over sized battery banks

Hi all,
I have noticed occasional references to "over batterying" pros and con's (mostly cons) as I have scanned and searched various posts on this forum and I apologize if I have missed a previous detailed discission of this topic.
With that said, I will start a thread for my first post here with my question concerning too big a battery bank.

I have been in my off grid cabin for a time now so have had an opportunity to observe the operation of the existing solar system. It is 24 volts comprised of 850 PV watts, 30 amp mppt charge controller, 3600 watt inverter, Honda eu2000i backup generator, and a large 1110 amp/hr battery bank (12 Trojan RE16Bs).

Usage averages ~1.5 kWhrs, ~55 amphrs per day. With lots of sun both winter and summer, the system fully charges nearly every day; 1/4 to 2 hrs absorb at 29.6 v (in winter up to 30.0 v which is the controller set limit temperature compensated), several hrs float at 26.4 v ( up to 27+ v or so temp compensated), max mppt output current up to 22 or so amps. The battery electrolyte can be heard "boiling" during the charge cycle. Water usage is typically around 1/2 gallon refill per month.
Clearly the battery bank never receives the "recommended" (~5% of battery capacity) 50 or so amps for charging.

I have never checked battery specific gravity as space allows room to get a hydrometer only to the first battery string.
In case you ask how does he water the back strings, there is an automatic battery watering system .

Question: Given the low watt-hr usage and the "apparent" frequent full charge (based on absorb and float voltages and times is it likely that the batteries are sulfating?
Comments on the pros and con's of large battery banks on low wattage use systems?
Thanks

Comments

  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: over sized battery banks

    You never said what the SG level on the one Battery. 3 Strings are not the best setup, but not the end of the world. If they were mine I would pull a couple out of all the strings and see how the SG's look. If at the end of absorb they are 1.250 - 1.260, I'd say they are ok. If they are low, then you need to up the absorb time and / or Voltage. If they are in the 1.1xx - 1.200 range then they need a EQ.

    I'd also pull them deeper once a month or so, You need to keep the electrolyte stirred up in them. The short Absorbs work against you.
  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: over sized battery banks

    You are drawing (a guess) around 10% of the power from your battery bank. Lead acid batteries will be a bit happier if you draw them down by 20% (or to 80% state of charge)--You can do that, as BC04 says, by simply turning off the solar charge controller for 1 day.

    Another issue is that you really need a minimum amount of charging current to stir up the electrolytes (and possibly even to help "unplug" the pores on the plates...
    • 1,110 * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,045 Watt array minimum
    • 1,110 * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,090 Watt array nominal
    • 1,110 * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,717 Watt array cost effective maximum

    Your 850 Watt array in a typical location that gets a minimum of 4 hours of sun per day (9+ months of the year), would generate a minimum of:
    • 850 watt array * 0.52 end to end system eff * 4 hours of sun = 1,768 Watt*Hours per day of 120 VAC power (minimum typical clear day)

    So--Here you are--The batteries, if everything is going well will last you--Call it 8 years as they are now.

    If you had 1/2 the battery bank, the batteries will last you ~8 years again. For 2x the amount of cash, and 2x the risk if those covered batteries you cannot check are actually "unhappy" right now.

    And you will have a much better rate of charge for the battery bank (well over 5%) without adding one solar panel.

    The amount of power you can pull from your bank if you use the 25% discharge rule of thumb (nominal--25% per day for two days, recharge 3rd day to keep battery from going much below 50% State of Charge):
    • 1,110 AH * 12 volts * 0.85 inverter eff * 1/2 days storage * 0.50 max discharge = 2,831 WH per day

    Or almost 2x your present load...

    Unless you have some large surge load (like starting/running a large well pump or similar)--You are just driving around in a bus when a 2 door car will do just as well.

    Not exactly wrong (although, I would still recommend you add some more panels to your array to get you >5% rate of charge), it is just not optimum.

    If you cannot run the genset very much in bad weather (hard to get fuel, don't like the noise and fumes, etc.)--Then, perhaps the larger battery bank is a good solution for your needs (again, I would suggest a larger solar array for better average charging current--And running down the batteries 1-2 days without recharging once a month like BC04 suggests to cycle the banks--Floating/shallow charging can be hard on lead acid batteries).

    Energy is a highly personal set of choices. What works for me (Grid Tied in a major metro-area) or a couple moderators in the middle of the Canadian wilderness, may not work for you.

    And, you may need to re-rack your batteries. Maintenance (measuring/logging temperature corrected specific gravity, cleaning/tightening wiring connections, checking/logging per batter voltages once a month, etc.) will help you better understand your bank and catch problems before they happen. In a bank your size, you could have an open cell or cable and ever even notice it--until years later when the bank capacity falls--and you find out 1/3rd of your bank is already long dead.

    Besides a hydrometer, I suggest a DC Current Clamp DMM (like this inexpensive one from Sears--good enough for our needs). You can monitor the DC charging and discharge current per string and make sure that everything is reasonably balanced and you don't have any shorted or open cells (or bad connections).

    Also, watch your automatic watering system--Do not let it run unattended... There are a few folks here that have had 5 gallons of distilled water filling a 1 gallon battery (all on its own).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: over sized battery banks
    If they were mine I would pull a couple out of all the strings and see how the SG's look.
    BB. wrote: »
    Besides a hydrometer, I suggest a DC Current Clamp DMM (like this inexpensive one from Sears--good enough for our needs). You can monitor the DC charging and discharge current per string and make sure that everything is reasonably balanced and you don't have any shorted or open cells (or bad connections).

    Welcome to the forum, hjerald1,

    If you don't want to do what Blackcherry suggests, then at least do what BB. suggests and get a DC clamp ammeter.

    Another option is to install a battery selector switch and run 1 or 2 strings at a time. If you can switch battery strings in an out at will, that will give you a chance to see how each string is doing, it will allow you to discharge one string deeper, and to get that one string fully charged with your existing array. If you charge up a string and leave it out of the circuit for a week at a time, there will be no harm done.

    It is especially important, when equalizing, to equalize just one string at a time... if your strings need equalization it is because they are not balanced. If they are not balanced, equalization can make things worse.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: over sized battery banks

    you could also do a poster "NORTHERN" has done and draw down to 50% +- SOC once a week, no charging for as many days as needed, and then do a full charge cycle , using gen if needed to BULK, and PV to absorb and float if you get enough sun...
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: over sized battery banks
    westbranch wrote: »
    you could also do a poster "NORTHERN" has done and draw down to 50% +- SOC once a week, no charging for as many days as needed, and then do a full charge cycle , using gen if needed to BULK, and PV to absorb and float if you get enough sun...

    And if he does that, he will need to run his generator because his array will not be capable of recharging his entire bank in a reasonable amount of time. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: over sized battery banks

    While the Selector switch works, I'd think the best would be to try and get rid of at least one string. If the batteries are not to old and you can find someone to buy them it would be a better so you can manage the others. I have done the selector before and even with the best intentions it becomes a pain. As VT alluded your going to have to separate them anyway if they have to be Equalized. You'll get much more life out the two strings then trying to keep all 3 going.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: over sized battery banks

    precisely, he is in a hard place, low PV charge rate which will lead to early cell deterioration, vs run gen set, caused by using 3 strings of LARGE (Ahr) batteries.


    Matter of fact I am amazed that the 3 strings of 24V batteries ever get enough to charge at 24V and 30A (charger limited) total, that works out to 10A per string or ~2.7% rate. He must be getting a lot of good sun everyday...

    If he removed 1 string from the system by using a selector switch he would have a much better balanced system.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • dhellerdheller Registered Users Posts: 6
    Re: over sized battery banks
    vtmaps wrote: »
    Welcome to the forum, hjerald1,

    If you don't want to do what Blackcherry suggests, then at least do what BB. suggests and get a DC clamp ammeter.

    Another option is to install a battery selector switch and run 1 or 2 strings at a time. If you can switch battery strings in an out at will, that will give you a chance to see how each string is doing, it will allow you to discharge one string deeper, and to get that one string fully charged with your existing array. If you charge up a string and leave it out of the circuit for a week at a time, there will be no harm done.

    It is especially important, when equalizing, to equalize just one string at a time... if your strings need equalization it is because they are not balanced. If they are not balanced, equalization can make things worse.

    --vtMaps

    Being in a similar situation too many parallel strings, I am interested in more details regarding installing a battery selector switch. Any tips on how to wire that in to an existing system? I am going to research the web for the needed switch I am interested in how you would make the electrical connections. I think I would have to add a common bus and use two switches or something similar.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: over sized battery banks

    here is one at our hosts store.

    http://www.solar-electric.com/blseabaswon3.html
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: over sized battery banks

    My recently retired battery bank lasted 11 years and due to 3 parallel strings, failed rather suddenly. Turns out the SG remained good throughout their lives, even at the end, but due to the 3 strings and often light loads, first one string lost capacity and I didn't notice, then the second lost capacity and I began to notice but as the SG was all good, I was kind of fooled. Finally the 3'rd lost capacity and I was in trouble.
    By the way, during those 11 years, I don't believe I ever took them down 20% and usually only about 5%, and recharged at 5 % rate. The new bank I'm trying to work a bit harder with some heavy loads for cooking and in Spring, the mini-split heat pump, but still, especially as the micro hydro basically floats all regular loads, the batteries stay near full most of the time. I do need the battery size though for water pumping etc, to supply the surges. The micro hydro only became part of my former battery bank's last few years of life, so we'll see how the new bank survives. BTW, the new bank now has available a 10% charge rate since more PV have been installed.
  • Ralph DayRalph Day Solar Expert Posts: 952 ✭✭✭✭
    Re: over sized battery banks

    Wayne,
    Are you able to shut down the microhydro to force a deeper discharge of the battery bank? Just once in a while for a deeper discharge. I know the techs at Surrette said that the odd 50% dod will "open up more reactive sites on the plates" . Getting back up from 50% was a challenge for me, not so much with utility power now.

    Ralph
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: over sized battery banks
    By the way, during those 11 years, I don't believe I ever took them down 20% and usually only about 5%, and recharged at 5 % rate. The new bank I'm trying to work a bit harder

    Why would you want to do anything different? Eleven years is a great success! Did you ever use a DC clamp ammeter to see how balanced your three strings were?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: over sized battery banks

    Yes, I've read where Surrette recommends a deeper discharge, and I can shut down the hydro, and perhaps i should, but then I am also mindful of how long the last bank lasted without doing what some might consider abuse. But then others could consider not discharging them more deeply, to be abuse. One of the great mysteries of battery life. So many different ideas, so many different ways of treating them.
    Re using a current meter on the previous bank, I did check once at about 3/4 life point when I finally got a current meter and all appeared fine. Checked again when I started having serious trouble and OH OH Huston, we have a problem. But then it was too late, the book had been written and new batteries had to be ordered. Would it have helped if I had caught it earlier? I don't know. They were acting like the active material had all been used up, leaving only the underlying lead grids. SG still good, just no capacity left. They most certainly owed me nothing and I had to say: "Well done my good and faithful batteries". Now, their pampered life over, they sit cold and alone on a pallet out back of a friends Machine Shop, exposed to the storms, waiting for the recyclers to take them away. :cry:
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: over sized battery banks
    Now, their pampered life over, they sit cold and alone on a pallet out back of a friends Machine Shop, exposed to the storms, waiting for the recyclers to take them away.

    Do the recyclers care if the battery freezes and cracks before they pick it up? --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: over sized battery banks
    vtmaps wrote: »
    Do the recyclers care if the battery freezes and cracks before they pick it up? --vtMaps

    Good question. They are with a bunch of other batteries, probably all frozen, but so far none cracked or leaking. Time will tell if my old ones crack.
  • Ralph DayRalph Day Solar Expert Posts: 952 ✭✭✭✭
    Re: over sized battery banks

    If they crack and leak out their electrolyte then part of the recyclers job is already done...disposal of electrolyte :p. But the weight for salvage/scrap would change too...hmm.

    Ralph
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