kWh required to keep batteries charged?

lasitterlasitter Solar Expert Posts: 56 ✭✭
Take a string of four or eight 48v Rolls 6CS 27P. It takes electricity just to keep them at a state of charge, even if you never call on them to supply power. I'd like to have some idea about how much power that is over the course of a year. And: How much difference would AGM be to keep charged vs flooded? I have a grid tied system and want to know how many panels to add just to generate the needed power that the batteries would consume, waiting to be called into action.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    Several variables here... Typically, for deep cycle lead acid batteries, it is recommended (by MFG) that you have >=10% rate of charge. Some even recommend higher currents.

    Practically speaking, for even a weekend system, 5% is really the minimum rate of charge I would suggest (various Lithium Ion chemistries do not have minimum rates of charge, so you can go less--However, 5%-10% is still a good minimum rate of charge just to have a reasonably useful/low maintenance/minimum genset time system).

    Some other "magic numbers":
    • 0.1% rate of charge (or less)/leakage current for AGM/GEL type batteries typical.
    • 1-2% rate of float charge (leakage current) FLA batteries that are old, near end of life, especially for Forklift/Traction batteries (which have higher leakage current generall)
    • 1%-2% is a good minimum rate of charge for floating a battery bank (no other loads, off season float, battery bank should be >90% state of charge before floating).
    • 1% or less rate of charge for FLA batteries, can probably get away without a charge controller.
    • 2% or greater rate of charge for FLA batteries, need a charge controller to ensure batteries do not over charge/vent/boil out the water (gassing)
    • 2.5% to 5% minimum suggested rate of charge for a flooded cell lead acid deep cycle battery Equalizing Charge.
    • >2% "float charge current" for lead acid batteries--Replace them. The battery bank will probably use lots of water/boil cells dry/run risk of fire/explosion.
    As always, read the battery manual/specifications for a starting point. Monitor battery voltage(s), specific gravity, and distilled water usage (if you have to fill the battery every 1 month or less, probably over charging. If you have to fill the battery less than every 3-6 months, probably need higher charging voltages. -- Note that water usage and charging voltages are very vendor/construction specific).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lasitterlasitter Solar Expert Posts: 56 ✭✭
    Bill: Thanks. Let me turn it around a bit. If you were selecting batteries for one or two power failures per year, and an extended failure (one week maximum) every five to 10 years, what kind of batteries would you get and how would you charge them? I want to go with batteries because I just don't trust generators. More accurately, I just don't trust the people you depend on to service them. Here in Massachusetts, the number of installed Generacs to installers is very large. I have a theory that in the first 24 hours of an outage, something close to 10 percent of these system will fail. Those that don't fail will require an oil change. There is no way that the installer can service all the failing systems at once, even if you have your annual service contract all in order. Batteries and inverters can fail, but I like my odds better.
  • lasitterlasitter Solar Expert Posts: 56 ✭✭
    But that does get me back to a revised question of whether AGM would be a better choice for me, or if an intelligent charge controller with the right flooded batteries would give me more bang for the buck. It seems that AGM would cost less to keep charged, but I don't know what kind of aH AGM could supply vs flooded, and how much extra of one kind of battery you would have to buy to equal the output of another ...
  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    It depends a lot on how much power you need during the outages (and where you live, how much sun, can you install genset+propane tank on your property, etc.).

    For example, if you only need to run a refrigerator, a few lights, etc... A 2kWatt genset and 2-3 gallons of fuel per day.

    Get a 10 kW genset, and you are looking at 10-15+ gallons per day (diesel is the most energy dense fuel, propane is much less dense--If you have natural gas, potentially you have an unlimited fuel source--Assuming your natural gas is not shut down in flooding/earthquake/etc.).

    Batteries will age out in 3-5 years (cheaper batteries, warm climates) to 5-8 years (better batteries, cool climates) to 15+ years (forklift batteries).

    Your electronics can fail earlier, but I would suggest planning on a 10+ year life cycle (i.e., money in the bank).

    For a pure emergency power system, I would highly suggest a genset--Propane will last forever--And if you use propane already (heating, cooking)--You already have the infrastructure.

    With any system, how cold it gets (diesel and propane are harder to start/run vs gasoline in very cold climates) and your actual power/energy usage needs to be nailed down. Conservation, in general, is much less expensive vs building bigger off grid power systems.

    An off grid power system that can give you significant levels of power usage (say 100 kWH per month, or 3.3 kWH per day for fridge, lights, washing machine, well pump, laptop+LED TV) is still not a "small system"... And the ongoing maintenance is a pain (checking water levels, running the system once a month for a day or to and cycle the battery bank, how much sun do you get during winter--more likely ice storms taking out your power???).

    AGM batteries can be good for standby power systems (no maintenance). However, AGM batteries tend to have a couple year shorter life vs equivalent FLA batteries.

    Gensets and fuel storage tend to be cheap. And as long as the genset is not running, no fuel costs (batteries need replacing ever XX years used or not).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lasitterlasitter Solar Expert Posts: 56 ✭✭
    Bill: Thanks. Don't batteries age partially based upon how often they are cycled? And how deeply? Do you think batteries fail just because they get older, even if never discharged to 50%? And discharged just a few times per year? And more about gensets: I have natural gas, but generators that could run for nearly a week without service are very expensive. Most cheap gensets are running at 3600 rpm, and you have to stop them and check/add oil every eight hours. Few can run for over 24 hours without an oil change. I just don't see myself getting out in 1-2' of snow and super cold temperatures to do that. A battery bank, on the other hand, would live at 55-65 degree ambient temperatures in the basement year round. Thanks for the info about AGM life expectancy vs flooded. For me, the probability that the system will respond when called upon is a critical factor.
  • EstragonEstragon Registered Users Posts: 3,965 ✭✭✭✭✭
    Batteries do die of old age, even with little or no use.  Telecom backup batteries, for example, are typically swapped out after some arbitrary period of time to ensure reliablility.

    Prime service generators are definitely more expensive than standby.  Mine is a 4kw 1800rpm Onan diesel from the early 80s.  It still runs fine, but if I had to replace it would run ~$10,000+.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    There are actually many ways batteries die... And much of that is usage and battery chemistry specific.
    • Aging... Batteries age and eventually die. Does not matter if they are cycled or not. The standard engineering rule of thumb is a factor of 2 (or 1/2) change in life for every 10C change in temperature. Run your 25C battery bank at 45C, it will last 1/2*1/2=1/4 as long (age 4x faster). If you store/use the battery at 5C, the battery will last 4x longer (2x2). We see this alot in Canada, cabins that are in deep freeze for 3-6 months of the year. Those batteries can easily last 2x longer.
    • Cycling... There are batteries designed for "float service" (sometimes as little as 4 cycles and 2 year life--such as UPS systems with generator backup... To batteries with 40 year life and x,000 of cycles, but they can only be drawn down to 85% state of charge for that long life). Standard deep cycle lead acid batteries generally like to be cycled (say to 75% SoC or a bit less) once a month for best life.
    • Charging... When you charge over ~80-90% State of Charge, the batteries gas (and generate internal heat). Can errode plates, O2 corrosion on positive plates/grids, lose enough water to expose plates (bad news for plates), and mix the electrolyte (a good thing, recommended for "tall" FLA batteries).
    • Charging for AGM... AGM batteries typically have a noble metal (like palatium) that is used to recombine the Hydrogen+Oxygen gasses back into water (basically low temperature combustion). There is a life for the catalysts (like XXX AH of "equalization current"). When the catalyst "wears out", the AGM batteries are on a road to death (loss of water).
    • Charging for GEL... Gel batteries have electrolyte plus silicates that make a "jelly" like electrolyte. Great for float service and short/high discharge current. In the US, most GEL batteries are limited to C/20 (5%) rate of charge maximum, or the batteries will form gas pockets in the Gel--And permanently lose capacity (for solar power systems, >10% rate of charge is usually required for systems that are used during poor weather/winter to recharge quickly.
    And there are other battery chemistries that have their own pluses and minuses... Nickel Iron batteries are pretty rugged, but you need to replace the electrolyte every ~7 years (carbon dioxide "poisoning").

    For off grid home power, the LiFePO4 (lithium iron phosphate) batteries are looking very interesting. Does not have near the drawbacks of Lead Acid batteries. Downside is expense and (usually) the use of a battery monitoring system (to ensure that the cells stay within the recommended voltage range during discharge and recharging).

    LiFePO4 batteries seem to be getting closer to "off the shelf" support with solar charging/inverter hardware. Downside tends to be the expense (but if batteries have a much longer life than FLA/AGM, the cost per kWH of cycled power gets down, or possibly below, FLA/AGM batteries).

    Batteries really only justify (cost / benefit ratio) their expense if you are cycling them (floating them, cycling them, eventually they will fail).

    As Estragon gives in his example... His genset has lasted >35 years. You will not get that long of life with batteries and typical power electronics. High temperatures and thermal cycling is very hard on any electronics.

    Some folks here have gotten some pretty nice gensets (different fuel types) from RV wrecking yards.

    Even a Honda eu2000i can last 2,000 to 6,000 hours with just standard maintenance. If you have 5 days of outages a year:
    • 2,000 hours 1/24 hours per day * 1/5 days = 16.7 years of life
    There are propane regulator kits for the smaller Hondas... One thing I have looked for an never found is a sump/oil filter system for the small Hondas. It would be nice to not have to shut down the genset every 24 hours to change oil (the small Hondas have an internal fuel pump--Place a sealed fuel cap with a hose leading a 5 gallon fuel jug/outboard fuel tank, and you can run days between refueling).

    Many of the solutions really depend on your personal energy needs (running in Mass winter, no sun, heat pump+resistive heater+wood stove+fan, etc.)... Cost wise, a genset (or even a pair of gensets) + fuel storage (or natural gas connection) is going to be much less expensive than any "high reliability battery" system.

    Once you have a handle on your daily power needs (by season), we can give more exacting answers (vs just a lot of hand waving). But the typical off grid solar power system really only justifies its costs if it is generating useful power. Running 5 days out of the year--Not so much.

    You can do things like they do in computer centers. Design a "small" battery system that will run for 30 minutes to an hour--Gives time to fire up the genset.

    I am not a battery engineer... There are people here that can give you much more detailed answers that would work for your system/needs (FLA, AGM, GEL, Li Ion, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lasitterlasitter Solar Expert Posts: 56 ✭✭
    BB. said:
    There are actually many ways batteries die... And much of that is usage and battery chemistry specific.

    CL: Great info. Now I just need advice on which of the batteries discussed below would be the best fit for my scenario.
    CL: Batteries die. So do people. If I can come up with a system that lasts until I'm 70 (eight years), I'll be very satisfied.
    CL: It sounds like I need something with occasional modest discharge and then maintain about a 90% SOC?
    CL: I'll be storing / using my battery bank at 12-14C.
    CL: One crucial requirement ... I need a system that can be sold and serviced by a vendor in Western Mass. That pretty much eliminates anything that starts of with "first you go digging thru a junkyard for XYZ"
    CL: I'd love to have a Perkins Diesel genset. I have 660 gallons of available fuel storage sitting next to me right now, as we converted our boiler to natural gas. But I can't find anyone near here that has any interest in selling me / servicing something like that.
    CL: Why would I want a pair of gensets?
    CL: I have no interest in a Honda / portable generator connecting via a manual transfer switch.
    CL: Everything fails, eventually. What I'm really purchasing with this system is peace of mind. I think for sure that it's worth $1,000 per year to me to have this peace of mind, and I think $1,000 per year would certainly keep me in batteries for a properly configured and maintained system.

    BB: Many of the solutions really depend on your personal energy needs (running in Mass winter, no sun, heat pump+resistive heater+wood stove+fan, etc.)... Cost wise, a genset (or even a pair of gensets) + fuel storage (or natural gas connection) is going to be much less expensive than any "high reliability battery" system.

    BB: Once you have a handle on your daily power needs (by season), we can give more exacting answers (vs just a lot of hand waving). But the typical off grid solar power system really only justifies its costs if it is generating useful power. Running 5 days out of the year--Not so much.

    CL: Again your location, experience and anxiety level are far different from mine. What's "worth it" to you is almost never going to be the same for me.

    About my power needs ... I need the well pump to run year round. If the power goes out  I can heat the entire house with our natural gas fireplace. Our boiler and circulators would need to be powered to take care of the hot water. I have more than average consumption from computer equipment. I would anticipate 2-3kWh continuous draw and surge for the well pumps.
  • lasitterlasitter Solar Expert Posts: 56 ✭✭
    I've had several people come out here and estimate installing generators. They all want to sell Generacs. That's basically all they offer.

    My own experience with generators? My sister lives in NC, and after hurricane Fran and an ice storm left her without power for a week each time, I bough a generator for her. She needed it to do her bit for the family business. It's been dead (battery?) for years. So when Florence came thru with another week long outage, it was just an expensive lawn ornament.

    For myself, I'd like to fail in a different way next time.
  • EstragonEstragon Registered Users Posts: 3,965 ✭✭✭✭✭
    https://locator.cummins.com/GlobalLocator/homeScreen.action?languageCode=en_US&regionId=6&countryCode=122&kmMiles=M&request_locale=en_US

    Onan-Cummins has dealers all over the place, likely one near you.  I'm sure they could service a unit out of a wrecked RV or whatever.  

    Once installed, there is a bit of regular maintenance needed, but easily done by a reasonably handy person.  

    Any generator should be run ("exercised") regularly to keep oil distrubuted, start battery charged, etc., and to catch any problems before you really need it.  You also need a plan to manage fuel.  Ideally you have a truck or whatever you can use aging fuel in so you can replace genny fuel periodically (eg annually for gasoline, 2-3 years for diesel).
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    edited October 2018 #12
    "I would anticipate 2-3kWh continuous draw and surge for the well pumps"....

    What do you mean by this? kWH is an "amount" of energy (like you have 10 gallons of gasoline).

    Watts is a rate ( like using 1 gallon of gasoline per hour).

    A 2-3 kWH meaning 2-3 kWatt continuous draw?
    • 3 kW * 24 hours per day * 30 days per month = 3,060 kWH per month
    • 3 kWH per day * 30 days per month = 100 kWH per month
    Roughly, a typical north American home uses around 500 to 1,000 kWH per month...

    I suggest 3.3 kWH per day (100 kWH per month) as a good aim point for an off grid power system. Large enough to run a full size fridge, LED lights, well pump (not a large pump), washing machine, laptop+LED TV (lots of conservation.

    Some math for a 3.3 kWH per day system using rule of thumb (relatively conservative system) design:

    • 3,300 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge (longer battery life) * 1/24 volts = 647 AH @ 24 volt battery bank
    https://www.solar-electric.com/fullriver-dc220-6-agm-sealed-battery-6v-220ah.html (6 volt @ 220 AH AGM battery)

    The bank would be 3 parallel strings of 4x6 volt 220 AH batteries for 12 batteries * ~$327 each = $3,924 (you probably should find a local vendor, Arizona is a bit far--Many vendors will drop ship from local wholesaler).

    To size the battery bank, two calculations. One based on capacity of battery bank, the second based on hours of sun and your loads... (note that charging can range from 5% to 13%+ rate of charge. 10%+ is good for full time off grid usage)
    • 647 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,437 Watt array nominal
    And then there is sizing based on hours of sun per day and your loads. Fixed array:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Chicopee
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 48° angle from vertical:
    (For best year-round performance)

    JanFebMarAprMayJun
    3.10
     
    3.87
     
    4.39
     
    4.48
     
    4.69
     
    4.86
     
    JulAugSepOctNovDec
    5.01
     
    4.91
     
    4.61
     
    3.94
     
    2.93
     
    2.77
     

    Note, you can go for 67 degree tilt (from horizontal) to shed snow better--If that is an issue and get a little bit more harvest in winter.
    • 3,300 WH per day * 1/0.61 AGM Battery off grid system eff * 1/2.77 hours of sun for December = 1,953 Watt array minimum (December long term average)
    So, for this size system (3.3 kWH per day), a 1,851 to 2,437 Watt array would be nice (larger array, less genset usage during winter).

    3,300 WH per day / 24 hours per day = 137.5 Watt average load (not even 2x 100 Watt light  bulbs).

    Say you run 10 hours per day, that is a 330 Watt load.

    A 647 AH @ 24 volt battery bank would support a 1,500 to 3,200 Watt AC inverter nicely (with ~2x surge current support). That is based on a flooded cell battery bank... An AGM battery bank can easily support >2x surge current.

    The above is just a starting point for the discussion... If you need 10 kWH per day, then 3x larger system and ~3x bigger costs. Note the battery selection/configuration is system size dependent, and you may choose some nicel longer life traction cells (FLA), or something else (each battery type/chemistry has its own weaknesses and strengths).

    I would tend to avoid the generic residential Generac generators (just for short outages generally--Not a long life set--In my humble opinion).

    In my area, last we lost power for days at a time was from a big storm in the early 1960s... Otherwise most of the outages have been a couple hours with several years in between. Our "big" one is an earthquake. And quakes (in our region) tend to have relatively small pockets of deviation and large areas where nothing bad has happened. Nothing like the major storms that you have in the east/south which can wipe out/flood many hundreds of square miles at a time.

    As you say, personal power needs are a set of highly personal choices. No one size fits all.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lasitterlasitter Solar Expert Posts: 56 ✭✭
    I have a server computer, pulling 200w idle and 400w w/monitor. 1.5 horse well pump on 2-pole 20 ... a little bigger than average? Kitchen circuits with gas stove/range. New efficient fridge. I'd like to be able to open the garage doors ... bedroom circuits to support operation of ceiling fans in the summer in absence of A/C. Before we got the solar (did 13 megawatts first year) we used about 28000 watt-hours per day when not heating or cooling. Hope this helps.
  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    The 24x7 loads (like a computer server) ends up being a pretty large load (vs a well pump or microwave, typically just 10's of minutes per day usage)...
    • 400 Watts * 24 hours per day = 9,600 Watt*Hours = 9.6 kWH per day (yes, using 400 Watts to make a point here)
    • 9.6 kWH computer / 28 kWH whole house = 0.34 or 1/3rd of your home's non-HVAC energy budget
    If the server is hosting--Will the data lines still be active during a power failure? Many times, the standard cable and internet equipment has zero to a few hours of backup (battery, fuel). Do you need to run the server during outages? Is the server needed for household tasks (home automation?). Could it be done with a laptop computer (~20-50 Watts, vs your present 200+ Watts)? Laptop comes with a built in UPS (battery) for short term outages/ride through failover switching.

    When are you likely to lose power? Winter or Summer (or anytime of the year)?

    One way to reduce some of your long term costs is to use a hybrid inverter-charger. It can either GT and feed energy back to the grid (if allowed with your utility), or just to supply power locally to the house (depending on hardware and configuration, some inverter-chargers seem to draw upwards of 2 amps from the utility even when solar+battery power is available).

    Or run your critical loads "off grid" and use the utility like your local generator (during bad weather/heavy loads)... Yes, using it this way will cycle your batteries and probably reduce their life somewhat.

    If you want a 28 kWH per day system, it would be almost 10x larger than the one I suggested above... Meaning that you are looking at $30,000 to $40,000 in AGM batteries (other batteries would cost more or less--Depending on what you specify). The total solar system cost (excluding genset, possibly excluding labor) is still getting somewhat close to $100,000 for your upfront system costs... Replace batteries every ~8 years, electronics every 10+ years, and you are looking at an amortized over 10+ years a "yearly" maintenance costs of something like $5,000 per year...

    I can size the system and give you some example costs... Or I can size the system and you can find equipment that would meet your needs (I am not in the solar business--And I do not get paid anything for moderating/posting here--All of us here are volunteering our time).

    I don't just want to throw numbers at you... There have been some posts here about "cost is no object" off grid solar power systems... When you get into the larger power systems (batteries, inverters, charge controller, solar arrays, etc.)--That sort of gets you into the professionally engineered and installed size of systems--Which adds costs--And does not always give you a well working system anyway (a few people have gone through multiple installers trying to get their systems working right).

    I would also suggest that large off grid/hybrid solar power systems (many with backup gensets) are not trivial systems to install and maintain. They can have some pretty complex parameters to setup and log to ensure that all is working well--And significant amounts of debugging if something goes wrong.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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