Grid Tie Battery Bank - AGM sealed or deep cell?

[MrEnergyCzar]

I know exactly how much power I use over a 4 day window excluding my plug-in Volt (40 KWH). I want a battery bank to cover me for this despite having a 5KW solar array back feeding the batteries during a power outage. This will cover me for more than 4 days. Since I'd be tied to the grid, would I get AGM sealed batteries? I don't want the maintenance. If the power went out 10 days per year, what kind of life could I expect from them? I have run numbers through the battery formulas but am confused regarding total amp hours recommended after using the formula and then taking that number and finding the proper battery. 48V bank. What is the best battery bank, brand, and specific size for what I need? I know the Rolls Surrettes are top batteries but can't figure out which ones and how many for my purposes. (I wouldn't be charging my Volt when the power goes out, as the draw would be too much and probably kill the batteries faster)

Thanks
MrEnergyCzar

[Sunking]

It is real simple.

1. First determine your daily Kwh usage. Say for example 5 Kwh.
2. Determine how many day of autonomy you want, then double that number so as to not ever discharge your battery more then 50%. In your example 4 days x 2 = 8 days.
3. Multiply Autonomy by Daily load to get total battery capacity. In your example 5000 wh x 8 = 40,000 wh or 40 Kwh
4. To find battery AH capacity = Watt Hours / Battery Voltage. So in this example 40,000 wh / 48 volts = 833 AH @ 48 volts.
5. Go broke buying batteries.

If using quality AGM batteries you can get up to around 2000 cycles or 3 to 5 years of service whichever comes first. A quality AGM battery manufacture are Concorde PVX Sun Extender series. Rolls does make AGM batteries but they are not suitable for RE system. For Rolls 4000 and 5000 series are RE batteries which are FLA.

[MrEnergyCzar]

It is real simple.

1. First determine your daily Kwh usage. Say for example 5 Kwh.
2. Determine how many day of autonomy you want, then double that number so as to not ever discharge your battery more then 50%. In your example 4 days x 2 = 8 days.
3. Multiply Autonomy by Daily load to get total battery capacity. In your example 5000 wh x 8 = 40,000 wh or 40 Kwh
4. To find battery AH capacity = Watt Hours / Battery Voltage. So in this example 40,000 wh / 48 volts = 833 AH @ 48 volts.
5. Go broke buying batteries.

If using quality AGM batteries you can get up to around 2000 cycles or 3 to 5 years of service whichever comes first. A quality AGM battery manufacture are Concorde PVX Sun Extender series. Rolls does make AGM batteries but they are not suitable for RE system. For Rolls 4000 and 5000 series are RE batteries which are FLA.

Thanks. So if the batteries are always topped off, except when the grid went down for 10 days per year, they would only last 3-5 years? How can that be if they are barely being discharged (50 discharges max over 5 years)?

MrEnergyCzar

[Sunking]

Because AGM batteries are very sensitive to positive plate growth when setting idle in float service, and not being cycled. Nor can they have an EQ charge applied to minimize the plate growth. AGM is damed if you use them, damned if you do not. In addition if not cycled when new the plates will never form correctly and reach their rated capacity. Batteries are just extremely bad news both for the pocket book and environment. You are not doing anyone or anythiong any favors using a battery system. Only person who will benefit is the one selling them to you and laughing all the way to the bank with your cash.

[Mike90250]

That's why, for just a couple days of backup, a modest 2-4KW genset & properly stored fuel, is the best solution. You can charge a 12V deep cycle battery, and use a small inverter to give you a couple lights all night, without the whine of the genset.

[MrEnergyCzar]

Because AGM batteries are very sensitive to positive plate growth when setting idle in float service, and not being cycled. Nor can they have an EQ charge applied to minimize the plate growth. AGM is damed if you use them, damned if you do not. In addition if not cycled when new the plates will never form correctly and reach their rated capacity. Batteries are just extremely bad news both for the pocket book and environment. You are not doing anyone or anythiong any favors using a battery system. Only person who will benefit is the one selling them to you and laughing all the way to the bank with your cash.

That explains the short life of AGM's. Looks like I'd have to get lead acid batteries then so they last longer and deal with the maintenance. I don't want any type of generator, it's against zoning.

MrEnergyCzar

[russ]

That explains the short life of AGM's. Looks like I'd have to get lead acid batteries then so they last longer and deal with the maintenance. I don't want any type of generator, it's against zoning.

MrEnergyCzar

I have seen many generators that you can not even tell if they are running without looking closely - are they against zoning?

[SunEagle]

That explains the short life of AGM's. Looks like I'd have to get lead acid batteries then so they last longer and deal with the maintenance. I don't want any type of generator, it's against zoning.

MrEnergyCzar

Really. An emergency generator is against zoning? I don't believe even a Home Owners Association can keep that from happening. A home owned backup power supply raises property values as well as reduces insurance rates. An HOA would be liable if a homeowner who needs emergency power for a medical issue couldn't have one because of their ruling.

My HOA tried to ban satellite dishes in the front yard. The rule was written back in the 80's when they were 6 feet in diameter and "ugly". Even though the dish is now only 1 foot in diameter they still try to enforce the rule. I told them they can't due to an FCC ruling saying no one is allowed to block or inhibit communication signals. The HOA backed down.

[MrEnergyCzar]

I have seen many generators that you can not even tell if they are running without looking closely - are they against zoning?

Everything is against zoning. I fought zoning for a year to get the pole mount array in the yard. Where I live, you need a variance for everything because the zoning laws are designed for big properties but mine is like a quarter acre. We're talking 100 foot set-backs from property lines, my property probably isn't even 100 feet wide. They said the solar will denigrate the community, I was able to lobby neighbors and others to get the 4 out of 5 board votes needed. They knew I would probably take legal action if denied since there were no specific solar zoning rules. It was counted as a swimming pool, they calculate % of total lot coverage etc... Some neighbors do not like it but I was able to get full permits in place right when it was completed. There would be near zero chance of getting a permit for a real generator, transfer switch or slab etc. The power has been knocked out for 3 separate weeks the past 18 months. The array was destroyed by a neighbors tree during one of them, I have a video of that but I won't post here obviously. About the only thing I could do is have a small 70db honda 1,000 watt generator that's quiet and portable, not ideal. Had I known about the voltage needing to be changed for a battery bank, I probably should of had that done when the replacement array was built.... lesson learned. Could of had the whole battery bank done in one swoop....

MrEnergyCzar

[Mike90250]

That explains the short life of AGM's. Looks like I'd have to get lead acid batteries then so they last longer and deal with the maintenance. I don't want any type of generator, it's against zoning.

MrEnergyCzar

Uh, lead acid batteries include:
GEL
AGM &
FLOODED types. So what you are looking at would be Flooded Lead Acid batteries, not AGM lead acid.

Without a generator, it will be impossible to save your batteries, if the grid fails & weather is bad, and you are not able to recharge the batteries. I have 3 backup generators, and 2 different chargers. Preserving the expensive batteries is paramount.

[MrEnergyCzar]

Uh, lead acid batteries include:
GEL
AGM &
FLOODED types. So what you are looking at would be Flooded Lead Acid batteries, not AGM lead acid.

Without a generator, it will be impossible to save your batteries, if the grid fails & weather is bad, and you are not able to recharge the batteries. I have 3 backup generators, and 2 different chargers. Preserving the expensive batteries is paramount.

Yes, the flooded ones, not familiar with the lingo.

Bear with me here with my questions, If you had a grid tied "small solar farm" that was say, 50KW, and your home only needed a 2 KW array, and the battery bank was large, 10 days autonomy, couldn't in theory you go without a generator in that scenario? Maybe have a small micro wind turbine to plug-in if it snows for a month straight? There must be someone out there that oversized everything and sells back solar. A bit extreme but wondering if it can be done. If I build a homestead from the ground-up, that's what I'd want to do. Of course, in a more normal battery bank scenario, couldn't I just have the battery bank shut-off at 40% DOD until the power comes back on or some other safe level that would require weeks without power before the batter bank degrades?

Final question. Assuming no lower than 50% DOD is ideal for battery life, if you had the battery bank shut off at 35% DOD, how long will it take to drop to 50%, assume winter.

MrEnergyCzar

[russ]

Do you realize the cost of what you are proposing? Even Bill Gates would blink at that.

[Sunking]

Bear with me here with my questions, If you had a grid tied "small solar farm" that was say, 50KW, and your home only needed a 2 KW array, and the battery bank was large, 10 days autonomy, couldn't in theory you go without a generator in that scenario?

OK Deep Cycle Flooded Lead Acid Batteries have a rather small window of charge current needed to keep them maintained properly. The minimum recommended charge current is C/12, and maximum is C/8 where C equal the battery 20 hour Amp Hour rating. For example if the battery is rated 100 AH the C/12 rating is 100 AH/ 12 H = 8.3 amps, and 100 AH/8 H = 12.5 amps.

The minimum charge current although not set in stone is because of what is known as stratification in flooded lead acid batteries where the heavier acid settles at the bottom of the jar and the water on top. It takes at least C/12 charge current to cause bubbling and agitation to stir the mixture up and prevent agitation. One way around that is to have a generator with a AC charger of sufficient capacity to apply the monthly Equalization Charge required for FLA batteries. A solar system is not capable of supplying an EQ charge because the EQ charge can take up to 24 hours.

The maximum C/8 charge limit is to prent the electrolyte from boiling over, gassing the batteries, and warping the plates.

Normally this is not a problem with a properly designed system with 5 day Autonomy, and where winter minimum Sun hour are 3+ or more hours, or summer above 6 hours. For example if you live in KCY with 3.3 Sun Hours in winter, need 1 Kwh/day on a 12 volt battery with 5 day autonomy. Battery required is 12 volts @ 400 AH. Panel wattage required with MPPT controller = 450 watts, Charge Current = 37.5 amps or a C/10 which is spot on perfect. However if you increased the Autonomy to 10 days like you suggest is a 800 AH battery and with a 450 watt panel generating 37.5 amps is only a C/20 charge current. Not going to work out and the work around is now you need at least a 850 watt solar panel and a very expensive 80 amp charge controller to maintain the batteries.

If you think it is hard to pull a permit with a generator, how hard do you think it will be to pull a permit with 2000 pounds of extremely toxic batteries on your property that will take a EPA permit and yearly inspections from your local fire department to inspect your spill containment system and check for leaks. You would be require to put one of those funny looking diamond shaped HAZ-MAT Signs with the 3 colored diamonds with number in them at your property entrance to warn people and First Responders there lives are in danger upon entering. Bet that will go over real good with the HOA.

[MrEnergyCzar]

I would only do that proposed set-up on large land in a less regulated state than CT. Can't you use the Grid each month to equalize the batteries? If the grid happened to be down that day you wanted to equalize, how long is the window of time to equalize again? One day, One week?

Thanks,
MrEnergyCzar

[Sunking]

Can't you use the Grid each month to equalize the batteries?

Certainly but if you do that then you have absolutely no reason to have solar. You would just buy an appropriate sized AC powered battery charger.

If you were my client I would advise you to install Generac 6 Kva EcoGen right next to your Heat Pump to power your whole house. At 52 db noise level you can sleep with it in the same room with you.