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  1. #1
    amateur expert dugimodo's Avatar
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    Default Battery question for solar power

    Any Lead acid battery bank gurus around here?

    I'm fairly confident with the basics but google has failed me on one minor point.
    I use 4 x 12V 60AH batteries set up with 2 batteries in series in parallel with the other 2 batteries also in series. I'm having minor issues with balance with one pair of cells being about .4V different at times.

    I'm wondering if I should tie the midpoint together to create two 12V parallel batteries in series.

    So I have this going on (it's a bit flat and still charging - ends up at 27.5V total)



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  2. #2
    pcsourcepoint
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    Default Re: Battery question for solar power

    You could try connecting all positives and all negatives, or daisy chain back to back - positive to negative. but I think the ratio of voltage/current changes for the two setups. I found this out with LiPO's when connecting them up both ways to my 18 V drill. So you might get nominal 24 V but double the current as opposed to nominal 48V (or 12V) but half/quarter the current in the other setup.

    Google Image search is what I use for diagrams - results for multiple battery connections...
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  3. #3
    amateur expert dugimodo's Avatar
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    Default Re: Battery question for solar power

    It has to be 24V, not 12V or 48V which is what you're suggesting.

    How it is works, I am just wondering if the middle strap would improve the battery balance so I don't get one cell lower than the rest like I have now.

    Google gave me a lot of results, most of which are like my current setup. What I couldn't find was any information regarding what I'm suggesting.
    Just searched again and found this http://www.batterytender.com/connect...el-connections which does seem to indicate I need to rewire a bit.
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  4. #4
    Awaiting Enlightenment R2x1's Avatar
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    Default Re: Battery question for solar power

    For batteries to be hooked up in parallel, the batteries must match fairly closely and be of similar types, ages, and capacities. No great value in putting mismatching batteries in a set. The weakest battery will determine your capacity, but during charging the weakest battery may well get stressed more highly at high charge rates and lose even more of its already low capacity. If there is dissimilar chemistry, eg flooded lead acid mixed with gel, or absorbent glass mat bad things are likely to happen. Calcium-calcium batteries (low maintenance types) must not be mixed with older style, nor should old batteries be matched with newer ones even if they are the same type.
    The batteries should be individually charged at the 10 hour rate until they have reached the desired voltage (charge state) then left to sit overnight. A short period (10 minutes?) under moderate load while monitoring the voltage, then a brief pause before measuring the voltage yet again. Do this for each of the four batteries and only use batteries that match very closely. Mismatched batteries don't just average out, they degrade each other.
    Any battery that has its rest voltage more than a small fraction of a volt away from the nominal for that type is only suited as a recycling prospect at best. Cheap batteries are an expensive way of storing energy, they waste lots of it, don't last long, and in eager cooperation with Murphy will fail at the worst time. Make sure your battery and charger are matched. Traditional lead acid batteries (12v nominal) should be float charged to 13.8V, Calcium batteries of the low maintenance type require 14.3V. Battery specifications are liable to be misleading, take 60 AH out of your 60AH rated brand new battery just twice and you've shortened its working life a surprising amount. That 60 AH by the way is specified at a steady 3 amps at the ideal temperature on a brand new battery, ie the 20 hour rate. Pull that rate for 10 hours and your battery is theoretically at 50% charge, which is as low as you want to take it (and then as seldom as possible if you wish to get a long service life). Cheap batteries over a 10 year period tend to be neither the dearest batteries available, and never the cheapest ones available new. Used batteries are like Lotto tickets, occasionally they are great value but normally they are duds financially.
    So, 60 amp hour batteries should only be used as 24 AH normally, occasionally 30 AH; kept at the same temperature you are comfortable at without adverse weather gear; and discharged / charged at the 20 hour rate maximum. It is not unreasonable to consider battery specifications are drawn up by the marketing department mainly to aid in marketing. Technical specs and data sheets have to be viewed with the same attention to the fine print and unspecified conditions as a lovingly crafted insurance policy. Solar panels are similarly specified - the rated output is the absolute maximum the very best panel they've ever made can possibly achieve for a few moments in the worlds most amazingly favourable spot on a better day than anyone has ever recorded when the sun is putting out lots of ideal radiation and no nasty heat. Watch very carefully for it may not even be done only once. In the lab is a different story, but it is tricky to get approval to set up your array in their lab permanently before even tackling the long connecting cable difficulties.

    The intermediate strap would only assist the mischievous dud cells to share the misery quicker. Sorry
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  5. #5
    amateur expert dugimodo's Avatar
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    Default Re: Battery question for solar power

    Well thanks for all that, but only the last sentence actually addresses the question. And it turns out from reading what I linked earlier that my setup needs some work and is likely causing a charge imbalance, hopefully no serious harm has been done yet. Only rewiring and individually charging to get everything back to the same level will get me to a place where I can see how it goes. Also one of my link cables is a lighter gauge than the rest, another error on my part as I was only considering current capacity and not voltage drop.

    Incidentally my batteries are a matched set of 4 AGM lead Acid 60AH nominal deep cycle batteries. And yes I had noted that my "500W" of solar panels rarely exceeds 300W on a bright sunny day. Mostly I try to use the whole setup when the sun is shining. It's something I did as an experiment for a bit of fun and it has been charging my lawn mower batteries and my E-bike once a week for about 18 months now
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  6. #6
    Awaiting Enlightenment R2x1's Avatar
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    Default Re: Battery question for solar power

    Well, free-range solar power is obviously much better for your gear than the factory stuff the power companies foist off on you. Mind you, as soon as it leaves the controller, it becomes battery farmed power "made from local and imported ingredients".
    Still, some of it for your E-bike is going to be recycled.
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  7. #7
    Senior Member piroska's Avatar
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    Default Re: Battery question for solar power

    Solar.....it's in the news a lot lately.
    I ran the calculator and got

    https://www.energywise.govt.nz/tools/solar-calculator/


    Estimated years to get your money back: More than 25 Years

    Estimated total earnings over 25 years: -$1,300(money lost)
    Ex-pctek

  8. #8
    Senior Member 1101's Avatar
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    Default Re: Battery question for solar power

    if the batts were truly matched, you wouldnt get one @ 13v & one @12.6 in the same series path.
    UN-matched batts right there

    Get some new batts or better batts
    Then use the lower circuit with the link. That will balance the 2 left batts & balance the 2 right batts . The batts need to be the
    same voltage before adding that link , else you'll get a dump of power from the RHS 13v into the RHS 12v etc.

    its a 2S2P

  9. #9
    amateur expert dugimodo's Avatar
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    Default Re: Battery question for solar power

    Batteries are expensive so I won't just be replacing them while they are still doing the job, if necessary I will dump 2 of them out of the pack and just use the other 2. But first I'm going to individually charge and test them and rewire them to be more balanced.
    Got a very good deal on these slightly used but almost new so I will get what I can out of them.


    Quote Originally Posted by piroska View Post
    Solar.....it's in the news a lot lately.
    I ran the calculator and got

    https://www.energywise.govt.nz/tools/solar-calculator/


    Estimated years to get your money back: More than 25 Years

    Estimated total earnings over 25 years: -$1,300(money lost)
    Yeah I played with the calculator and it makes solar seem not very worthwhile. My setup is not intended to save me any money or do anything for the environment, it's just a small setup on my garage roof I built for fun to experiment with. Charging my lawn mower and E-bike is just a bonus because I might as well use the power for something. I also run the garage door opener off it in summer or if the power is out for some reason. In winter the batteries don't recover fast enough and tend to go flat if I use the door opener too much so I plug it back into the mains.

    I am watching solar prices though, in a couple of years when the mortgage is gone I might look at it seriously on a household scale.
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