How to Find Happiness with an Off-Grid PV System

JNCrain

Solar Enthusiast
Joined
May 30, 2021
Messages
9
We rely on well water. Last winter, our beloved utility company suffered a prolonged power outage (I believe the polite term is “Electrical Disfunction”). Without electricity, the 115V AC jet pump that pressurizes our plumbing did not run. No electricity, no flowing water, the pipes froze, and we were very fortunate that they did not break. Quite a nerve-wracking situation. The utility company disavowed any responsibility – the usual “act of God” excuse. As a result, we decided to relieve God of the responsibility, and install a small (RV-size) off-grid PV electrical system to provide power to our jet pump. God would only be responsible for making the sun come up once per day.

We considered replacing our AC jet pump with a DC pump, but quickly discovered that DC pumps are very expensive and much harder to obtain than cheap, common AC jet pumps like the one we have, so going with an inverter seemed like the logical choice.

Here’s the system we spec’d out:
  • The Sun
  • (4) Rich Solar 100W polycrystalline panels
  • Epever Tracer4215BN 24V 40A MPPT charge controller w/MT50
  • (2) SOK 100Ah 12V batteries in series for 24V
  • Giandel 3000W pure sine wave 24V inverter
The Sun was free. The size of the inverter was chosen because we measured an inrush current of about 35 amps, and continuous running at 10 amps. This translated to an inverter that could handle at least 4200 watt surges. Our jet pump automatically switches on somewhere around ten times a day and only runs for about two minutes each time to keep our water pressure between 30 and 50 psi. We figured the two SOK batteries, at about half the cost of the entire system, would provide more than enough stored power.

The biggest unknown was how to best program the Epever charge controller for our LiFePO4 batteries. A lot of research ensued. Almost all sources, including SOK, provided suggested settings that assume it would be desirable to keep the batteries fully charged. Those settings include a Charging Limit Voltage of 29.2 to maximize the charge of a 24V lithium battery. After all the equipment was installed, we set the charge controller to the commonly recommended settings for the User mode of the Epever. That’s when the problem arose.

Our 400 watt array would quite easily charge the batteries to full capacity early in the day. That was good, but the high 29.2 volt SOC would sometimes trigger the High Voltage Disconnect for the inverter. Apparently our 24V Giandel does not like more than about 30 volts and would shut down with a High Voltage Disconnect as soon as it detected a spike. We were having to frequently power down and restart the inverter to clear the error and get the inverter inverting again. In the meantime, during these events, the jet pump wasn’t getting power and water pressure would fall to zero. Exactly what we were trying to avoid.



So, we reprogrammed the Epever, changing the Charging Limit Voltage to 26.8 volts, corresponding to about a 90% SOC. Now, the batteries never get charged beyond that voltage, which has completely cured our problem with the inverter shutting down. And yet, the batteries continue to be maintained at, or very close to, the 26.8 Charging Limit Voltage. There’s another benefit to setting the reduced Charging Limit: It’s known that lithium batteries last longest if the SOC is kept between, say, 10% and 80%. Our upper limit of 90% seems like a good compromise.

From direct observation, we estimate that our system will be able to handle nearly a week of overcast days – quite sufficient, especially here in New Mexico. And as a safeguard, since we left the existing utility grid AC circuitry in place, if we ever need to, we can just unplug the jet pump from our off-grid system and plug it back into the grid (assuming it’s available even if the Sun isn’t). We could also use that grid power for an occasional top balancing of the batteries and other things too, if necessary.

To see a summary of our Epever charge controller settings (very thorough), check out the attached .pdf of our spreadsheet. It shows recommendations from various authorities, the settings we arrived at for our specific system, and includes the Epever parameter setting rules as well as the best chart we could find showing 24 volt SOC versus battery voltage. The chart and much of our understanding about lithium batteries comes from the excellent and often-cited article “How to Find Happiness With LiFePO4 (Lithium-Ion) Batteries” (https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/). And of course, Will Prowse.

Moral of the Story: Don’t fret too much about maximizing the charge controller settings for lithium batteries. Understand what those settings are for, and don’t take other people’s recommendations as gospel. Use settings that are suitable for your energy usage and try to maximize the longevity of your expensive lithium batteries. Under-stressing your system is the best policy.

After the initial growing pains and kink-removal, we expect this system will be reliable and almost maintenance-free for years, maybe decades to come (God willing. Buddha could care less).

(Though I do happen to write fiction, this is not one of them. References to any utility companies, persons, or deities, real or imagined, are not coincidental but may be irrelevant. For a 12 volt system, cut everything in half except the Sun.)

1630804064789.png
This above-ground structure tops an 8-ft diameter culvert jammed vertically into the ground. Interior temperatures are stable at about 68 degrees F, with only about 2 degree variation. No worries about lithium battery low-temp charging cutoff.

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Angle-adjustable, home-made panel mounting system using IronRidge rails on Superstrut hardware. Cheap.

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Inside of pump house, showing PV system components (not-so-cheap). Rack made out of scrap ¾” plywood and sections of metal conduit held together with all-thread (super-cheap). Surprisingly sturdy and compact, with room for expansion, tools, documentation, etc.
 

Attachments

  • PumpHouseEpeverSettings.pdf
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sunshine

Solar Addict
Joined
Apr 24, 2020
Messages
515
Don’t fret too much about maximizing the charge controller settings for lithium batteries.
I have noticed some changes to the recommended voltages of the last couple of years as real case users start to influence the debate.

Looking at your settings if one stays with the lead acid settings the life cycle of LiFePO4 would be maximised.

(maybe there is a typo in the low voltage dis/reconnect)
 

JNCrain

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Joined
May 30, 2021
Messages
9
(maybe there is a typo in the low voltage dis/reconnect)
I confess I'm pretty new to solar - especially small, off-grid systems. I'm always open to suggestions! What settings for those parameters seem right to you?
 

sunshine

Solar Addict
Joined
Apr 24, 2020
Messages
515
What settings for those parameters seem right to you?
I am swapping lead acid for LiFePO4 so looking at all the recommended voltages but have noticed there is a tendency to lower voltages to extend LiFePO4 life cycle and having only two charge settings, boost and float.
 

JNCrain

Solar Enthusiast
Joined
May 30, 2021
Messages
9
there is a tendency to lower voltages to extend LiFePO4 life cycle and having only two charge settings, boost and float
If you mean lower settings for the Low Voltage Disconnect and Reconnect, lower settings would allow the battery go to a lower SOC (e.g. Low V Disc = 22 would permit discharging to less than 5% SOC). That would not extend the battery life cycle. The main thing for extending life cycle is keeping the battery in its Goldilocks zone, not too highly charged and not too drained. The Charging Limit Voltage setting will lower the peak SOC, and I believe the Low Voltage Disconnect will prevent it from being overly discharged. In my system, over-discharging doesn't seem to be likely - but charging to the limit of LiFePO4 would happen constantly if the Charging Limit Voltage isn't set to a lower value, and that would shorten the life cycle capacity of the battery, according to the experts.

I'm not really sure what you mean by "having only two charge settings, boost and float". The Epever has many other charge settings whether you modify them or not. And it will demand that you follow its rules for setting parameters. If you don't follow the rules, there is a good possibility that it will not allow you save your new setting and will tell you something like "Invalid Parameter" in the MT50 display.

Am I misunderstanding something?
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,217
References to any utility companies, persons, or deities, real or imagined, are not coincidental but may be irrelevant.

May be irreverent?

An alternate solution is an above-ground tank with small 12V booster pump, like Surflo. Unlike yours, it wouldn't operate indefinitely, but could ride through short-term outages and provide water for key needs.
 

JNCrain

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Joined
May 30, 2021
Messages
9
above-ground tank with small 12V booster pump
We do have above-ground tanks, kept full with rain catchment. We're in the process of tying them into our water system to supplement the well water. Would a small, inexpensive 12V booster pump be capable of pressurizing a house plumbing system? Nope. The pressure is needed to keep pipes from freezing by letting all our faucets trickle. We looked into DC pumps, and anything capable of pressurizing our plumbing would cost an arm and a leg.

There was another reason we went with the off-grid system that I didn't confess to in the write-up: The learning experience. We have some acres, with buildings of various kinds scattered around. The idea of setting up little solar energy islands is very appealing. And fun. Lots more fun than tearing up our property with massive trenches for electrical lines. :)

May be irreverent?
Truly! Hopefully, no one was offended. Least of all, any deities.
 

sunshine

Solar Addict
Joined
Apr 24, 2020
Messages
515
Am I misunderstanding something?
Yes. The low disconnect and reconnect v are reversed in the first two rows, Minor issue and not part of maximising the life cycle.
Will answer in depth later.
 

JNCrain

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Joined
May 30, 2021
Messages
9
The low disconnect and reconnect v are reversed in the first two rows
Ah! I think I see what you mean - the Low Voltage Disc/Rec settings listed in the first two (SOK Recommendations) columns are backwards, i.e.,

Low Voltage Disc = 11 , 22 (for 12V and 24V systems, respectively)
Low Voltage Rec = 10 , 20

Is that what you're referring to? They do appear to be reversed. The Epever rule is:

1630938572998.png

I copied the settings in those columns from someone else's file on the SOK Facebook group just to cite an example. Never tried using them. If I had, the Epever would probably not have allowed me to save the settings and given an "Invalid Parameter" error. The settings I'm actually using are in the last column, "My Settings". They're also reflected in the Rules section.

Good catch!
 
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