Will my Batteries be damaged by a light load

KJ7YMKJ7YM Solar Expert Posts: 30 ✭✭
Several months ago we self installed a 1000W array with 4 Crown Batteries to run my refrigerator off grid. I have had all my lights off grid for a year or so now. I must say my electric bill really plummeted, and I could not be happier. But the system for the refrigerator was designed around my 25 year old refrigerator that pulled between 200 and 300 Watts to run depending on the internal/external heat load. Well recently it started pulling real high startup current, to the point of occasionally tripping my 3500W Conext Schnieder inverter. I was about to install a hard start kit and a delay on make relay on the old fridge, thinking that it was short cycling in the much higher summer temps.

But my wife pointed out that after a quarter century of use, it might be a good idea to think about replacing it. I was off like a shot guys, and returned shortly with a new unit. OMGosh are they more efficient now or what? I see that the new unit only pulls between 85 to 100 Watts, with the end result that the Solar System we built is just idling now. My logs show that it never gets down to even 25 Volts at night. Even with the cloudy days we just experienced.

We all know that in a perfect world you do not want to discharge below 20% DOD, but is there such a thing as to much of a good thing in this area? Does this new super low discharge rate mean I need to watch for other perils to the battery life? Or should I just consider myself lucky and look forward that mythical 8 to 10 year battery life span we all read about, but none of us have ever seen? :-)

Thans in advance for your comments guys. Your the best!

Larry

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    More or less, the ideal range is to cycle the battery bank to 80% state of charge between charging cycles and try not to go below 50% state of charge.

    If you only cycle to 90% state of charge between charging cycles--That is not great (shallow cycling for a deep cycle battery). And try to avoid going below 50% State of Charge on a regular basis (longer battery life). Although with fork lift batteries, you can cycle them deeper--But with solar, it can take several days of sun to get a deeply discharged bank back up.

    And try not to let the batteries set below ~75% state of charge for any length of time--They tend to sulfate (faster) at lower state of charge.

    There is an alternative way of cycling/operating a lead acid battery battery bank (please research/ask your battery vendor--This is a less common method of cycling).... Cycle the batteries between 50% and 80% state of charge, and bring the battery back over ~90% state of charge once a week (Rolls suggested at least once every 28 days maximum).

    Also keep an eye on electrolyte levels and specific gravity--Charging at >~85% state of charge can use more water (gassing). And shallow cycling can cause issues with specific gravity (cells having a spread of >~0.015 to 0.030 sg units or more between high and low cells, or possibly low specific gravity when you think the battery bank is full).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    KJ7YM wrote: »
    Does this new super low discharge rate mean I need to watch for other perils to the battery life? Or should I just consider myself lucky and look forward that mythical 8 to 10 year battery life span we all read about, but none of us have ever seen? :-)

    You are a perfect case study of the dictum: it's cheaper to conserve power than to generate power. If you had bought that fridge before you designed your solar power system....

    To answer your question: yes, repeated shallow cycling will damage your batteries. Lead dioxide clumps will form on the positive plates.

    The best way to deal with an oversized system is to skip charging on any day where your SOC is above a certain level in the morning. For example, if you are at 90% SOC in the morning and skip charging for the day, you might expect to be at 80% the next morning. I don't know what batteries you have, but if (for example) your batteries were rated to have 2000 cycles at 80% SOC and each cycle took two days, you would go 4000 days on those batteries. That's pretty good!

    Furthermore, most/many batteries are considered dead when they are at 80% of their original capacity. Since your batteries are oversized for your needs, you can expect that after they are "dead" you will still have a couple of years of service.

    Several manufacturers offer solutions that involve skipping a day or two of charging. The best way to do it is to use a battery monitor that controls the charging decision by SOC.

    By the way, it's probably a good idea to float on the non-charging days. If you start (for example) at 90% SOC in the morning and go into float (without charging), you will end up at 90% in the evening and then the next morning be at 80%. If you don't go into float, you might drop down to 80% during the day and be at 70% the next morning.

    --vtMap
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • lkruperlkruper Solar Expert Posts: 115 ✭✭
    vtmaps wrote: »

    You are a perfect case study of the dictum: it's cheaper to conserve power than to generate power. If you had bought that fridge before you designed your solar power system....

    To answer your question: yes, repeated shallow cycling will damage your batteries. Lead dioxide clumps will form on the positive plates.

    The best way to deal with an oversized system is to skip charging on any day where your SOC is above a certain level in the morning. For example, if you are at 90% SOC in the morning and skip charging for the day, you might expect to be at 80% the next morning. I don't know what batteries you have, but if (for example) your batteries were rated to have 2000 cycles at 80% SOC and each cycle took two days, you would go 4000 days on those batteries. That's pretty good!

    Furthermore, most/many batteries are considered dead when they are at 80% of their original capacity. Since your batteries are oversized for your needs, you can expect that after they are "dead" you will still have a couple of years of service.

    Several manufacturers offer solutions that involve skipping a day or two of charging. The best way to do it is to use a battery monitor that controls the charging decision by SOC.

    By the way, it's probably a good idea to float on the non-charging days. If you start (for example) at 90% SOC in the morning and go into float (without charging), you will end up at 90% in the evening and then the next morning be at 80%. If you don't go into float, you might drop down to 80% during the day and be at 70% the next morning.

    --vtMap

    Would you mind providing the names of equipment that charge by SOC? What is that feature called, that I might google it?
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    lkruper wrote: »
    Would you mind providing the names of equipment that charge by SOC? What is that feature called, that I might google it?

    I know that some SMA equipment can do that. I believe (not absolutely sure... based on what think I remember) that victron, some outback and some schneider equipment can do it.

    Midnite charge controllers can be programmed to skip a day or two of charging, but it is not based on SOC.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,838 ✭✭✭✭✭
    Run a heat pump for loading your system! You should be getting down to about 24.5 volts overnight.
    I have had 2 sets of batteries in 23 years BTW. My clients rarely do that. The first set really was not bad but had lost capacity.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • KJ7YMKJ7YM Solar Expert Posts: 30 ✭✭
    Thanks everyone for your comments. So Dave with the old system I always shot for 24.8 Volts by morning. It was not uncommon to go down to 24.6 some time and I would panic. I have never been sure how to compute the running discharge voltage against what it would be at if I just let my food spoil for a 24 hour period and then tested the charge voltage after the rest period. :-)

    I could easily load the system down a little more by adding another device to the load, if you think it a good idea. My lights have been on a separate system for years. (Cost me $960 for that one VTMAPS). So that's out but maybe one of TVs would do the trick.

    Thank you all guys really, I so count on your experience a lot in trying to get things right.

    Surely the R.O.I. will be a long time coming, if I just count only the cost of electricity, but we have gotten much more value than that with these great homemade projects. The lighting system ran me $960, the bigger system ran me $4500 with the batteries.

    Since we first started working the solar angle into our lives, we have reduced our electric bill from $365.00 a month to $162.00 flattened out over the year. At first blush you would think $2400 a year in savings would make for a great ROI. But a lot of the solar savings is passive stuff, like the 40 foot false wall on the West side of my house, built 2 1/2 feet away from the original wall. It cost me 1500.00 and is responsible for nearly half the monthly savings. And as vtMaps certainly has figured out, the replacement cost of batteries at $1100.00 a pop will be what makes or breaks the solar power generator savings.

    My goal is to get that 8 year lifespan. That will make the difference between a savings project, or an expensive hobby. Either way I will still be happy. The independence of never paying APS again, for our lights and refrigerator and freezer electricity ever again is a neat thrill and gives me an ROI of a different but real kind. And considering that we live in a high power failure area. The cool factor of seeing the grid go down around us so often, while all our lights blaze and basic services still go on without a flicker is a value in itself also.

    Again guys, I cannot say it enough. Thanks for sharing your knowledge and experience with us.

    Larry

  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Definitely NOT trying to start an argument, rather I'm looking to be educated. I don't know everything - - -
    My last battery bank, six "Power Battery" L-16, lasted 12 or 13 years. Throughout their life they very rarely went below 90% fully charged, and after the mini-micro hydro came on line during the last 5 years of their life, except for supplying surges, could probably be considered to live mostly in float mode. For instance, at 5 O'clock this morning, like every other morning before the sun comes up, the system voltage is12.6 with one freezer, computer, air exchanger etc running. If one of the freezers isn't running (they and the "fridge" are interconnected with relays so only one ever runs at a time), the voltage goes back up to at least 13 volts during nighttime. Of course during daytime, the voltage is up around 14.4 to 14.6 (higher still in Winter) until falling back to float.
    My experience doesn't seem to connect with the generally accepted "truth" that off grid batteries should be taken down at least to the 80% fully charged state every, or almost every night. Is there real scientific information behind this accepted "truth", or is it possibly like the story about how we should always drink 8 glasses of water every day, a story started by one Doctor many decades ago, never questioned or scientifically investigated, yet unquestioned until recently, and accepted as truth by both Doctors and the general public? Again, NOT trying to start an argument, just looking for where this belief comes from, and if there is real scientific truth to it, does it apply to all deep cycle batteries, or to only certain makes or designs.
    Always looking to learn more.
    Wayne
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    during the last 5 years of their life, except for supplying surges, could probably be considered to live mostly in float mode.

    I see at least three possible explanations for the long life of your batteries:

    1) A battery that lives in float mode is never really being cycled... therefore it will not suffer the depredations of shallow cycling.
    2) A battery that has lost most of its capacity may still function perfectly well in float, buffering your surges.
    3) Your batteries are cool. The lower temp reduces their capacity, but they have enough to meet your needs.

    The question in my mind is: after a decade, could you still draw off half of its rated amphours without the voltage collapsing? Or, in other words, was it still a deep cycle battery? It sounds like you could get by with an automotive type battery... only drawing down enough to buffer surges and then immediately recharging it from your hydro.

    I have seen quite a few systems with batteries that last a decade or more. These installations are mostly 'old school' design with undersized solar arrays and oversized batteries. That type of design was common when batteries were cheap and solar panels were expensive. These batteries were undergoing chronic deficit charging, but could last a decade because even with half their capacity lost, they still had adequate capacity remaining to meet the needs.

    I hope that my batteries, which just barely meet my needs, will last 7 years... that would be a better outcome than having twice the battery and getting 10 years. Time will tell.

    Of course, this is all speculation on my part... I have no basis to argue the point, and even if I had a basis, I have no desire to argue :D

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • lkruperlkruper Solar Expert Posts: 115 ✭✭
    vtmaps wrote: »

    I see at least three possible explanations for the long life of your batteries:

    1) A battery that lives in float mode is never really being cycled... therefore it will not suffer the depredations of shallow cycling.
    2) A battery that has lost most of its capacity may still function perfectly well in float, buffering your surges.
    3) Your batteries are cool. The lower temp reduces their capacity, but they have enough to meet your needs.

    The question in my mind is: after a decade, could you still draw off half of its rated amphours without the voltage collapsing? Or, in other words, was it still a deep cycle battery? It sounds like you could get by with an automotive type battery... only drawing down enough to buffer surges and then immediately recharging it from your hydro.

    I have seen quite a few systems with batteries that last a decade or more. These installations are mostly 'old school' design with undersized solar arrays and oversized batteries. That type of design was common when batteries were cheap and solar panels were expensive. These batteries were undergoing chronic deficit charging, but could last a decade because even with half their capacity lost, they still had adequate capacity remaining to meet the needs.

    I hope that my batteries, which just barely meet my needs, will last 7 years... that would be a better outcome than having twice the battery and getting 10 years. Time will tell.

    Of course, this is all speculation on my part... I have no basis to argue the point, and even if I had a basis, I have no desire to argue :D

    --vtMaps

    The subject of over-sized battery bank and loss of life is something I have not heard of before. Not surprising since I am still learning. I make it a practice to read the manufacturer's usage manual with all their charts, etc., and re-read them as I gain knowledge. Do they (Trojan, Concorde, Rolls, etc.) cover this in any fashion in their documentation with usage/curve estimates?

  • Dave AngeliniDave Angelini Solar Expert Posts: 4,838 ✭✭✭✭✭
    To be more clear ! I am saying to cycle the battery bank fairly deeply on occasion. I do this in winter and summer with cooling and heating loads at night from a heat pump. The rest of the year I am much more like Wayne. The 24.5 V reading is before sunrise with just a refrigerator load cycling & inverter load. I am not saying I have rested the battery bank as that would be a deeper discharge.
    At lease once or twice a year I will get 3 days of poor solar and go deeper. I tell my 48 V customers 49 Volts early am. This has worked along time BTW.

    I agree with vtMaps also and there is no argument from me on any of this. I do tend to oversize battery and always solar because some of my clients are really remote and replacing batteries is alot more difficult than paying for them. I want them to last 7 to 10 years. What I want and what I get are often two different things because of maintenance and not following the simple rules I give them.
    I always tell them to put a copy of the battery check they wrote on the wall near the bank! Even that does not always work. Some can afford this but I think it is wasteful.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

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