE-wrenches]

Travis Creswell, Ozark Solar ozsolar at ipa.net
Thu Jan 19 05:08:21 PST 2006


Hi Allan,

We reluctantly installed 1hp VFX3524 and two 12 volt/225 Ah gel batteries
for 1hp well pump.  It was the inverter I was worried about, not the
batteries.  It's a three wire pump which greatly reduces the starting
current.  Using my 337 it surges to 29-30 amps AC and runs at just under 10
amps which is lot different then the numbers you're getting.  Your customers
pump could be a two wire.  I've seen the DC amps anywhere from 75-85
depending on battery voltage.

I'm thinking the static level of the water is close to 300'.  The inverter
barely grunts and the batteries are trucking along fine after a year and
half.

And as Windy wisely pointed out to me several months back, starting a pump
under load makes no difference in the surge.

My suggestion would be to change it out to a 3 wire pump 3/4 hp pump along
with a large pressure tank (at least 25 gallons of drawdown) to reduce the
pump cycling time.


Best,

Travis Creswell
Ozark Energy Services


-----Original Message-----
From: Allan Sindelar [mailto:allan at positiveenergysolar.com] 
Sent: Wednesday, January 18, 2006 7:24 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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