agm battery questions

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lamplight
lamplight Solar Expert Posts: 368 ✭✭✭✭✭✭
Hi folks,

ive abused the heck out of my t105's and while ive gotten over 5 years out of them (which is a miracle itself) theyre showing signs of damage and im looking strongly at AGMS. or a couple large wet cell surrettes. trying to decide how big i can go with the bank size. obviously bigger with agm. Its hard to say as i dont have all the variables needed (explained below) but i do need to get something. ive got 675ah now and charging with 700w of PV. I did ok charging that bank on 525w of PV last winter (occasionally needing supplemental charge from AC when we'd get 7 days of gray). Now I have the array higher (a LITTLE less shade) , and have added another panel for 700w. to compound winter's ~50% reduction in sun, i have some partial midday shading in winter, so i want to be conservative on bank size. but i want it as large as possible. so i plan to rate the bank size slightly less than what id get out of a 4 sun hour day.

I get 450-500w out of the array in the heat (depending) , im hoping for around 550-600w in winter sun x4 hrs=2400wh. so looking at maybe 1000 AH bank. do these numbers sound about right? I am looking at wet cell large batteries too, but really think ill go with agms for the extra charging efficiency : looking, now anyhow , at either 848ah or 1020ah from concord. (looking at bigger batteries, dont want to get 8-10 batteries). i know this is a big spread but at the battery size i want thats the jump. my logic on being a little too big is i may have to give some supplemental charge in winter. i havent done this stuff in 5 years and math is NOT my best subject so thought id checking with folks here. any thoughts on my plans and bank size ideas?

my main load in winter is simple: a web server thats about 11w 24x7. with significant additional loads as the sun allows, mostly spring, fall & summer (home office, kitchen lighting, window ac in summer, fridge, etc). basically i dont run anything else real dark winter weather.

anyhow i have some specific questions related to agms :

1) ive read that agms have charge losses as low as 4%. is that a number i can work with and that people agree with?

2)ive read here about 5% being minimum charge current for wet cells. whats the lowest percentage of charge current I can consider for agms? (ie: i want a bank as large as possible with as much standby days as I can get).

3) whats your favorite brand agm, sun extender from concord seems to be pretty popular.

thanks!

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: agm battery questions

    you are confusing me to a point with all of that stuff, but for your questions, i'll put my 2 cents in.
    1> i think that's a safe number and i've seen 2% i believe for sunxtenders.
    2> you should still go with at least 5%, but they can handle higher percentages if you should want to. the extra efficiency may gain you up to a half of a %, but it still will take that much longer to charge if you do.
    3> (your 2nd 2) sunxtenders are good, but there are other agms that are good too. the mk/dekas and c&d just to name a couple. it's your choice as to which one you may prefer. they do cost more and you don't want these abused so if you think there's a chance you might abuse them then stay with fla types as there are many good ones to choose from there too.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: agm battery questions

    After playing around with the numbers in the thread:
    Time to Question the 3 day Rule ?

    I have come to wonder if it works out (roughly) that buying 2x-4x the batteries will let them last 2x-4x as long... Yes--deep cycling does "wear out the batteries" faster. But--if you, for example, deep cycle 50% 1 day for 500 cycles, vs shallow cycle 12.5% for 4 days for 2,000 cycles (4x as much battery storage)--the end result is still the same $$$ battery per year usage.

    From working the numbers, all you have done is buy (for example) 4 batteries which will last 4 years, 8 batteries for 8 years, or buy 16 batteries which will last 16 years (all things being equal, which they rarely are). You have made the choice to buy all of your batteries up front, or spread the purchase out over the next decade or so...

    So--the actual sizing of batteries gets back to your needs--how long do you want to go between external charging cycles ("cloudy days") or other specialized needs (say genset and battery bank, and no solar panels).

    There are some very heavy duty batteries (forklift type, etc.) that claim 20 year life... And other batteries that can get out to 40 year life if very shallow cycles (telecom where you can only draw down 10-15% or so of their capacity for long life).

    Or, you can go with "cheap/good quality" flooded cell batteries, abuse the heck out of them, and replace them again in 5 years. Then use the "extra cash" to build out the rest of your needs (more solar panels, upgrade to a battery monitor, backup genset, etc.).

    Really depends on what your plans for the future are... Stay on grid with emergency backup. Grid Tied. Moving off of the Grid. End Times or whatever...

    In my area, my ideal system would probably be a Grid Tie Hybrid system. A Xantrex XW Inverter+XW Charge Controller(s)+3.5 kWatt of solar panels (already have them with a GT 3kW inverter)+400 Amp*Hours of 48 Volt Battery bank (at least initially).

    Keep battery bank (relatively small) and a good backup up genset for winter power failures (if they ever come). The "small" battery bank would allow me to put the cash into the balance of system (new inverter, charge controllers, possibly some sort of backup genset other than the Honda eu2000i) -- and keeps my maintenance costs small (small battery bank, lower losses and "cheaper" to replace). Hybrid/GT inverter keeps the batteries from cycling--so less losses there too.

    That is 19.2 kWhrs of storage--and works out to 1.5-3 days of backup power (with minor-major conservation in my home) down to 50% discharge.

    More than enough storage for those what the heck power failures/rolling blackouts (keep the fridge and freezer running) with a normal life-style... And allows time to really roll back the power usage and have genset backup for when bad times hit (large storm--winter when we can go 1kWhr per day for a week instead of 10-18--or earthquake which can hit in any weather).

    And still have Grid Tie for "optimum" power storage (as long as we have 1 year net metering). Which keeps us happy 99+% of the time.

    Keeping the battery bank "small" with a hybrid inverter system--I might then look at AGM to reduce my battery maintenance to near zero (no monthly watering, little cleaning needed). All advantages for my "install and forget" whole house UPS+Solar system.

    Although, the costs of those Trojan L16 is pretty nice when you have to buy 8 or 16 at at time...

    And, I keep coming back to my original decision--GT Inverter + Solar, and a small backup generator with enough fuel for a couple weeks of light usage.

    -Bill

    PS: I should add--that as you purchase more/larger batteries for longer life--the expected life for good pay back ($$$/year) exceeds the warranty of the battery (7-10 year for "good" batteries)--placing more risk on you that the batteries will last 10-20 years.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
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    Re: agm battery questions

    I have AGM, I would never spend a penny on wet cell ever again, they are in completely different leagues performance and maintenance wise.

    Go to your local battery supplier, you should be able to find house brand AGM's for not to much more than typical wet cells, I purchased 24, 6V, 220ah AGM's ( 3 strings of 8 ) and that what is on my XW-6048 ... the battery's require no maintenance, work well for Gridtie and the times I have cycled them, have had no issues ( run then down to 50-60% DOD so I can run tests the next day without inverter ripple load )

    The charging efficiency of AGM vs wet cell is 20-25% difference, that's allot of PV to make up for that difference.
  • lamplight
    lamplight Solar Expert Posts: 368 ✭✭✭✭✭✭
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    Re: agm battery questions

    thank you guys , plenty to think about. i appreciate you got what i was asking in general and specifically.

    still doing battery research and will probably be back with more questions. thanks neil , my dealer has c&d on his site , but not the larger batteries, ill look into them too. i do recall deka had inferior warranties from the last discussion i read about agms, but im still looking into this all and gaining some info.

    wet vs agm is another whole issue in addition to the bank sizing analysis you did (that thread was very informative thanks for doing it bill) - but still , the extra efficiency of agm is VERY appealing.


    re my ideal system, i have it, : i have the gridtie (except id like to be a larger someday) and i had this existing offgrid system even though we have reliable grid. i just like to use the offgrid system as much as possible, its fun, reduces the grid usage and is nice to know we have some backup power if needed for emergency.

    the 3 day rule discussion definitely simplified the issues im considering , which somehow seems innumerable when figuring all this stuff. its probably that im not sticking to some general rules as you guys often do, but i keep thinking of all the real world variabilities, thus confusing myself..
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: agm battery questions

    The 3-day rule (really 6x daily usage when 50% max discharge is included) encompasses so many issues (charging, discharging, max cycling, sulfate hardening, equalization, stirring of electrolytes, cost of solar panels vs balance of system costs, etc.)--that you really need good reasons to not follow the rough rule of thumb--such as when a bank will be 99% of the time used as a UPS against a grid connected system and you can justify a smaller bank just for emergency use.

    And good quality AGMs have so little maintenance requirements and relatively high efficiencies/low losses--they are close to the perfect battery (excluding the costs for perfection).

    Which ever you decide--try to swing justifying a good battery monitor for your system (almost 100% required for AGMs).

    In the end--like life--there is probably no one correct/optimum answer; but there are lots of bad ones. :roll:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: agm battery questions

    sg,
    what make and model agms did you get? if they are just slightly higher in cost than the fla type this would be a great option for matt and a good buy too.
  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
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    Re: agm battery questions

    Power Sonic PS-62000B

    I paid 150 for the first 16, when back about 4 months later and paid 205 each ...

    My point was find a battery store local and see what they have in stock. At the time I bought these they has a similar battery, but 12V 100ah AGM's, never used for 25 bucks a battery ( old stock ) ... I almost both them but it would have been to many strings for the XW

    Local stores always have something that you won't find online and chances are you can find a good deal
  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
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    Re: agm battery questions

    I just purchased four East Penn (Deka/MK) 8AGC2s (6V 200AH @ C100) from a local dealer for $180 each. I asked the dealer for a spec sheet but they had none. I had already gotten some specs from East Penn's website but was looking for more detailed information regarding three stage charge set points and temp compensation. I fired off an email to the mfg and, lo and behold, the next I received a reply with precisely the information I had requested. I'm very pleased.

    Craig
  • Brock
    Brock Solar Expert Posts: 639 ✭✭✭✭
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    Re: agm battery questions

    I have eight Deka 8A8D's, basically 500A at 48vdc and I will not go back to flooded again. I am still amazed by the charge loss. I am still very close to 99% in and out after 3.5 years of cycling them between 10-25% a day.

    The no fumes (H2) to worry about, the adding water, and other testing is all gone, just a clean dry battery, worth every penny.

    I also would find a local dealer and see what they have in stock or what they regularly carry and research from there.

    As far as size I would agree with Bill, it's how often you want to change them out as to how deeply they would get discharged.
    3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • nigtomdaw
    nigtomdaw Solar Expert Posts: 705 ✭✭
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    Re: agm battery questions

    What about cost versus life, Im in to my 4th year soon on a set of S/H traction FLA batteries, probably 10/14 yrs old in reality I havent made many mistakes with them but when these fail I intend to replace with Rolls FLA as an expected 20 yr life expectancy with a knowledgable owner . They will see me kick the bucket, I fit and like AGMs for customers and part timers (holiday home owners ) but for cost and long life with a little bit of maintainance aka effort and I mean a little bit of maintainance FLA still punch there weight IMHO.


    I need to spend 30 minutes a month on maintainance on a large x 36 units of 2v 700ah cells, I spend more time each day looking and caring for my dog. Both are a pleasure!

    AGMs are are retailers preferred stock item due to there low self discharge rate.
  • rplarry
    rplarry Solar Expert Posts: 203 ✭✭
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    Re: agm battery questions

    Lamplight
    I have had both FLA and AGMs (my FLA's were t-105s) and I agree with Brock and Solarguppy, I would never go back to FLA's. My CD 1500's are 6 years old. They are still 97-98% efficient, never need any maintaince, no equalizing, no cleaning corrosion, no checking or adding water, nada. They also still charge down to 1% of their capacity (C/100). I charge with a 1040watt stc array. Granted the t-105's are at the lower end of solar batteries and probably need allot more maint. than larger FLA batteries. But to me the zero maint feature makes AGM's very desirable.
    Just my 2 cents
    Larry
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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    Re: agm battery questions
    1) ive read that agms have charge losses as low as 4%. is that a number i can work with and that people agree with?
    It depends. AGM battery coulombic efficiency (Ah out / Ah in) can be as high as 98% or so, but energy efficiency (Wh out / Wh in) is probably in the 85% to 90% range. Remmeber, it takes ~14.4 V to push the fully charged battery’s resting voltage to ~12.8 V.
    2)ive read here about 5% being minimum charge current for wet cells. whats the lowest percentage of charge current I can consider for agms? (ie: i want a bank as large as possible with as much standby days as I can get).
    I like the 5% minimum from a design perspective. In other words, using published specs, an array may be spec’d to deliver "5%" charge current . For example, the match suggests that a 574 W STC array delivering 20 A into a “24 V” battery bank at 28.7 V would be a good minimum match for a 400 Ah battery bank.

    The catch is that such an array would rarely deliver 20 A continuously at 28.7 V under typical operating conditions. My system with a 944W array typically delivers ~25 A to 28 A (give or take, depending on outdoor temperature, Sun angle, charge mode, and how clean the array is. :roll: My system seems to be well balanced.
    3) whats your favorite brand agm, sun extender from concord seems to be pretty popular.
    My size 4D AGM’s are from Deka (East Penn, also available from MK Battery), and are going on four years old. So far, so good!

    See: http://www.outbackpower.com/forum/viewtopic.php?f=1&t=848

    HTH,
    Jim / crewzer
  • Island Mon
    Island Mon Solar Expert Posts: 29
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    Re: agm battery questions

    Another related AGM battery question here.

    I am installing a system on a coastal island in Maine. The residence is seasonal, being used only in the summer months (may-september). The battery bank is an AGM 48V 800AH bank.

    For the remainder of the year, the house will be dormant with no loads/heat.

    I was planning on leaving the inverters and charge controllers on, and set to float charge all winter.

    I know that the self discharge rate decreases as temperature decreases, and that AGM batteries are nearly immune to freezing temperatures.

    Would it be better to leave the batteries on a float charge all winter or to turn off the inverters and charge controllers and let the batteries sit dormant (in freezing temperatures) all winter?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: agm battery questions

    Turn off the inverters, as they draw when idle. The charge controllers should be able to keep the AGM's 'happy' all winter with no trouble.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: agm battery questions

    And that your charge controller is set for AGM's (typically lower "final voltage" so as not to "boil" the battery)--and (ideally) you have a remote battery temperature sensor on your charge controller to set the voltage vs the battery bank temperature.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lamplight
    lamplight Solar Expert Posts: 368 ✭✭✭✭✭✭
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    Re: agm battery questions

    thanks much everyone, still researching this, and am basically putting off the purchase until fall. i am reading all this carefully and have decided to definitely stick close to the 5% charge rate spec.

    i just read a statement on here in another older post, you caN DISCHARGE the agms deeper than wet cell's without damage, is this true?

    re: the one lonely voice advocating the larger rolls/surrette batteries: i have considered it but im just so forgetful and have so much going on with kids, family & business etc that im going to try the agms i think (barring a financial emergency requiring the MUCH cheaper cost). my solar dealer is a huge fan of the t105s for watts stored over time (i think, is how he puts it). if you dont take into account the cost of extra panels needed (but of course we do) and only look at longevity issues forgetting about the charging source cost / size issues, the big wet ones definitely offer a better $ investment. at least as spec'd.

    NAW&S shipping quote was about $400. my dealer locally can ship much better and in fact has the battery manufacturers ship from their closest warehouse. still have to checkout the car shops for a local source i might get even better pricing.