Two Battery Banks

Wilis
Wilis Solar Expert Posts: 84 ✭✭✭✭
in General Solar Power Topics #1
Hi,

First-I would like to thank everyone who posts on this forum. I have learned so much about my solar system. I was one of the start backward ones. Because of the Forum I am getting things almost figured out.

At this time I am doing a weekly grid charge to keep my batteries well mixed. All my SG is fine and the batteries are balanced. On sunny days I am able to charge my batteries, but at a lower amp rate than is recommended. My CC goes through its cycles and usually floats for at least an hour. I want to be able to charge the battery bank at the 10% rate. To do this I will need to have two banks of batteries and switch between them.

The question is:
Having two banks of batteries, I understand to keep each bank charges as much as possible, but once a bank is charged and removed from the CC how long is it good for it to set? Sometimes we have many days of sun, so I could run from one bank all of these days and leave the other charged bank setting. How often should I change between the two banks? Once one string is charge and just setting, how long should it set before I switch it back into service? Will a weekly change be ok or can I just change them monthly? How do others do it that has two banks?
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Two Battery Banks

    Interesting question/concept...

    Perhaps, I would only switch to a single bank when equalizing (if you cannot make ~5% with both banks in parallel).

    And you should only be equalizing when the batteries need equalizing (if cells are over 0.050 different in specific gravity, or if all the cells are are low specific gravity (you should be able to get to 90% state of charge at least once or twice a week?).

    Equalization should be rare (once a month to once every six months) if all indications are OK.

    If you look at the cycle life charts... If you cycle 1/2 as often (separate banks) but twice as deep (only one bank on-load)--the overall cycle life of the battery is roughly the same (1/2 the number of cycles, 2x life; make 2x deeper cycles, cut life by 1/2...).

    I would suggest getting enough panels to make 5% charge rate on your "final bank" and let it go at that--don't worry about switching A/B once a week... It probably is not worth the hassles.

    If you find yourself needing more power (having to use backup power source to bring the batteries to full charge) outside of the winter months--then you would probably want to look at more solar panels (or more conservation) instead. Switching between banks is not really going to do much in that regard.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Wilis
    Wilis Solar Expert Posts: 84 ✭✭✭✭
    #3
    Re: Two Battery Banks

    Thanks Bill,

    The one question I was trying to get answered was, how long should a fully charged flooded battery sit without being charged? I guess I didn’t communicate as well as I should have. I have read most of the post for the last few months and understand many things about running a solar system that I didn’t know 6 months ago. Thanks to you and others that have posted.

    I have been checking my SG regularly and it has been fine. I have 4 parallel strings. All balanced resistance. I would like to have only two. I purchased what I have before I knew better. I should have purchased larger batteries to start with. Now have to live with what I have until their live cycle is thought. Two parallel strings will run my small load for three days in an emergency. I am paying less than 9 cents per KW, so solar is just a backup for me.

    I will be purchasing more PV’s but have to save the money. I want to do the best for the batteries until then. We have a mostly electric house but our electric stays under $50 per month except when we run AC. I will be going to a small AC unit for the bedroom. That will help use less power.

    So again what I would like to know is-How long can a fully charged, flooded battery sit between charges? Thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Two Battery Banks

    You're wondering about the 'self-discharge rate'.
    Unfortunately that varies with the individual battery and the conditions it exists in.

    As a rule, AGM's and gel batteries have an extremely low self-discharge compared to flooded cell types. The amount can be anywhere from 1% per month for an AGM to 1% per day for an FLA. Sorry that's not a better answer.

    I can see what you want to do: divide your existing bank into two smaller banks so that the charge current level is increased to help keep the electrolyte stirred and reduce over-all sulphation. The easiest way to do this is to get a switch like this:

    http://store.solar-electric.com/basw1300amp.html

    Then just switch between using/charging each bank half daily. If you trade off every other day there should never be a problem with self-discharge.

    I have to ask: you have a mostly electric house with utility power yet you have a battery-based solar electric system as well? Why? Emergency power back-up?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Two Battery Banks

    Monitoring the resting voltage and specific gravity will tell you that... If the batteries get below ~75% state of charge, then they will begin to sulfate.

    So--New batteries self discharge less, older batteries self discharge more. AGM's have very low self discharge (a few percent a month) and forklift batteries (when they are old) can self discharge 1-2% a day...

    So AGM's can go for 6-12 months between charges. Flooded Cell perhaps a few months. And Forklift batteries a few weeks.

    I would look at either:
    • hitting them with a charge when you think they are dropping below ~85% state of charge.
    • getting a float charger. Keeping the batteries between 13 and 13.6 volts or so will not take much energy and use hardly any water--Does not require you to monitor/control the state of charge of the bank every few weeks.
    What I do for cars that sit (and especially have those annoying loads like On-Star, GPS, Lojack, etc. which can draw 0.180 amps or so)--I take a small battery charger and connect it to a lamp timer. I set the timer for 1-4 hours or so of charge per day ("dumb car"--maybe 1 hour per day; "smart car"--maybe 4 hours per day).

    That has saved me (and relatives) lots of batteries--those that did not go "dead" and also those that did not boil dry from the cheap wall wart chargers I got (back before I knew that there were good wall wart float chargers).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Wilis
    Wilis Solar Expert Posts: 84 ✭✭✭✭
    #6
    Re: Two Battery Banks

    I have to ask: you have a mostly electric house with utility power yet you have a battery-based solar electric system as well? Why? Emergency power back-up?

    Thanks again for the answers.

    Coot-We do have several power outages a year. In winter we can have ice that takes power out several days. I live out far enough we are the last to get power back. I know my gen would do, but I have used this solar as a learning experience. We heat with wood and now we could live several days without grid. My loads are refrigerator and small freezer. My water is well and now we can get that. I would not want to live without grid forever, but could for several days. People in the US are so used of the grid power being there. What if we have a earthquake or other natural disaster how many could live until things get back to normal? Anyway I just need something to keep learning about and solar has been a good learning experence. Thanks:D
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #7
    Re: Two Battery Banks

    Having two banks makes life more complicated. I would not worry that much about not hitting a 10% of A-H rating current for charging unless your batteries are not getting fully charged in a normal day from your panels.

    When you have two banks you will end up with unequal charging and you will lose the instantaneous power supply capability of the the combined battery bank.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #8
    Re: Two Battery Banks
    RCinFLA wrote: »
    Having two banks makes life more complicated. I would not worry that much about not hitting a 10% of A-H rating current for charging unless your batteries are not getting fully charged in a normal day from your panels.

    When you have two banks you will end up with unequal charging and you will lose the instantaneous power supply capability of the the combined battery bank.

    BINGO!!!!


    Adding a second bank only adds to the troubles and will make it more likely to have a failure for any number of reasons.

    Just for the record, we have been off grid for over ten years, and just this past winter have we gotten to the point where we can charge from the PV at 5% rate. For many years we would only charge at about 1%! In spite of that, we have had very good battery longevity. In fact the original set of T-105s is still going strong on a second system 10 years in.

    Make sure the get charged after being drawn down, if you got the grid or a genny, plug them in to a big current charger once a month or so and don't worry about them.

    Remember KISS.

    Tony
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Two Battery Banks

    Sorry to disagree with you gents, but a dual bank system isn't some horrible maintenance nightmare.

    Besides which, what the OP is asking about is splitting his existing bank into two more manageable parts.

    If he uses the switch I suggested, he can leave them together (1+2 setting) for daily needs and separate one or the other out if he needs to boost charge current to get EQ done.

    It is a viable solution.

    For clarification, the reason I always tell everyone to "shoot for" 10% charge Amps when designing their system is because everyone forgets about panel de-rating and the inevitable draw of loads while charging. Shoot for 10%, you'll probably get your 5% minimum.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #10
    Re: Two Battery Banks

    Marc,

    I am not trying to suggest that it will be some maintenance nightmare,, I just think it a solution in search of a problem, ultimately leading trouble.

    One might do a calculate the cost of a second battery bank, factor in the longevity of two battery banks, and discover it is cheaper to run one, have it fail faster, but since the cost is 1/2 of two it might be net/net cheaper.

    Tony
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  • Wilis
    Wilis Solar Expert Posts: 84 ✭✭✭✭
    #11
    Re: Two Battery Banks

    Thanks again everyone. I think it would be a nice experiment with the switch. I will be able to see the different affect just 450 AH and with 900 AH make. I learn the best when I see things happening. My switch from NAWS was shipped today. It is coming UPS ground so about 5 days I expect. Will post results if I see something others will be interested in hearing. I have buss bars now so with the additional cables I ordered I will be able to keep the resistance in the cables balanced. ;)
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  • jeffkruse
    jeffkruse Solar Expert Posts: 205 ✭✭✭
    #12
    Re: Two Battery Banks

    I have a question about connecting the banks together. What happens if the batteries are not at the same state of charge? What if one is full and the other is at 80%?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: Two Battery Banks
    jeffkruse wrote: »
    I have a question about connecting the banks together. What happens if the batteries are not at the same state of charge? What if one is full and the other is at 80%?

    Not much. It is a bigger problem to "simulcharge" (new word! Do you like it? :D ) two dissimilar batteries than two of the same type that are at different SOC.

    Mostly, you try not to let them get too far apart. My method is to keep them tied together 99% of the time, and only switch one bank out when it's time to EQ. Note: this uses two switches; one for the inverter connection and one for the charge controller so that the bank being equalized is not supplying loads.

    If you were going to do this sort of thing to have a 'back up' bank, it would be better to use AGM's at least for the auxiliary. This would reduce the self-discharge problem. But the extra cost of AGM's makes adding even one more panel more attractive, unless doing so would require major revamping of the whole system.

    Ironically, my next battery bank plan will eliminate the two bank system. Not because it doesn't work or is too much hassle, but because I keep experimenting with different set-ups as a matter of course. Not a recommended course of action, btw! :p
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  • Wilis
    Wilis Solar Expert Posts: 84 ✭✭✭✭
    #14
    Re: Two Battery Banks

    What I am planning is to charge all batteries most of the time. About once a week do just ½ of the batteries. This way I will make sure they get a good charge. I am not required to replacing any water, so must not be getting quite enough charge. I will EQ as needed as I see SQ difference. I want to experiment but not hurt my batteries. When I do have to replace the whole battery bank I will replace with one series string. Now I know how to size the batteries for the number of PV’s. Thanks.
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