Building a standby power system for a well-pump

I still want to be able to take a shower during a power failure

Nov 14, 2021

One of the great advantages of country living is that our water is supplied by a underground well. Unfortunately we don't always have the flow that we need, especially in the summer months. It’s not enough of a problem that it warrants us digging a deeper well. Instead we opted to install an underground cistern storage tank. The well pump fills the cistern until the float indicates full. Not shown in my diagram below, is the Symcom PumpSaver that I use to prevent the well pump from running when it’s dry.

The cisten ultimately feeds the house and to achieve appropriate domestic water pressure, an additional pump is used to pressurize a tank until the cut off switch turns off. The pumps are typically 1/2 to 3/4 hp and run at 220 volts.

All of this is contained in a separate pump house structure from the dwellings.

This is a pretty standard setup, but it won’t maintain water pressure for long during a power failure. Like most folks, we got around this issue using a backup generator that I would have to manually start when I wanted water.

But this seemed both wasteful a hassle and unnecessarily noisy to run the generator for such an intermittent load. Once the tank has achieved appropriate pressure, you shut it down only to have to start it again when the pressure drops below the limit.

A obvious solution to this problem is to use some form of inverter/charger and backup battery as a standby power supply or UPS. I am not attempt eliminating the backup generator, but only using it when our batteries have drained. Optionally, you could even add solar panels if you have enough sunlight.

Well-pumps and surge current

A few words about well pumps. A lot of the submersible pumps in rural locations are single phase three wire units, and they use something similar to the Goulds CentriPro starter controller box. These controller consist of a potential relay and a start capacitor.

Well-pump motors have relatively high starting torque and thus have two motor windings, one for running and one that assists the start. The potential relay coil is wired in parallel with the motor start winding and the relay contact is normally closed.

As the motor speed increases, back EMF is that is generated across the start winding. This causes the relay to energize and take the start capacitor out of the circuit. At about 75% motor speed and the pump proceeds on the run motor winding only.

Sizing the inverter

The typical 4” submersible well pump motor runs at 220 volt and falls into the 1/2 to 3/4 hp range. With a running current around the 5 - 7 Amps this puts us in the 1.5 KW range. But because of the high torque involved in starting, these motors can pull quite a bit of surge current. Pump manufacturers refer to this as locked rotor amps (LRA). You have to keep this in mind and size your inverter appropriately.

For my application I went with the Sigineer Power APC6024D 6KW Dual Phase Pure Sine Inverter Charger. It might seem like overkill, but I wanted the options of running both pumps in the event of a power outage.

The APC6024D doubles as a charge controller, and will also take AC from the utility or generator charge the batteries and keep them topped off.

The folks at Sigineer have a fairly good reputation and build solid equipment. I have also have had some reasonable luck with tech support.

I will warn you though, this thing is a built like a tank. It's HEAVY, about 105 pounds of heavy. With most of that weight coming from a large power transformer which handles the surge. So keep this in mind when finding a place to mount it.

Another factor is that these inverters have the potential to generate some heat. According to the folks at Sigineer Power the APC6024D needs about 50 cm clearance around it, and they suggest ventilation to support a minimum of 145 CFM air flow. Just to be safe I mounted the inverted on a piece of cement backerboard and added a thermostatically controlled ventilation fan to cycle the air out of the pumphouse.

Battery selection

The batteries for this system will be most likely spend most of their lifetime in a charged or standby state. This was a major factor in choosing the appropriate battery technology for this project. At first I considered using LiFePO4 batteries. But due to the current state of supply chain problems I had a difficult time getting hold of reliable cells.

I ending up going old school with a quality brand of flooded deep cycle lead acid cells. The Surrette Rolls S6 L16 SC arranged in a 24 volts array. These things can crank a lot of power. In addition to the simple fact that they were available when I needed them, they also have a long history of reliability provided you do some reasonable maintenance now and then.

I decided to host the batteries just outside the pump house, both because of space considerations as well as safety and ease of access. I constructed a battery box from marine plywood with a front cover that I can slide out to access the batteries.

By the way, the L16 units weigh about 125 pounds each, so that sliding plate comes in handy. I also designed the box with a elevated top with stovepipe vent and fresh air intake to prevent hydrogen from accumulating.

The batteries are wired in a 24 volt configuration and fused with a Blue Sea class T fuse block. The batteries can produce an amazing amount of current, and given the possibility of hydrogen in the box, you don't want to cut corners on this.

In addition I added a Victron Energy SmartShunt so I could monitor the state of charge.

System Wiring

There are two separate circuit breaker panels in this system. One is dedicated to selecting the AC input source: utility power or standby generator. This requires a safety mechanical interlock to prevent any power feeding back to the utility. This also feeds the inverter input used for charging the batteries.

A second panel is used to channel the output of the inverter to the critical circuits like the well-pump and pressure tank-pump. The inverter also acts as a bypass when power is available at the inverter input.

I also wired a bypass breaker to allow me to take the inverter offline for any maintenance and power the pumps from the power panel directly. Once again assembled the breakers with a safety mechanical interlock.

Inverter setup

The Sigineer Power APC6024D is relatively easy to setup, once you get your head around the documentation. There is a lot of good technical information there, but it could use some editing.

There are two places you need to do setup: the dip switches on the left side of the inverter and the battery type and charge current setting on the front panel.

In my case, I am using the inverter as a standby power supply or UPS. This means that when AC input is present from the utility or generator, the battery will be charged, and the inverter will be bypassed and transfer the input AC to power the load. When AC power is removed, the inverter will be activated and generate power from the batteries. Sigineer refers to this as AC Priority mode. Thus we set SW 5 into position 0 (off). For 60 hz operation set the SW 4 into position 1 (on).

The only other dip switch of consequence is the Power Saver override SW 3. I turned this off. Some of my other loads were too low to be detected.

The second set of relevant settings was the battery type selector. This is straightforward for setting the boost and float voltages for your battery. Once you have hooked everything up, you need to adjust the maximum charge current knob to fine tune for your battery setup.

Performance

I spent some time monitoring voltages and currents and I am impressed how well this system works. The inverter transfers from standby to inverter mode very quick enough to keep my networking and computer systems running. It also has a built in delay of about 15 seconds before transferring back to avoid any issues when the utility is unstable. The important thing was it generated enough power to easily start and run both pumps at the same time.

System status.

The APC6024D has a set of LEDs that do a good job of indicating what is going on as well as built in LCD display. You can tell when the system is in bypass or inverter, what kind of load it's under and current mode of charging.

The SmartShunt has a bluetooth interface and by using the VictronConnect app you can get quite a bit of data about the state of charge of the battery.

Computer interface

Both the SmartShunt and the Sigineer Inverter have the ability to be remotely monitored via a serial line.

The SmartShunt API is defined by the VE.Direct Protocol. All you need to do is attach the USB interface cable and the SmartShunt will periodically stream a key-value set of parameters indicating battery status. This is pretty well documented by Victron and turned out to be very reliable API.

The Sigineer Inverter on the other hand required us to do a little bit of work to get data from the Inverter.

Sigineer Power does provide some documentation. But in order to communicate to the inverter you need to purchase the PSW7 remote control module which provides a DB9 RS232 interface.

While I believe it is possible to connect directly to the inverter using an RJ-45, I haven't worked out the details yet.

Once you have a serial connection to the inverter, you must poll the device. The only command that seems to work is the Status Inquiry 'Q1'.

<-- Q1<cr>
-->(240.0 240.0 238.0 000 61.0 26.8 00.0 00001001<cr>

The return value is in the form of

Start Byte: ‘(‘
Input Voltage: MMM.M
Input fault voltage: NNN.N
Output Voltage: PPP.P
Output current (percent of max current): QQQ
Output Frequency: RR.R
Battery voltage: SS.S or S.SS
Inverter Temperature: TT.T (turns out this does not work, ignore it)
Status bits: B7 - B0 (defined in documentation)

In my initial testing I have found the Inverter's serial interface to be mostly reliable, but it will hang every so often. It could go a day or two then it stops responding. Sometimes you can retry the query and it will recover and other times I have had to unplug the PSW7 remote console from the RJ-45 cable going the inverter.

But to be clear, even though the serial communications from the PSW7 fail, the inverter continues to run fine run and switches back and forth from inverter to charger mode. Something is up with the remote console serial interface.

I am working on a fix to this and will post info as I get some progress.

Conclusion

For the most part I am pretty happy with this setup. I would have liked to use a more modern battery technology. I also would have liked the Inverter computer interface to be a bit more robust, but I can derive a lot of information about battery state from the SmartShunt. All and all it was a win. I now have been running through two seasons so far without any issues from the system outside of the serial port problems with the PSW7.