[RE-wrenches] Design Question re Battery Bank Sizing

Wed Feb 15 10:38:28 PST 2012

Alan,

See if you can find the locked rotor amps (LRA) for this pump. Design to provide that much current for starting the pump. My guess is about 7-9kW for a 3HP.

I'm not a fan of multiple parallel strings but you are correct that parallel strings will produce more instantaneous current. However in this case, I don't recommend it. Even with other loads it is not likely you will demand more than 250 amps @ 48Vdc draw on the battery to start the pump. That is not excessive for an AGM battery, especially since it is very short duration. 

If you need more battery capacity, opt for lower voltage batteries instead of paralleling.

Larry Crutcher
Starlight Solar Power Systems
(928) 342-9103

On Feb 15, 2012, at 10:53 AM, Allan Sindelar wrote:

Esteemed fellow Wrenches,
We need to size a battery bank for a GTWB just for starting a well pump. The well is on its own meter, so as long as the battery voltage does not crash when the pump starts there will be plenty of capacity left over for minimal loads.  We'll be using sealed batteries, most likely Concorde SunXtenders. According to theSunXtender Technical Manual, Concordes can handle up to a C/0.2 charge rate (500A for a 100Ah battery), but there is no discussion of maximum discharge rates or voltage drop due to large surge loads.

The pump is 3HP and is conventional 3-wire, not soft-start. The well is 940ft. and the depth to water is unknown. It pumps into a pressure tank, not open discharge. Customers are not interested in replacing the pump, but might be talked into adding a soft start Franklin controller, although at this preliminary design stage we're not certain one is available for this particular model. The inverter would be a Radian, so system voltage is 48V.

I would think that configuration would make a difference, as well as size. Using 12V batteries in series/parallel would minimize interconnect and terminal resistance. I think that it would also allow multiple paths for discharge current, reducing the voltage dip.

What's your best guess at the smallest size battery bank that would reliably start this pump, and how would it best be configured? 

Thank you,
Allan

Allan Sindelar
Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com

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