Sizing a new battery bank

Endurance
Endurance Solar Expert Posts: 40 ✭
in Off Grid Solar & Battery Systems #1
My 3 year old 760 amp hour battery bank is due for replacement. It has 8 AGM GC2 batteries wired in series/parallel for 12 volts. This is on a boat with a lot of 12v equipment, so I need to stay at 12 volts even though life would be easier at 24 volts. My boat is at Lake Powell with 100 degree temps pretty standard in the day. For that reason and a few others I won't bore you with, I probably need to stick with AGM batteries.

My biggest load is a 110v residential fridge. It can go through 400 amp hours in a hot 24-hour period. The 12 volt draws add another 100 or so amp hours of use in 24 hours.

To feed my loads, I have:
Honda eu3000is Generator
Magnum 2812 Inverter/Charger
100 watt panel with a SS-10 controller (more of a maintainer than a real source of power).

To limit my parallel connections, I am thinking of going with L-16 batteries. I could easily duplicate my current size with four L-16s in two parallel sets. Six L-16s would put me over 1200 amp hours. I am thinking of doing this to be able to charge after a night's use at 80% SOC rather than dipping to 50% like I often do now. I hope that this will give me more than three years out of a set of batteries.

If I were going to stay with generator power forever, it seems bigger would be better for a battery bank. But I have plans in the next season or two to add 1200 or so watts of panels and a Midnite Classic 150 controller to limit my generator run time to hopefully a few hours of bulk or acceptance charging each morning and let the panels keep up with daytime use and float charge all day. It seems like I've read here that too big a bank doesn't work well with a solar system.

Bottom line - for my planned generator/solar system, should I go with a "bigger is better" battery bank like 1200Ah or will a smaller battery bank work better from a solar standpoint?
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Sizing a new battery bank

    You don't really have a "solar standpoint" to work from. One 100 Watt panel? Not even much of a maintainer on that much battery. 5.7 Amps, perhaps? Not even 1% of 760 Amp hours.

    So you need enough capacity to supply your loads between recharging from generator source, however often you wish to do that. Are you going to have enough charging capacity to handle the larger bank? You're looking at roughly 120 Amps @ 12 Volts or 1440 Watts peak. The EU3000 will handle that. Not sure of the 2812's output capacity.

    If you did have more solar, significantly more, it could provide some of the power use during the day once the batteries are charged. 1200 Watts of solar would be nice: roughly 77 Amps current or 6% of 1200 Amp hours.
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  • Endurance
    Endurance Solar Expert Posts: 40 ✭
    #3
    Re: Sizing a new battery bank

    Many thanks 'coot. The Magnum MS 2812 is rated for 125 amps dc charging, so that should be good. It also sounds like I "need" that 1200 or so watts in panels sooner rather than later.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Sizing a new battery bank

    Ironically, with the bigger solar array you could allow less battery capacity (depending on usage patterns). The array could charge the batteries and supply daytime power, so the stored capacity would only need to be enough to last "overnight".
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #5
    Re: Sizing a new battery bank

    I wouldn't see a bigger bank as a problem, you have generator power and will only be charging back what you use. If you have 800 amp hrs now and your pulling them below 50 % ( 400 +100 ) then a bigger bank would help you. You only have to charge back what you use and make up for any self discharge. I assume once you leave the dock you'd be operating in the 85%-50% range and do a full charge back on shore power or once a week with your generator, so your usable amp hrs are even less 800 * 85 % = 680 * 50% = 340 usable amp hrs.
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  • Endurance
    Endurance Solar Expert Posts: 40 ✭
    #6
    Re: Sizing a new battery bank

    It sounds like the takeaway points are two: 1) A bigger battery bank won't hurt a thing except that it will cost more money. 2)I will use my batteries less, have better battery life, limit generator run time, and be generally better off to add solar panels as soon as I can. I will start another thread so I keep the board clean with one topic per thread title.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #7
    Re: Sizing a new battery bank

    Not entirely...

    A battery bank is for storage only... If you are using a generator (only) for power, then the size of the battery bank will not reduce generator run time (unless you get a bigger genset and battery charger to pump current back into the bank at a higher rate).

    And, large batteries require higher charging current and still have their self discharge losses of a few percent per week to 1-2% per day--such near end of life forklift batteries (which in a large bank with smaller loads/low powered solar+generator can be very significant in themselves--a very old forklift battery could use 5% rate of charge from a solar array all by itself--and would obviously be ready for recycling).

    For the most part, a 2x larger bank will last something like 2.2x longer (very rough guess). So, that 2x larger cost still works out to about the same cost over time (plus, if there is an "oops" with the battery bank, you have 2x larger costs to replace the bank).

    Unless I had very special needs, I would tend towards the 2 day @ 50% maximum discharge--Or 4x daily energy use--As a starting point for the battery bank. Keeping the system "balanced" (loads to battery bank to charging source(s)) seems to be the optimum way to go.

    With solar panels being so "reasonably priced these days", it is much easier to go to the 10-13%+ rule of thumb for optimal charging vs the 5% that many people used to use (and still our minimum recommended in the range of ~5-13% rate of charge).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #8
    Re: Sizing a new battery bank
    Endurance wrote: »
    ... I am thinking of going with L-16 batteries. I could easily duplicate my current size with four L-16s in two parallel sets. Six L-16s would put me over 1200 amp hours....

    I don't think your calculating your Amp Hours of storage properly. If you have 4 - 300 amp hour 6 volt batteries in series and parallel, you add the voltage only in strings and amp hours only in parallel, so you would have a 600 Amp Hour battery bank at 12 volts. If you do the calculations this way what size is your current battery bank?
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Endurance
    Endurance Solar Expert Posts: 40 ✭
    #9
    Re: Sizing a new battery bank
    Photowhit wrote: »
    I don't think your calculating your Amp Hours of storage properly.

    There's a very good chance I am calculating my amp hours wrong since math isn't my strong suit. Here are my figures:

    Current bank:
    Eight GC-2 AGMs with 190 Ah each at 6 volts. Each set of two batteries should have 190 Ah at 12 volts. Four of those sets should have 190 x 4 or 760 Ah.

    Same capacity with L-16:
    Four Concorde Sun Xtender PVX-4050T with 405 Ah each at 6 volts. Each set of two batteries should have 405 Ah at 12 volts. Two of these sets should have 405 x 2 or 810 Ah. This might be pretty close to what I have now since the 190 Ah rating on my current batteries is a 20 hour rating while the 405 Ah on the Concorde L-16 is a 24 hour rating.

    Increasing bank size with L-16:
    Six Concorde Sun Xtender PVX-4050T. Same 405 Ah at 12 volts per set of two. Three sets in parallel should have 1,215 Ah at 12 volts.

    If this is wrong, please let me know. Batteries cost too much to guess and I need the education anyway.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Sizing a new battery bank

    That is right: batteries added in series increases Voltage but not Amp hours, batteries added in parallel increases Amp hours but not Voltage.

    Not sure what the 4050T is at the 20 Hour rate, but it should be just under 400 Amp hours. Three strings of them around 1170-1200 Amp hours.
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  • Endurance
    Endurance Solar Expert Posts: 40 ✭
    #11
    Re: Sizing a new battery bank

    Thanks. Looks like I'm in the ballpark. The Concorde Sun Xtender PVX-4050T is from NAWS. A local supplier (in Salt Lake City) has a Full River AGM L-16 that is 415 Ah (20 hour rating). Are Full River batteries any good?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
    Re: Sizing a new battery bank

    There aren't that many battery manufacturers in the world. Chances are very good that "Full River" is made by someone else and/or in China (the name sounds like the sort they like to use on things over there).
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #13
    Re: Sizing a new battery bank
    Cariboocoot wrote: »
    There aren't that many battery manufacturers in the world. Chances are very good that "Full River" is made by someone else and/or in China (the name sounds like the sort they like to use on things over there).

    the answer: http://jgdarden.com/batteryfaq/batbrand.htm
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
    Re: Sizing a new battery bank

    Thanks for that vtMaps; I've added that link to the relevant thread in the FAQ section. :D
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