stand alone system for water well runs hour, then shuts down

[hal53]

I am puzzled about a solar system that I installed to run a 400’ water well for my duck impoundment. It will start easily and run for about 1 hour and then shut off. The error code reads “low battery.”



It is a 48 volt system using four VMAX tank SLR125 AGM 12V batteries in series (each is 125 mAh). There are 6 solar panels set up as two series of three panels, each series generating 135 volts. They run through a Midnight Solar charge controller.



The pump is 2 hp; I understand it should consume about 13.5 amps to run. Information on the Internet indicates a 2 hp pump takes 7500W to start and 3750W to run.



I calculate that the system should be generating about 5000 watts.



Am I missing something?

 

[pollenface]

Observe the voltage when it cuts out.

Likely your 2hp pump is pulling the voltage down to the low voltage shut down point of your inverter. Very normal with lead-acid batteries. You would need to double/triple the battery bank size running that kind of load continuously.

Forget the calculation of 5000w and observe what it actually generates under real world conditions.

Where did the calculation of 5000w come from? What are your panels?

 

[sunshine_eggo]

Since you provided no array data, looking at just the battery:

12V*4* 125Ah = 6kWh

125Ah is at a rate of 125/20 = 6.25A

At 3750W:

3750/.85/48V = 92A - about 15X your C20 rate meaning your batteries will deliver nowhere near 125Ah.

As an example, 150Ah Trojan T-1275 batteries will only deliver 70Ah @ 75A, which is < 50% of rated.

You are hammering the shit out of those batteries. They are WAY undersized for the application. You're lucky to get an hour out of them.

 

[n2aws]

You don't mention what panels, just the voltage.. nor do you meantion the voltage of the pump... but just quick napkin math:

48v x 125ah (I assume you meant ah, even though you said mah) = 6000 watt hours (3,000 usable, due to AGM and not discharging below 50%)
13.5a x 240v 3240

So the math adds up, in 1 hour, you've essentially consumed all of the available battery capacity.

You also didn't say which inverter. but if it's overloaded, it may have the ability to "surge" for 30 or 60 minutes, before it overheats and shuts down.

Either way, you're driving those batteries *hard*. It sounds like when you are using them, they are actually likely in excess of their rating, and possibly damaging them.

 

[hal53]

Magna Sine 4400 watt 48V inverter, previously used to run 12 350W panels for campsite. Panels are Hellene 66C 360 watt panels; each was outputting 45 volts, according to voltmeter

Appreciate all the information. Do I need a second bank of batteries then?

 

[sunshine_eggo]

An inverter converts DC to AC. What are the panels connected to, and how are they connected?

 

[time2roll]

Appreciate all the information. Do I need a second bank of batteries then?

Runs for an hour? Double the battery and double the solar would be my first inclination.
If spending money on battery go with the lithium.