Inverter/Battery Question [RE-wrenches]

[Ray Walters]

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Hi Allan;
I'm surprised the VFX3648 wouldn't run a 1 hp pump. I've got a pair of 
VFXs running a 3 Hp pump. My understanding was that the Outbacks would 
limit current at 50 amps, while letting the voltage dip to allow 
starting large inductive loads.
The battery voltage could be dipping too low, what is their instantenous 
voltage dipping to? You can hit a T105 pretty hard. We regularly 
discharge them at C/0.5 rate without problems.... (well better not have 
any questionable connections at 450 amps)  The voltage drops pretty 
radically under this load,  (a 144 v nom. bank at full charge will go 
under 100 v)
What was the inverter's failure mode: over current or low voltage?

For residential, I actually prefer the single pump system to the 2 pump 
& cistern approach primarily for water quality issues. Sounds like you 
covered your self fine though, and the guy just didn't listen, so now 
its time for him to get the check book out and have the NABCEP solar 
bozo main man make it right. More batteries and a second inverter will 
still not be as much as the 2 pumps, pump pit, and  cistern would be, so 
he should be happy.

Ray


>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
>
>
>  
>

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