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