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battery question

Discussion in 'Photovoltaics' started by Douglas Gaulin, Feb 18, 2004.

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  1. Last year I put together a small starter solar package for my summer
    cottage. I originally started with four T-105 batteries. I am considering
    purchasing two more T-105's to add to the system, but was told that adding
    new batteries to an existing bank is not good. Is this true and is there
    anyway around it?

  2. SQLit

    SQLit Guest

    Adding more capacity with out more panels will eventually harm the
    efficiency of the system. As you add more batteries you need to add more
    X number of panels can only charge X number of batteries. Unless you
    undersized the batteries on the original installation you will need more
    panels to charge the additional batteries. Sorta like you change your
    battery from a 100 amp to a 1000 amp. The charger stays the same at 10 amp.
    You will never be able to charge the new battery completely because the
    charger is not big enough. example exaggerated
  3. Bughunter

    Bughunter Guest

    I'd think that you would be reasonably safe to add a couple batteries to
    your bank. One years use at a summer cabin probably has not worn down your
    existing batteries too much, unless you have been really discharging them to
    the max r otherwise absing them.

    The issue is that if your existing batteries are aged (many deep cycles,
    abuse, time) then they have used up some of their useful life. When you add
    new batteries, they will be limited by the useful life left in the 'oldest'

    Think of it like adding a single new 60k mile tire to a set of tires that
    already have 50k miles on them. You are going to have to get a new set of
    tires in another 10k miles. The new tire does not add to the life of the
    whole set. It is even a bit worse with batteries, because they are
    interconnected and a bad battery will place additional strain on a good

    As another reply stated, you do want to make sure that you are adequately
    charging the bank. Adding additional batteries will not compensate for a
    situation where you have an indaequate charging setup.
  4. Guest

    Wow Mr Bughunter, you gave me something new to worry about. While I
    only have 140 Watts of panels, I also only have 1, 75ah deep cycle
    (Marine starting) battery, with a 400 watt inverter. As soon as I am
    working again I do plan to get proper (Real) deep cycle batterys. But,
    how many ah's should I concider (I am still in the experimental
    stage)? I would like to boost my wattage to 500 before next winter
    starts. I would like to balance my batterys to my (Panel) Wattage. I
    can always plug in more appliances as I produce more power (and learn
    Should I get more panels, or should one get batterys sized to ones
    "estemated" eventual panel Wattage size?

    Please remember that there are no stupid questions (I hope)

  5. Beyond a certain point (six months?) the conventional wisdom is that
    it is a Bad Idea, but outside of folklorish rules of thumb like this,
    I don' know. I could keep a set of batteries floated for six months
    and they wouldn't be the same batteries as if I was sucking them dry
    and recharging them every day... or one could propse an infinite
    number of intermediate variations, so a simple time formula seems
    useless to me.

    I posed the question to Trojan via their website. I'll let you know if
    I hear... though perhaps if some other people ask them the same
    question they'll be more likely to respond and/or add it to their FAQ.
    Never asked them anything before since the site is pretty informative,
    so I don't know how quickly they'll respond.

  6. Bughunter

    Bughunter Guest

    You don't really have much to worry about. About all that you are risking
    with your current setup is the value of a 75ah marine battery. My guess is
    that it is way undresized, and that you are probably well on your way to
    sending it to a recycling center. Not a big deal. I have burned up countless
    marine batteries in my lifetime. A polititian would claim that it is a
    stimulis to the economy.

    Your concern seems to be to design a well balanced system. While it's not as
    complcated as rocket science, you do need to have a good undrestanding of
    your usage patterns and needs, as well as being able to translate those into
    numeric values. There are a number of spreadsheets around that you can use
    to calculate your esitmated loads, battery sizing, inverter size, and
    charging requirements. Try

    The first place to start is to determine what your loads will be. For
    example, how many lights do you use, at what wattage, and for how long each
    day. Repeat for other types of loads. One advantage of an experimental setup
    like yours is to keep records of the loads that you actually use. Keep a
    log. Your particular usage patterns are a critical factor in the design that
    only you can estimate with any accuracy. Time of usage is an important
    metric. You post never mentioned it at all, so it is something that you do
    need to consider in a dynamic system.
    Loads are expressed in watt-hours, battery capacity is expressed in
    amp-hours. Time must be taken into consideration in charging, and
    discharging of batteries, as well as in estimating loads.

    You loads can help you estimate how large an inverter you need. You want to
    buy a large enough inverter to meet future needs, but not so large that you
    are only utilizing a small fraction of the capacity. Keep in mind that all
    of your loads will not be on at the same time. You can estimate how many you
    do desire to use simultaneously, and then size to that. Leave youself some
    headroom for future expansion in loads. Upgrading inverters multiple times
    might be more expensive in the long run that getting one large enough in the
    first place. A few simple estimates and calculations can help you choose.

    The battery would be my next consideration. How big it needs to be depends
    on your loads, as well as how long you hope to run from it without needing
    an input in the form of a charge. Maybe you are happy to have enough
    charging in a single sunny day to keep your lights and TV running for the
    evening hours of the same day. Or maybe, you want to have 3 or 4 days of
    capacity to get you through a few cloudy days. Most systems make provisions
    for at least a few days of battery autonomy. Incremental upgrades to a
    batery bank is not a good idea, so take care in sizing it on the first shot.

    Think of your battery bank as a water resevoir. You don't want to go thirsty
    just because it doesn't rain for a few days. Think of your charging system
    as analagous to water collector (roof?). If it is too small, the resevoir
    will never fill. Too big, and you just waste the excess production. It's a
    balancing act.

    Finally, there is the charging system. This is often the most costly part of
    the system, unless you are grid connected. For alternative sources like PV
    and wind, you need to consider the environment in which they will run. Not
    every site has ideal wind conditions, and the same is true of the sun. An
    good system will keep just ahead of your ability to run the batteries below
    50% DOD. Some systems, will augument alternative sources, which are capitol
    intensive with genertors (not quite as capitol intensive), but more
    expensive to run.

    Now, the calculations are not complex, but there are so many variables that
    are tricky to estimate that the output will never be "perfect". Shoot for
    good enough.

    So, my bottom line suggestion woud be to add a spreadsheet to your
    experimental setup.
    You might also look at the specs on some "packaged systems" to get a feel
    for what other designers thought were reasonably balanced systems.

    my $.02
  7. Bob Adkins

    Bob Adkins Guest

    You can bet Trojan will err on the side of selling more batteries. :)


    Remove "kins" from address to reply.
  8. I should think you'd be able to watch a lot of TV with the four batts
    you have. How much juice does the TV want?

    You have four T-105's, 225AHr, 6V nom, assembled into two strings to
    make two 225AHr 12V batteries, paralleled, giving you 450AHr at 12V.

    That's about 12V x 450AHr = 5400WHr (conservatively rated, or use
    12.5V and call it about 5600WHr)

    I dunno about your TV, maybe it's a monster. Let's say it wants 100W
    just for a round number to see what ballpark we're in.

    So at 3 hours / day and assuming a 90% efficient inverter (again,

    3Hrs x 100W / 0.9 = 333WHrs per day, or 2333WHrs per week.

    Most folks don't recommend discharging lead-acid batts more than 50%,
    but 2333WHrs isn't quite 50% of your capacity. The batteries will live
    longer if you don't discharge them that much, but again, we're looking
    for ballpark numbers here.

    The real numbers will depend on the actual requirements of the TV, but
    it looks to me as if you don't really need more batteries.

    I'm sure someone will check my math and probably my units too.

  9. Yeah, maybe. If so it will be interesting to see if they provide a
    detailed justification. Dunno, we'll see if they respond.
  10. Bughunter

    Bughunter Guest

    I agree with Scott.

    I have the same sized battery bank, and use it to run a 13" color TV, a few
    CF lights, a DVD player, stereo speakers. I can run for about a week before
    I need to use the generator and a 40amp automotive charger to recharge the
    batteries. I need to run the generator to use a big 110v well pump, and the
    charger runs simultaneously. So, the batteries get a charge whenever I run
    out of water, which usually occurs before the batteries get too low. The
    schedule of my battery charging needs just happens to coincde with my water
    replenishment needs.

    It is a pain to run a noisy generator for the sake of a TV or a few lights.
    I try and avoid running it at night completely.

    How big is your battery charger? If it's one of those little 10amp jobs, you
    might spend your money in getting a larger one to reduce generator run
    times. You might also consder adding a PV panel. Your goal should be to
    reduce generator run time. A better bet is an inverter/charger combination
    although they are quite a bit more expensive than off the shelf inverters
    for automobiles. I run a cheap little 350 watt inverter. It works, but my
    end goal is a bigger inverter charger. Eventually, I'll get my 600 watts of
    PV wired into the system.

    My camp is wired conventionally for AC. I highly suggest you go this route,
    even if you only have a few applances. You should consider AC wiring for
    inside the camp instead of 12v wiring. You will pay an unecessary premium
    for 12v applances, wiring and switches.

    One disadvantage of the cheap inverters for automotive applications is that
    you cannot have neutral bonded to ground as required by NEC for homes.

    It would be no problem to add batteries now if that is what you really want
    to do, even if only for anticipated future needs. Add it now if you think
    you will need it. It sounds like you have been very easy on them wih your
    current usage and charging regimine, so they would be better matched if you
    do it now rather than later.

    Oh yeah, get a power strip for the TV so that you can actually turn it off.
    Almost every TV ( and other electronic gadgets) draws a substancial phantom
    load with the power off.

    my $.02.
  11. I thought it has to do with the efficiency when it comes to discharging or
    consuming a fully-charged battery. IIRC, a fully-charged battery will
    provide a longer continuous power to its load(s) when discharged at a rate
    less than 50% of its capacity.
  12. Steve Spence

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