Battery bank sizing

GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
What's a good formula for figuring battery bank size to array output requirements ?

Let's say I had a total of 1kw produced from my wind turbines average. How many batteries would I need to maintain a good charge on them ?

Is there a rule of thumb ?
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Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank sizing

    There are three considerations in recharging batteries.

    1). Voltage (0f course). You have to have more than the 'nominal' battery Voltage available in order to charge.
    2). Current rate. You must be able to charge at a sufficient rate to prevent sulphation and keep the electrolyte 'stirred'. That's expressed either as the "C/#" rate or 5%-15% of the battery's "20hr" Amp/hr rating.
    3). Time. You have to be able to maintain a good charge rate long enough to Bulk charge and Absorb.

    Your wind turbines produce '1kW' over what period of time? 1kW steady as a photovoltaic array, or 1kW/hour in a day?
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Battery bank sizing

    Normally, the rule of thumb is take your daily load, 3 days of no sun (wind) and 50% maximum discharge. Or 6x your average daily load.

    There are lots of reasons behind picking this rule of thumb and you can go more or less.

    If you are sizing it the other way--optimum collection of power from a power source... You can look at maximum charge rates...

    Typically, C/20 to C/8 would be correspond to the to the 5%-13% charging rule of thumb (minimum charging current for proper equalization to maximum typical recommended charging current).

    Those would be the two places to start with.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Battery bank sizing

    Bang ! You got me.

    I never calculated the size of my battery bank required for the array. Just kept building panels and turbines until I got an excess charge rate.

    15 deep-cycle SLA's main battery bank. 120Ah each. 12v system.

    1.125 kw solar panels.
    0- 1.8kw capability on wind turbines.

    Example: Charge was at 12.75 volts yesterday on a cloudy but very windy day. Over night, voltage went down to 12.46 volts. Inverter idles around 30-50 watts when not in use 24/7. Today at 5pm, plenty of sunshine and charge was at 13.00v.

    Question is: Am I close to achieving the optimum charge rate ?
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank sizing

    What are you using for a charge controller on the panels? If you're using a good controller like an MX60 it will log actual kW/hrs produced.
    The wind turbine is a crap shoot: almost impossible to pre-determine output from one.
    You've got 15 12V 120 A/hr batteries in parallel? That is a wiring nightmare; they probably are not being used/charged equally.
    15 *120 = 1800 A/hrs total. That bank would want around 90 Amps minimum charge current. It is unlikely you are achieving this with just the array (1125 Watts * 80% efficiency = 900/12 V = 75 Amps roughly).
    If you pick a "middle number" for the wind turbines you get the same result as above. That would assume an average wind speed of probably 12.5 mph, as most turbines don't reach maximum output 'til at least 25 mph. To get significant charging from them, the rate would need to be sustain for around 5 hours (same as with good insolation).

    BTW, the proper charge Voltage for a 12V bank is significantly higher than 12.75. You should be able to sustain 14.4 V (+/- depending on batteries) for two hours in order to completely recharge them.

    My guess is you've got more than one sulphated battery by now.
  • KamalaKamala Solar Expert Posts: 452 ✭✭
    Re: Battery bank sizing
    ... 15 deep-cycle SLA's main battery bank. 120Ah each. 12v system.

    I think SLA stands for Starting-Lighting-Accessory, which implies an automotive battery. Such are not "deep cycle."

    Also, with an odd number (15) of batteries in the bank, I assume that they are all 12V and connected in a parallel configuration. Large parallel banks have problems with charge/discharge distribution among the constituent units. Elaborate wiring schemes are needed to ensure that charge/discharge currents are shared equitably by all members of the bank. See link below:

    http://www.smartgauge.co.uk/batt_con.html
    ... 1.125 kw solar panels.
    0- 1.8kw capability on wind turbines.

    Plenty of capability here. Although, I tend to view 0-x.xx as 0. :-)
    ... Example: Charge was at 12.75 volts yesterday on a cloudy but very windy day.

    I use AGMs and consider their "full charge"voltage to be 12.7. FLAs, maybe 12.6. These are "resting voltages. You are not getting nearly enough potential difference (150mv) to charge effectively. Even on a "windy"day.
    Over night, voltage went down to 12.46 volts.
    12.46V is a discharge condition for any 12V bank.

    Inverter idles around 30-50 watts when not in use 24/7.

    My ProSine 2.0 uses 25W with no load (load sense does not work with the devices I have connected to it.) However, if I merely disable the inverter, (which I do for no less than 5 days per week and 8 to 10 hours on the weekend) its consumption drops to 500mW. Once I install the remote shutdown switch, the consumption will drop to 24mW, that is about 1/4 of a watt.
    Today at 5pm, plenty of sunshine and charge was at 13.00v.

    Not even close. My bank floats at 13.35V. And, yes, you do need to measure to the 1/100th of a volt! My bank needs 14+V to absorb. TC of course.
    Question is: Am I close to achieving the optimum charge rate ?


    Dunno.

    Good Luck!

    K.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Battery bank sizing
    Kamala wrote: »
    I think SLA stands for Starting-Lighting-Accessory, which implies an automotive battery. Such are not "deep cycle."

    Sealed Lead Acid, so either AGM or Gel.
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Battery bank sizing

    Yes, they are indeed Deep-Cycle marine batteries. They are not sulphated. The Morningstar TS-60 uses PWM to charge every day.

    For everyone who wishes to know, my method of 12v parallel hookup is by far the most advanced I've seen and heard. This is new to the NEC and hasn't been printed yet. I'll share the idea, however will not accept any responsibility if someone tries this and gets injured.

    All 15 batteries are hooked up in parallel with the NEGATIVES ONLY. 1/0 wire is hooked up to one of the Negative posts and leads to Negative on Inverter.

    The other 15 Positives have EQUAL length 8 awg wire isolated to a Bus Bar (Aluminum 2" round stock) where the main Positive wire is 1/0 directly to the Inverter.

    The Positives are rated at 8 awg because they're acting like a separate fuse in case of over or under voltage. Over voltage - the Positive wire breaks on the battery affected. Under voltage - Same affect & takes the bad battery out of the loop. End result is an efficient battery bank without unnecessary explosions.

    This method is nice because you don't have to search for a dead battery in the loop. The broken wire leads right to it. A regular parallel hookup would try to charge the bad battery, heat it up and possibly explode causing the entire bank to have bomb potential.
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank sizing

    Okay, for one thing "Marine Deep Cycle" batteries aren't true deep cycles: they are a hybrid, and their plate design is half-way between that of an SLI (Starting Lighting Ignition or "automotive") and a Deep Cycle (as used for electromotive force in forklifts and golf carts and also for Renewable Energy applications).

    Your innovative wiring technique isn't innovative, nor is it the best. The best for multiple batteries is separate, equal-length wires from BOTH (+) and (-) going to respective "common points" to connect to inverter & charge controller. Wiring circuits are just that: a loop. The resistance on the negative side is just as important as the resistance on the positive side.
    All 15 batteries are hooked up in parallel with the NEGATIVES ONLY. 1/0 wire is hooked up to one of the Negative posts and leads to Negative on Inverter.

    There's the problem: whichever battery has the negative inverter lead attached to it will be doing most of the work. Any "further out" electrically will have more resistance in the line and won't contribute/accept as much.

    Also, you 8 AWG wire on the positive posts is insufficient for significant current flow. Most automobile batteries are connected with 4 AWG - or more. Think about that.

    Right now you could go to your local automotive store and buy 30 pre-made 4AWG battery wires and improve your systems performance by 100%. But it's probably too late to save those batteries.

    For fuses, USE FUSES! Do not EVER rely on a piece of small wire burning out to protect a circuit (actual "fusible links" being the exception). The insulation can catch fire, right above a bunch of batteries expelling hydrogen gas in some cases. Very bad scenario!
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Battery bank sizing

    OUCH ! That hurts !

    Try not to admire my work too much.

    Will have to consider the negative hookup strategy similar to the Positive one.

    I'll give myself much credit for reading several books prior to building a battery bank. None of them gave any clue as to how many & size batteries needed for a specific amount of power produced. Fact is that I did it and it works OK, not great, just OK.

    All part of the fine tuning.....
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank sizing

    Yup; you gotta start somewhere. Nobody every learned mathematics by starting with calculus.

    That's why we have "rules of thumb" and "nominal values": to get a rough idea of what you're dealing with. The closer you get to a complete system and the more complex the system the greater the need to fine tune.

    Take heart: there are still issues that come up here which perplex the engineers. :D

    Me, I've been working with circuits for forty+ years - and haven't electrocuted myself yet.
    But I keep trying! :p
  • mike95490mike95490 Solar Expert Posts: 9,363 ✭✭✭✭✭
    Re: Battery bank sizing

    It's hard to do any better than the Smartguage link contents:
    http://www.smartgauge.co.uk/batt_con.html
    A basic read, and why to dispose of the asymmetrical wiring
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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  • bryanlbryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: Battery bank sizing

    The smartgauge site has good stuff! His tech notes are some of the best I've seen for conceptual understanding as well as content.
    Normally, the rule of thumb is take your daily load, 3 days of no sun (wind) and 50% maximum discharge. Or 6x your average daily load.
    The only quibble is that the issue is energy draw, not load. The distinction between power and energy is a major source of confusion in many discussions.

    Another rule of thumb is to have 1 watt of solar for each pound of battery. Since wind may have a higher duty cycle, I'd suggest perhaps a watt of wind for every 2 pounds of battery.

    On the fuse: a fuse _is_ a carefully selected short piece of wire as are fusible links. If you plan to make your own, though, you need to make sure you use the right wire with the right insulation. Often, a lower gauge wire can be more of a current limiter than a fuse and will serve an adequate purpose in that role.

    I see the 'deep cycle' vs 'marine' myth shows up again here. What does it take to get folks to realize there is no such thing as a 'true deep cycle' battery in common use for RE or RV? (see the "50%" in the rule of thumb quoted above and think a bit). Look at the battery specs and things you can actually measure that are pertinent to the use intended!
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Battery bank sizing

    Another rule of thumb is to have 1 watt of solar for each pound of battery. Since wind may have a higher duty cycle, I'd suggest perhaps a watt of wind for every 2 pounds of battery.

    Now that makes more sense. My average battery is 65-70 pounds and every Solar panel is 72 watts each. I have 15 solar panels hooked up to the main battery bank. Right now I'm getting a slightly higher charge rate than draw, however at night time I loose more than it can recover unless it's windy enough. Now it's windy and overcast today and I'm still getting the banks charged, just not as fast. The system has been on its own for 3 full days and nights. Battery voltage was at 12.31v this morning and is now around 12.5v.

    With all due honesty it sounds like I need another 3-4 solar panels (200-300 watts) to make a balanced system.

    I need to clear things up from previous posts. These batteries are not Marine type. They're true deep-cycle SLA's. My backup bank is 14 Gel batteries and charged by only two solar panels and one wind turbine. Since it's the backup, I only need the system when I'm using it in the Barn.

    I'm going to tear apart both banks and wire the Negatives to a Bus bar just like the Positives, but only on a cloudy, windless day. Hopefully this setup will make a difference in the current drawn from the banks and improve the resistance to discharge rapidly.
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank sizing

    I've never heard of measuring a battery's power by its weight before. Normally they are measured by Voltage and whatever current standard is relevant to the application (as in Cold Cranking Amps for starting or "20 hour" Amp/hr rating for deep cycling).

    There is no "true deep cycle vs. marine myth": there are three basic types of flooded cell batteries. SLI - automotive type designed for delivering short burst of very high current, Deep Cycle - designed to deliver a steady current over a long period of time to a significant depth of discharge, and Hybrid - whose plates are half-way between the 'holey' SLI batteries and the 'very thick' deep cycles.

    What marketers call them in order to gouge a gullible public is another issue. I expect to see "Deep Cycle Alkaline Penlight Cells" any day now (yes, I'm a cynic at heart).

    As for how much Wattage in panels you need to charge the batteries, the only formula is "enough to provide sufficient charge current for long enough to recharge the batteries." Weight doesn't enter into it.

    Fuses are not just short pieces of undersized wire. Nor are fusible links. They are quite precise pieces of undersized wire designed with a specific maximum current load/time ratio. They are also encased in non-combustible material - unlike the vinyl insulation on a wire.
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Battery bank sizing
    I've never heard of measuring a battery's power by its weight before.

    I have, though I can't remember where I first heard of it...

    "Energy Density

    The amount of energy that can be contained in a specific quantity of the fuel source. Typically quoted in watt-hours per pound, wh/lb, or watt-hours per kilogram, wh/kg. For example, flooded lead-acid batteries generally have about 25 wh/kg, the latest advanced lead-acid designs claim about 50 wh/kg, and newer battery technologies such as NiMH and LiON are in the 80-135 wh/kg range."

    http://www.rtpnet.org/teaa/battery.html
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Battery bank sizing

    What size fuses would be good for 8 awg wire ?
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  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Battery bank sizing
    Carrying how much current?

    Wire size can handle a maximum current over a given distance. Fuses are sized to protect devices (including wires) by being the 'weak link' in the chain. But you have to know the whole chain first.

    In the context of *wiring*, fuses, and breakers, are to protect the wire - not the devices. Devices which need protection have their *own* overcurrent protection.

    It's common to wire a house with 20a circuits for the receptacles in the wall, and using #12 wire - but the *devices* (the receptacles) themselves are only rated at 15a.

    I have a couple of 12v power receptacles (cigarette lighter type) in my RV. These are both fed from a single 20a breaker - using #12 wire. What "devices" do I plug into them? Well there is a 12v oscillating fan - which has its own 10a fuse in the plug. There is a small charger for AA and AAA batteries, which has its own 5a fuse.

    I also have a 200w MSW inverter fed by another 20a breaker and #12 wire. The inverter has its own fuse as well (I don't recall the size). The Norcold 323 fridge draws 1.4a when powered by AC - yet it is plugged into a 15a receptacle fed by #14 romex and protected by a 15a breaker.


    Therefore, the answer to the question is: #8 is rated to a max of 40a, so a fuse or breaker to protect that wire will be 40a or less.
  • nigtomdawnigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Battery bank sizing

    off topic off grid my golden rule in sunny locals is HERE........... THERE IS NO SUCH THING AS TOO MUCH PV EVER , disagree run your unrealiable noisey expensive generator as long as you can afford the repair bills........ I hate generators with a passion, am I repeating my self. Nigel Guess I am as I do.

    Sos for the rant:blush:
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Battery bank sizing

    Why is it that DWH is the one speaking my language ?

    40 amp fuse rated for 8 awg wire. I'm assuming that I want the fuse to pop before the wire heats up. Knowing that, I could use say, 30a inline fuses. I'll give it a shot.
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank sizing

    After my nap I re-read what I posted, and then deleted it because it was utter gibberish! :blush:

    Honestly! I don't know where my brain gets to sometimes. But I hope it enjoys itself while it's there. :p

    dwh's answer is correct - and sensible.

    (I think I'm going to have to add a disclaimer to all my posts; Warning: I might be talking nonsense. You never know.)
  • bryanlbryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: Battery bank sizing

    It amazes me how much measure and objective definition can be disdained in these conversations.

    As far as weight as a measure of energy capacity - go look at a spec sheet and you'll find that the range is rather narrow (i.e. design is highly optimized). Using weight rather than amp hours or such things can be convenient for rules of thumb as it avoids all sorts of arcana and gets to the root of the matter.

    As for deep cycle, the plain fact is that you don't deep cycle lead acid batteries if you expect any life out of them. There is no specification or objective definition that can be used to type batteries by the term deep cycle. The term refers to a discharge state, not a battery type.

    As often used 'deep cycle' asserts something about longevity of battery life. But, in practical and real world terms, battery life has much more to do with use and maintenance practices that it does simply with discharge state.
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Battery bank sizing
    bryanl wrote: »
    It amazes me how much measure and objective definition can be disdained in these conversations.

    As often used 'deep cycle' asserts something about longevity of battery life. But, in practical and real world terms, battery life has much more to do with use and maintenance practices that it does simply with discharge state.

    Not true.

    The term deep cycle battery refers to batteries with much thicker plates than starting batteries, which enables them to withstand many more cycles, whether deep or not. It just turns out that cycling a deep cycle battery down to around 50% on average is usually the best economic tradeoff when comparing cost vs energy stored and used.

    You can do perfect maintenance on a starting battery, but if discharged down to 50% each day, it will die much sooner than one designed for that purpose.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank sizing

    I'll add that the the thicker plates of the true deep cycle means that they would weigh more, which ties in with the energy density theory. (I'd never heard that used for batteries before, I confess - solid and liquid fuels yes, just not batteries.)

    Bryanl's weight idea might be useful in comparing similar batteries too: if two brands claim a battery is "X" Amp/hrs and one of them weighs more - it might be the better choice.

    Although it might be due to heavier casing as well.

    Anybody else remember "tar top" batteries? :blush:
  • bryanlbryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: Battery bank sizing

    re "Not true." - please give me something I can measure that is useful and pertinent. Talk about 'thicker plates' don't mean squat to anything involving actual use.

    There is no distinct difference in any commonly available measure I have yet found that can be used to distinguish a so called deep cycle battery from others. From what I can tell, there is a reason for this and that reason is that the 'deep cycle' mantra is worthless in real terms compared to other factors.

    All of the discharge depth versus life graphs I have seen are similar and all lead acid batteries suffer the same results. The differences I have seen indicate that batteries towards the SLI end suffer after a hundred or two deep cycles while those on the 'deep cycle' end suffer after a few hundred. The difference is not a matter of distinct type but rather of trade-off of battery parameters. The lesson is that anyone wanting battery life does not deep discharge any lead acid battery as a routine thing.

    So, please - don't just give out bland assertions like a salesman's hype as if they proved anything. Give me something I can actually measure that has some utility for making decisions. IMHO It is a disservice to toss out empty assertions about things that make very little measurable difference in any actual practice.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank sizing

    You don't see the association between thicker plates (the hallmark of deep cycle) and the heavier weight you see as the best way to measure a battery's potential?

    This issue has been explained over and over again. It's getting tiresome.
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Battery bank sizing
    bryanl wrote: »
    re "Not true." - please give me something I can measure that is useful and pertinent. Talk about 'thicker plates' don't mean squat to anything involving actual use.

    Nobody is forcing you to use one battery or another. You are welcome to use K-Mart starting batteries for your golf cart or off-grid solar system if you so desire.

    There is nearly 100 years of history of deep cycle vs SLI batteries, if you dispute the claims about deep cycle batteries then I would suggest the onus is on you to show why it is a fallacy.
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Battery bank sizing

    One interesting thing about plate thickness, which I still don't understand is that many forklift batteries can be bought at the same Ah rating, but a different number of plates and plate thickness. Take a look at this supplier's page for example:
    http://www.sunlight.gr/x_pr_range.asp?gid=4&cid=41&id=43&pd=74&ex_id=243&lid=2

    At the one end of the spectrum they offer a 600Ah battery with 10 plates, each plate rated at 60Ah. At the other end there's a 600Ah battery with 4 plates and 150Ah for each plate. Does anyone know how these different battery architectures influence the performance - and which is more appropriate for renewable energy?
    I.e. is fewer thicker plates any better or worse than more but thinner plates - even if the total amount of plate material stays the same?
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Battery bank sizing

    My guess is that you have several issues... One is that "peak current" is proportional to surface area of exposed lead. Making a battery surface "spongy" and/or lots of thin slices (think of the surface area of the outside of a block of wood vs the surface area of all of the pages of a book the same size a a block of wood)...

    Also, the more "slices you have, the more insulators/acid you have in between each slice (as part of a whole). A block of lead may weigh 10 lbs... But a 10 sheets of lead + insulators cannot have 10 lbs in the same area as a large percentage of the space is insulator/electrolyte.

    So--more surface area, more current, less total lead, less energy storage.

    Also, you have "strength" of the material... There is a grid inside of each plate (at least as I understand it) to both support the soft lead/oxide/sulfates and to conduct current... The thinner the plate, the less structural support per plate. Easier to shed materials. More material to shed as there is more surface area. Would lead to a battery that does not perform well when deeply cycled.

    If, you desire is total capacity and that is pretty much dependent on how much lead is in the battery to convert--then both types of battery (at limited current levels--i.e., similar amps per sq.ft of exposed lead surface)--then, by weight, each should have similar AH/WH of capacity.

    However, thicker plates would mean a denser battery per unit volume, and would store more useful power--if the application was space limited.

    Picking ratings for a task would help define the difference between an "ingition battery" and a "deep cycle" battery.

    For a car, the battery is asked to supply large amounts of current for short periods of time C/1 or larger and to be light weight (weight reduces fuel efficiency). So a battery with lots of surface area would meet the high current / light weight needs of a car (and cheap--must not forget price when designing a car).

    For a forklift--lower, but constant current demands of C/16-C/10 (assuming 8 hour shift), small size, and lots of weight (keep fork lift small and heavy) indicates a battery design with a few, thick plates to meet the design needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bryanlbryanl Solar Expert Posts: 175 ✭✭✭✭✭
    Re: Battery bank sizing

    re: "This issue has been explained over and over again. It's getting tiresome" and "no one is forcing" - Yes indeed. The question is why the mantra is accepted to this point without regard to measurable result? Why do repeated requests for something I can measure that is relevant come up with this sort of response instead?

    re "You don't see the association" - I see the explanation as a convenient teaching fiction that has been extrapolated way past its usefulness. The actual data - the only reality that can be seen - does not support any major distinction such as usually portrayed.

    I do note that the efforts to avoid my request have started to go far afield from the RE and RV nominal (thanks for the link BTW). That is indicative. I'll take heed.

    Bill is on the right track, I think. Quantity of lead with a proportionate amount of electrolyte determines capacity. Surface area of lead exposed to electrolyte determines peak current (this gets into surface charge). Ability to move in 'fresh' electrolyte is going to have something to do with average current.

    The physical strength of the plates to be able to handle the expansion and contraction that occurs with cycling and the shedding from use is not just a lead issue, though. That is why there are dopants such antimony and calcium. I think there are other factors involved in the mechanical design of the battery as well.

    The 'thick plates' conveniences goes towards the simple model that a battery is a plate of lead immersed in a bath of acid. That model is OK for getting across some concepts but I don't think it really applies to modern battery design nor to the issue at hand (durability of modern batteries).

    I have seen these measures such as CCA and warranty life change over the last few decades. My guess is that there have been incremental improvements over time to optimize these factors. That is why even 'deep cycle' batteries have fairly large CCA and why SLI batteries can have 8 year warranties. It is also related to changes in battery failure mechanisms over the years.

    Sorry to be stubborn, but when you insist on telling me not to believe my lyin' eyes, I do tend to have a problem.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Re: Battery bank sizing

    By the way--the term "surface charge" -- at least as it is used around here -- is the the higher than normal cell voltage reading that people see when a battery is just removed from the charger...

    From my understanding, the high cell voltage is the result of higher than normal specific gravity of the electrolyte right next to the surfae of the plates vs the balance of the solution in the cell.

    And because the cell voltage is directly related to the specific gravity--the battery appears to be at a higher state of charge when freshly removed from a charger vs if the battery "rests" for three hours (electrolyte concentrations balance throughout cell), then the cell voltage(s) are measured.

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