Battery string switching

Marc KurthMarc Kurth Solar Expert Posts: 453 ✭✭✭✭
Has anyone done any battery bank switching between strings in order to increase charge rate? In other words, charging each string separately.

I'm working with a guy who wants a huge battery bank compared to the PV array. It's for a weekend getaway place, so the system will have at least 7-14 days (often more) to charge, then BIG power draw over two to three days. He's had his system up and running nicely, but wants to triple the battery bank.

I have some ideas how to do it, but I figured that someone else has already worked out the kinks.

Thanks,
Marc
I always have more questions than answers. That's the nature of life.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery string switching

    Welcome to the forum, Marc. ;)

    Manually it's pretty easy: http://www.solar-electric.com/basw1300amp.html
    Automatically it gets more complicated. But you can use a charge controller like the Outback FM series with an AUX function that can control a relay to swap one bank for another. You would still need a switch as above to isolate the two banks in respect to the inverter, or else find a relay capable of handling an enormous amount of DC current. The AUX output of charge controllers are usually capable of only a few mA's and so would have to trigger one relay which would control the higher current relay.

    Personally I wouldn't do it, and I have done it. Better off examining the whole system and see if there isn't a simpler approach to the problem.
  • BB.BB. Super Moderators, Administrators Posts: 29,360 admin
    Re: Battery string switching

    Big batteries, deep discharge, and slow charging, more or less, add up to short battery life due to sulfation.

    The longer the batteries spend below ~75% state of charge, the more/faster they sulfate ("fluffy" lead sulfate grows into hard crystals that do not participate in battery charging/discharging any more).

    Ideally, I would suggest that you get a lead acid battery bank back above 75% state of charge within 1 day... A large bank with small array deeply discharged is not going to do that.

    I would suggest that deep cycling (to 50% or even down towards 20% on random weekends with a small battery bank and relatively large array) is a better deal.

    Otherwise--the only other way is to recharge with a genset back above 80% state of charge and then let the solar finish it off...

    But if a genset is already required to quickly recharge a big battery bank--then why not use a genset to "assist" the smaller (cheaper) battery bank that would probably last just as long as a 4x larger battery bank at 1/4 the cost and maintenance (if flooded cell).

    Basically, the smaller bank would die due to deep cycling (but only 20-50 times a year vs a 500 or so deep cycle life) and the larger bank would die from sulfates (if not charge quickly) or just aging (as any battery can eventually fail).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Marc KurthMarc Kurth Solar Expert Posts: 453 ✭✭✭✭
    Re: Battery string switching

    I was thinking in terms of monitoring voltage levels and current coupled to time in order to create a simple control system with interlocks to avoid opening/closing under high load.

    Conceptually, when the charge rate on string "A" falls down low enough, go for bank "B" etc. (He's got nice dump load system of pumping water to a livestock tank that I can use to transition between banks)

    If you can think of a simpler way - I'm all ears! :D

    Existing 48v system is (20) Sanyo 180 watt with (24) 2v 915 amp batteries. He wants to triple the battery bank size to three strings of batteries.

    Marc
    I always have more questions than answers. That's the nature of life.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Battery string switching

    adding batteries to older batteries is not a good idea so the battery banks would have to be separated. my guess is he likes big weekend parties at a cabin.:cool: i would suggest going with a generator to supplement the big power draw that the system will see, but i am assuming that there is already a fairly good sized amount of ah capacity present without even thinking to ask how much is already there and how much pv is charging it. tripling the ah capacity is quite the undertaking for the present pv system to deal with for while 1 set is charging the others lay in wait to sulfate. if it takes all week for that pv array to charge all of the batteries this guarantees some of those batteries will start to sulfate and every week it'll get slightly worse so it will shorten the battery lifespan somewhat.

    edit to add: see what happens when i take too long to post as several posts went in with info i asked for. doh.
    go with a generator and the inverter generators do a nice job while conserving on gasoline and are a bit quieter than most of the old standard generators.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Battery string switching
    Marc Kurth wrote: »
    I was thinking in terms of monitoring voltage levels and current coupled to time in order to create a simple control system with interlocks to avoid opening/closing under high load.

    Conceptually, when the charge rate on string "A" falls down low enough, go for bank "B" etc. (He's got nice dump load system of pumping water to a livestock tank that I can use to transition between banks)

    If you can think of a simpler way - I'm all ears! :D

    Existing 48v system is (20) Sanyo 180 watt with (24) 2v 915 amp batteries. He wants to triple the battery bank size to three strings of batteries.

    Marc
    Maybe I'm just dense but I don't see what this will buy him. The net discharge is still the same whether it's from 1/3 of the bank at a time or distributed across the whole, and the net amount of charging needed to bring the whole bank back up won't have changed, either.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery string switching
    ggunn wrote: »
    Maybe I'm just dense but I don't see what this will buy him. The net discharge is still the same whether it's from 1/3 of the bank at a time or distributed across the whole, and the net amount of charging needed to bring the whole bank back up won't have changed, either.

    Ordinarily you'd be right. But here the use will be on one or two days, allowing recharge over five.
    It's not a great idea in any case, as it inevitably means batteries will sit in a discharged state for up to perhaps three days before it's "their turn" to be recharged.

    Right now there's a 3600 Watt array and 915 Amp hours @ 48 Volts. This begs the question: "What does he need almost 22 kW hours worth of power for?" and by extension why would he need even more than that for one or two days? That is massive amounts of power for off-grid living.

    Furthermore, that array size isn't even big enough to support the existing battery bank:
    92 Amp hours * 59 Volts charging = 5428 Watts before compensating for derating. That bank should have about a 7 kW array on it. He's not doing those batteries any good by charging them at a low rate over a long time.

    My advice: spend more money on panels and less on batteries. I'd bet he's dissatisfied with the performance of the existing system because the batteries have already suffered significant sulphation and have lost quite a bit of capacity.
  • Marc KurthMarc Kurth Solar Expert Posts: 453 ✭✭✭✭
    Re: Battery string switching

    Yes, I agree that more power production would be in order. But it’s not my system.... He will be adding a small Bergey wind generator, but I just don’t think it will do much for him in his location. He has a genset onsite, but refuses to use it except to exercise for maintenance.

    His existing battery bank is still pretty new (about 24 months) and doesn't show any signs of sulfation yet. (We capacity tested 6 of his batteries with a Beta D-50 at came up at 96% of rated) He's good about not pulling them down below about 20%-30% DOD on average. Some weekends he needs to use much more for a variety of reasons, but not every time.

    He wants to maintain a reasonable DOD but is adding more loads and really wants more autonomy time. Since he has firmly decided that he is going to add the batteries: I was simply wondering if bank switching the strings to keep a charge rate of something like 5% of C/24. So, 3600/57.6 = 62.5 x .80 for real world = nominally a 50 amp charge rate.

    I am very familiar with his Concorde batteries in both theory and field applications and I do not foresee any substantial sulfation forming in a few days at shallow DOD’s. In this particular application, I’m more concerned about the cumulative effect of repetitive slow charge rates over years.

    I do appreciate the input given, but it looks like the consensus is that it's a bad idea.

    Marc
    I always have more questions than answers. That's the nature of life.
  • BB.BB. Super Moderators, Administrators Posts: 29,360 admin
    Re: Battery string switching

    AGM batteries do not need even 5% current to properly recharging.

    Flooded cell batteries are the ones that need 5 to 10% rate of charge to mix the electrolyte.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery string switching

    Marc;

    I'd also agree with you that the Bergey is likely a waste of money. Most sites do not have the kind of wind necessary to get any usable power out of turbines. The cost of this with the tower ... Why not spend the money on PV? :confused: It just doesn't make sense.

    The more batteries added the worse his already bad charging situation will be. The sulphation isn't going to rear its ugly head immediately, but in my opinion the existing set-up has cut 50% off the potential lifespan of some expensive batteries.

    I think you're trying to give him good advice and he's ignoring it. Either charge double :p or walk away.
  • Marc KurthMarc Kurth Solar Expert Posts: 453 ✭✭✭✭
    Re: Battery string switching

    Bill,

    I will respectfully disagree and say that Concorde would prefer to see at least 20% for deep cycle applications - more would be better.

    Marc
    I always have more questions than answers. That's the nature of life.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Battery string switching

    It's really simple. If he wants to add more batteries he's going to need to add more PV to charge them. Wind turbine? Fuhgeddaboudit.
  • BB.BB. Super Moderators, Administrators Posts: 29,360 admin
    Re: Battery string switching
    Marc Kurth wrote: »
    Bill,

    I will respectfully disagree and say that Concorde would prefer to see at least 20% for deep cycle applications - more would be better.

    Marc

    Why?

    AGM batteries can take 20% rate of charge-But they do not need it.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Marc KurthMarc Kurth Solar Expert Posts: 453 ✭✭✭✭
    Re: Battery string switching
    BB. wrote: »
    Why?

    AGM batteries can take 20% rate of charge-But they do not need it.

    -Bill

    Bill:

    Per the Concorde engineering guys: When deep cycling their batteries, 20% is the minimum in order to achieve long battery life. Very shallow cycle or float duty reduces this requirement in a non-linear fashion.

    Please understand that I'm not meaning to challenge theory or common practices. Most of my work involves industrial/scientific/OEM applications where the limits really get pushed.

    Example: A customer removes the vent caps (voids the warranty) on 300 amp 6v batteries (PVX-3050's) - on three trays of (24) at a time in unmanned deep sea vessels. Then they bring them back up to the host ship to re-seal then charge at 250 amps while another (72) are discharging below. This is per vessel and there are eight vessels.

    Example: Fleets of buoys deployed in the arctic ocean (6 PVX-1530's each) with special hulls designed to pop-up on the ice in lieu of getting crushed. We helped layout the PV/CC/battery system to survive the winter. Since we got involved, most of the of the buoys now start reporting back to the satellite as soon as there is enough solar to wake up the system in the spring. It used to be 20% because of catastrophic battery failure - before we got involved.

    There are countless really cool applications to tell stories about, BUT there are also some very ugly failures from misapplications too. My point is that we do a lot of stuff so I have learned to listen to the very sharp folks at the factory.

    The main thing I've learned is that I have a lot yet to learn!

    See page 19, under charging: http://www.sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf

    Best,
    Marc
    I always have more questions than answers. That's the nature of life.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Battery string switching
    Marc Kurth wrote: »

    Great info! Thanks for sharing.
  • BB.BB. Super Moderators, Administrators Posts: 29,360 admin
    Re: Battery string switching

    Not a problem, I too am always looking to learn.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Battery string switching

    i find it interesting that they specify a 20% rate of charge and seems contradictory to almost every lead acid battery i know of. i haven't had any problems with lower charge rates and have been having a good lifespan. agms are a more efficient battery so it does seem to handle the low %s well imho. i doubt many people will opt to expand their pv systems just to meet this 20% goal, but if one does it would take advantage of being able to fully charge one's batteries in the few hours of full sun available to most of us.
  • Marc KurthMarc Kurth Solar Expert Posts: 453 ✭✭✭✭
    Re: Battery string switching

    Th key is depth of discharge. Note that the statement in the manual refers to deep cycle applications needing 20% to maintain battery lifespan. In practice I've found this to be true - the deeper you cycle them, the harder you should charge them.

    General rule of thumb: we see more premature battery failure from undercharging than anything else.
    I always have more questions than answers. That's the nature of life.
  • The Only SargeThe Only Sarge Solar Expert Posts: 164 ✭✭✭✭✭
    Re: Battery string switching

    I will not comment on any of the previous comments...but will throw out a thought/question on the subject.

    If I read and comprehended correctly the question on the table is a fella wishes to add a battery bank to his existing battery bank. Keep the same PV Array and charge both once a certain voltage has been reached switch (auto) to charging the 2nd (new) battery bank.

    Could a fella not use something like this?
    http://www.colemanair.us/vp_asp/scripts/shopexd.asp?bc=no&ccode=DLC_CKIT

    Parallel another relay to dump in the event both battery banks are fully charged?

    Just thinking out loud.:roll:
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery string switching
    Marc Kurth wrote: »
    Th key is depth of discharge. Note that the statement in the manual refers to deep cycle applications needing 20% to maintain battery lifespan. In practice I've found this to be true - the deeper you cycle them, the harder you should charge them.

    General rule of thumb: we see more premature battery failure from undercharging than anything else.

    That's exactly what we see around here too.
    Your 20% recharge rate means I get to say "you need more panels" even more often than I do already! :D

    Sarge:
    That 12 Volt diversion controller might be adaptable to the application, but it's not an off-the-shelf solution for it. In this case were looking at a lot of Amps on 48 VDC so there's still a need for a big relay. Frankly that's the part I wouldn't trust.
  • VicVic Solar Expert Posts: 2,970 ✭✭✭✭
    Re: Battery string switching

    I almost always seem to be on the wrong side of these two topics.

    To me, The substantial mixing occurs in Asorb, and then, really, at the later stages of Asorb.

    The reasons for high charge rates -- ten or more percent of C/20, is when deep cycle batteries are deeply cycled, and this high rate of charge helps open/keep open active plate material.

    The MIXING occurs when the charge efficency is the lowest -- where the major gas production occurs.

    Just MHO, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Battery string switching
    Vic wrote: »
    I almost always seem to be on the wrong side of these two topics.

    To me, The substantial mixing occurs in Asorb, and then, really, at the later stages of Asorb.

    The reasons for high charge rates -- ten or more percent of C/20, is when deep cycle batteries are deeply cycled, and this high rate of charge helps open/keep open active plate material.

    The MIXING occurs when the charge efficency is the lowest -- where the major gas production occurs.

    Just MHO, Vic

    sorry vic, but we are talking about sealed agms and they have no mixing or gassing with proper charging.
  • VicVic Solar Expert Posts: 2,970 ✭✭✭✭
    Re: Battery string switching

    Hi niel,

    YES, I do see the AGM reference starting at about post #8 ... Was just referring to the glancing reference to Charge Rates of 5% of Capacity equating to mixing ... and just throwing out my thrice-yearly comment that my personal opinion regarding mixing equating to higher charge voltages and higher state of charge and so on This if of course for FLA, vented batteries.

    That is all, was NOT referring to AGMs. Thanks, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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