Suggestions for off-grid back-up system

[TimC]

I want to have a solar system to back up electric power during very infrequent outages (two short outages in five years). I'd like to power my fridge and freezer full time, which are rated less than 3 KWh/day combined and have enough reserve storage for very conservative power usage if the grid goes down. I'm shooting for three days of storage for the fridge, the freezer, a minimum number of LED lights and in winter my furnace (900w peak and 600w continuous when running.) I no longer have actual per day consumption of the furnace. An educated guess would be the furnace runs for two to three hours per day Dec thru Feb. I have a Kil-o-watt meter that I can plug the furnace into, but, being fall and currently 70 degrees I don't see getting actual consumption numbers right now.

Here's where I am for equipment... I have two 100w Renogy panels (12v) and one Epever 30a charge controller. I do not yet have a battery bank for use in this effort. I'll likely chose AGM batteries as I have had excellent service life with them in the past when kept at full charge. I agree that in the long term some sort of LI-Ion battery may be more cost effective, but, I am trying to build a more reasonably priced system right now.

I am not saying I need to use the two Renogy panels, I realize the 300+ watt panels available are probably more efficient and will build the system around just the Epever if needed. I am not sure how the higher voltage panels work with a battery bank. I believe the Epever is good to 24v.

So, using the Epever and knowing I'll need batteries, a very clean inverter and solar panels where should I start calculating the size of the system? If you could advise in layman's terms please. I am familiar with solar for my 12v camper (the source for the charge controller and panels) but the higher voltage systems are beyond my expertise.

Thanks
Tim

 

[snoobler]

You're almost to the starting line.

Bottom line a number, e.g., "I need to be able to provide XkWh/day."

Then decide how many days of operation you want WITHOUT solar charging, or identify that you're okay with providing less solar than needed, but will end 72 hours with 50% of your AGM battery remaining.

Then use link #5 in my signature for your location, panel orientation and tilt. Pick the worst case month where you expect to have to engage your system for the 3 days, and provide that month's solar hours.

Then, answers shall flow forth with the magic of arithmetic.

 

[DThames]

I am testing such a system right now. I am not finished with it yet. I have two used LiFePo4 batteries that I ran down using my freezer. Power used at the Kil-o-watt meter was a hair under 5kwh when I stopped the discharge test. I have an old freezer that uses about 5kwh per day. I have 2, 295watt solar panels set at an angle good for fall/winter. They are older panels and on a good day they will output 250 watts each. I am using a EPever Tracer 40amp. 380watts (14 amps) are going in the battery according to the BMS. This is the second day (maybe third) day of charging and the battery is not fully charged yet. That is with near zero use since the discharge test ended. I think I need about 3000 watts of panels to do a quicker charge and to also run the freezer and frig while charging...while the sun is up....so to make zero load on the battery during the day.

Note overcast dark day you will get about 5% of what you get on a sunny say. If the sky is overcast but bright, you might get 50%. That is based on experience with my large grid tied system. So if the power is out, you need overkill if you want to keep the freezer going. I also have a generator.

 

[DThames]

<snip>

I'll likely chose AGM batteries as I have had excellent service life with them in the past when kept at full charge.
<snip>

Thanks
Tim

While the AGM might "last" if you take good care of them, they will not deliver the rated amp hours for very long. I feel that they are like pouring money on a fire. Still just my opinion but you would be far better off to spend time studying and learning how you can build your own LiFePo4 battery from raw cells.

If you are willing to replace the AGMs every five years or so, I am sure they will do okay if you can get the capacity that you need. I would challenge you to get a small AGM and a charge/discharge charger and see if it will get its rating.

 

[snoobler]

I'll likely chose AGM batteries as I have had excellent service life with them in the past when kept at full charge.
<snip>


While the AGM might "last" if you take good care of them, they will not deliver the rated amp hours for very long. I feel that they are like pouring money on a fire. Still just my opinion but you would be far better off to spend time studying and learning how you can build your own LiFePo4 battery from raw cells.

If you are willing to replace the AGMs every five years or so, I am sure they will do okay if you can get the capacity that you need. I would challenge you to get a small AGM and a charge/discharge charger and see if it will get its rating.

LFP only has a price advantage when you talk about $/cycle.

There are two primary battery applications:
1) Cyclic use
2) Standby

1) is for typical daily off-grid use. 2) is for backup or UPS purposes where the number of cycles is VERY low.

For a backup system used for "infrequent outages," i.e., backup or standby systems per the OP. AGM charged/floated at 13.8V can last a decade and retain 60-80% capacity.

For backup/standby systems, LFP looses the value battle unless you PLAN to transition the batteries to a cycling application in the future.

Personally, I wouldn't even consider LFP for this application unless I planned to expand it into something more AND use it in a cycling application. I'd likely rather go with cheaper FLA and take care of them. I'd also revisit the idea that I can only discharge them to 50% because I don't plan to need 1000+ cycles out of them. I'll probably be happy with 300.

 

[TimC]

While the AGM might "last" if you take good care of them, they will not deliver the rated amp hours for very long. I feel that they are like pouring money on a fire. Still just my opinion but you would be far better off to spend time studying and learning how you can build your own LiFePo4 battery from raw cells.

If you are willing to replace the AGMs every five years or so, I am sure they will do okay if you can get the capacity that you need. I would challenge you to get a small AGM and a charge/discharge charger and see if it will get its rating.

Not sure if this applies, but, I have eight 18aHr Mighty Max batteries (12v in parallel) in my camper. They have been used and fully charged for five years. They consistently show voltages of over 13v on the MT50 (Epever). I think what you are saying is the batteries may show over 13v, but, the actual aHr capacity at full is much less than the original 18aHr (which I never verified and don't know how that is accomplished). So, not sure what all that means.

I understand that building a battery bank is doable. I just don't see it happening as the instructional videos I've watched are above my skills/interest and the cost savings is not substantial. So, as I mentioned I probably will stick with AGMs. I appreciate your advice, I just don't want to complicate this any further than my current skill set of buying off the shelf and wiring it all up.

 

[TimC]

You're almost to the starting line.

Bottom line a number, e.g., "I need to be able to provide XkWh/day."

Then decide how many days of operation you want WITHOUT solar charging, or identify that you're okay with providing less solar than needed, but will end 72 hours with 50% of your AGM battery remaining.

Then use link #5 in my signature for your location, panel orientation and tilt. Pick the worst case month where you expect to have to engage your system for the 3 days, and provide that month's solar hours.

Then, answers shall flow forth with the magic of arithmetic.

Thanks Snoobler. I will sit down and figure that all out and see if I can come up with numbers that make sense. Will P makes it sound easy, and I know he is right. I just have to get by that stinkin math problem : )

I'll have more questions later.

 

[snoobler]

Not sure if this applies, but, I have eight 18aHr Mighty Max batteries (12v in parallel) in my camper. They have been used and fully charged for five years. They consistently show voltages of over 13v on the MT50 (Epever). I think what you are saying is the batteries may show over 13v, but, the actual aHr capacity at full is much less than the original 18aHr (which I never verified and don't know how that is accomplished). So, not sure what all that means.

I understand that building a battery bank is doable. I just don't see it happening as the instructional videos I've watched are above my skills/interest and the cost savings is not substantial. So, as I mentioned I probably will stick with AGMs. I appreciate your advice, I just don't want to complicate this any further than my current skill set of buying off the shelf and wiring it all up.

I have those in my 2X 48V scooters. 4 in each.

Keeping them topped off and not cycling prolongs their life.

And voltage is the same for a 10000Ah battery and a 1Ah battery at the same state of charge, so you're correct. Voltage gives no clue as to capacity.

If they are only infrequently used and not depleted to below 50% SoC (about 11.6-11.8V under a 7A/90W load), they likely are in pretty good shape.

If you've left them discharged for months at a time, they're probably suffering.

 

[DThames]

Not sure if this applies, but, I have eight 18aHr Mighty Max batteries (12v in parallel) in my camper. They have been used and fully charged for five years. They consistently show voltages of over 13v on the MT50 (Epever). I think what you are saying is the batteries may show over 13v, but, the actual aHr capacity at full is much less than the original 18aHr (which I never verified and don't know how that is accomplished). So, not sure what all that means.

I understand that building a battery bank is doable. I just don't see it happening as the instructional videos I've watched are above my skills/interest and the cost savings is not substantial. So, as I mentioned I probably will stick with AGMs. I appreciate your advice, I just don't want to complicate this any further than my current skill set of buying off the shelf and wiring it all up.

My limited experience with sealed/gel lead acid is in computer backup supplies which almost never get used. After 2 or 3 years they are no good. It could be that the charging system is punishing them, as they often are bulged. I have just learned to avoid them.

 

[DThames]

Not sure if this applies, but, I have eight 18aHr Mighty Max batteries (12v in parallel) in my camper. They have been used and fully charged for five years. They consistently show voltages of over 13v on the MT50 (Epever). I think what you are saying is the batteries may show over 13v, but, the actual aHr capacity at full is much less than the original 18aHr (which I never verified and don't know how that is accomplished). So, not sure what all that means.

I understand that building a battery bank is doable. I just don't see it happening as the instructional videos I've watched are above my skills/interest and the cost savings is not substantial. So, as I mentioned I probably will stick with AGMs. I appreciate your advice, I just don't want to complicate this any further than my current skill set of buying off the shelf and wiring it all up.

Understood. It seems to be in my nature to go after stuff that require me to learn something. Not everyone is that way and time (to do other stuff) may be more important. Because you can't take lead acid down to full capacity, be sure to include that fact in your bank calculations. Multiply what you need by 150%-200%.

 

[TimC]

Yup. thanks guys. I'm a DIYer myself. But in my 6th decade I am less apt to test my limits too much. Wood working yes. Electronics, not so much
: )

I know this is a rough comparison, but, I'm looking at possibly two 100 ahr SLA/AGM batteries ($174 per) and if they last five years the cost per year is around $70. Then, comparing that to one commercially made (ExpertPower 24V 100Ah Lithium LiFePO4) battery that the mfr suggests will last ten years, it's looking like $150 per year. Rough calc I know, but pretty convincing to me since I don't have the money to build a system that expensive anyway.

Its kinda funny cause several mfr descriptions of LiFePO4 batteries state that AGM/SLA batteries only last three years (not my experience even taking a beating in my vehicles). I think they are being a little disingenuous. Caveat emptor!

As Snoobler suggested I will get some guesstimates of capacity I need and do the math and then throw some questions at the forum.

Tim

 

[DThames]

Yup. thanks guys. I'm a DIYer myself. But in my 6th decade I am less apt to test my limits too much. Wood working yes. Electronics, not so much
: )

I know this is a rough comparison, but, I'm looking at possibly two 100 ahr SLA/AGM batteries ($174 per) and if they last five years the cost per year is around $70. Then, comparing that to one commercially made (ExpertPower 24V 100Ah Lithium LiFePO4) battery that the mfr suggests will last ten years, it's looking like $150 per year. Rough calc I know, but pretty convincing to me since I don't have the money to build a system that expensive anyway.

Its kinda funny cause several mfr descriptions of LiFePO4 batteries state that AGM/SLA batteries only last three years (not my experience even taking a beating in my vehicles). I think they are being a little disingenuous. Caveat emptor!

As Snoobler suggested I will get some guesstimates of capacity I need and do the math and then throw some questions at the forum.

Tim

Take a look at how the lead acid life is tied to the level that they are commonly discharged. If you get large enough batteries so that you rarely discharge a lot, might help extend the life. You can Google and found those life charts.

 

[snoobler]

Take a look at how the lead acid life is tied to the level that they are commonly discharged. If you get large enough batteries so that you rarely discharge a lot, might help extend the life. You can Google and found those life charts.

I want to have a solar system to back up electric power during very infrequent outages (two short outages in five years).

Cycle life isn't the driving factor with 2 cycles every 5 years.

After thinking more about it, I'd get cheap FLA and keep them at 13.2V with a grid AC powered $20 power supply and not rely on the solar to charge them every day as that results in micro-cycles.

 

[DThames]

Cycle life isn't the driving factor with 2 cycles every 5 years.

After thinking more about it, I'd get cheap FLA and keep them at 13.2V with a grid AC powered $20 power supply and not rely on the solar to charge them every day as that results in micro-cycles.

Once upon a time, I walked into the communications room of the security police on our Air National Guard base. There was a large rack with flooded lead acid batteries for emergency radio power. The cases were clear and the top would come off, lifting the plates out of the electrolyte. I thought, now THAT is the way (serviceable) to make a lead acid battery.

 

[snoobler]

That sounds slick!

I have no other reason to love it, but I have a group 24 Interstate FLA in my trailer... can see the water level through the translucent side. Love it.

 

[Hedges]

You hardly even need batteries. Refrigerators can be run while the sun shines.
Some of the hybrid inverters are batteries optional. Of course, with no battery the PV panels and capacitors have to deliver 2000W for an instant to start the motor.
Recent model SMA Sunny Boy grid tie inverters have a "secure power" outlet, 120V up to 2000W, which can be manually enabled during grid failure.
But a 2000W or larger pure sine wave inverter, small battery, battery charger, and 600W or more of PV panels could probably do it.
Put the fridge and freezer on timers so they don't drain battery at night and stagger their start-up in the morning.
Avoiding discharge at night, could put a good size automotive battery into this service, already in your driveway, with backup generator attached.
The trick is making sure PV is always carrying the load.

 

[rin67630]

Then use link #5 in my signature for your location, panel orientation and tilt. Pick the worst case month where you expect to have to engage your system for the 3 days, and provide that month's solar hours.

... and think about the potential divisor if you must consider some shades (trees, own house, other buildings, even mountains in the winter)

 

[rin67630]

You hardly even need batteries. Refrigerators can be run while the sun shines.

...and when the sun does not shine?
On a casted weather day your PV will hardly deliver 10%.
Two casted days in sequence + a power outage and you can throw away your freezer content.

 

[Hedges]

...and when the sun does not shine?
On a casted weather day your PV will hardly deliver 10%.
Two casted days in sequence + a power outage and you can throw away your freezer content.

"two short outages in five years"

"... could put a good size automotive battery into this service, already in your driveway, with backup generator attached."

 

[rin67630]

"two short outages in five years"

"... could put a good size automotive battery into this service, already in your driveway, with backup generator attached."

Which does not mean that a bigger outage cannot happen.
When did your house burn away last time?
A good size automotive battery is ~ 1KWh.
Nominal. Expect realistically 600Wh.
How long will you run your house on that?