Parrallel Batteries and charging

Rngr275
Rngr275 Solar Expert Posts: 127 ✭✭
in Off Grid Solar & Battery Systems #1
As my "Affair" with my batteries continues to grow I am beginning to finally feel like I am getting a grip on the whole charging, or as I like to say "Keeping the Girls HAppy". I have noticed though that as time goes by (a month or so) that the cell to cell SG seems to be drifting appart. It seems and I will verify this by doing another "Every Cell" measurment at the end of the month.... that the ends of the battery bank typically have a little higher SG than the center of the bank and the outter cells of a battery are a little higher than the center cell. We are not talking huge differences but measurable (0.010-0.015 range). Can or could this be happening or is it a random thing that I am seeing a pattern (or making it up)?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: Parrallel Batteries and charging

    you're learning. and just like girls, they are never totally happy.:roll: even in a simple multi-battery series arrangement that one would consider perfect that for some reason unknown to me the ends of the bank tend to charge/discharge differently ever so slightly. maybe heat differences? who knows. now you have paralleled batteries which is even harder to maintain equality in. an eq should bring that into line. your alternative may be to switch battery positions at some points putting the highest into the lowest positions based on sg. not so easy to do with heavy batteries. as usual, check all connections periodically.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: Parrallel Batteries and charging
    niel wrote: »
    the ends of the bank tend to charge/discharge differently ever so slightly. maybe heat differences? who knows.
    Seems like a viable idea. Have you tried measuring temperatures (if you care)? Do they drift toward a lower or higher SOC than the inner ones?
    SMA SB 3000, old BP panels.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: Parrallel Batteries and charging

    my setup would not be ideal to test such a thing and most could just give a small eq once in awhile to straighten up this phenomenon as it slowly appears. it's either that or rearranging of batteries could help. if the difference occurs frequently then suspect something in the connections or wires to be throwing it off farther. as i said, parallel connections are even more difficult to keep equal than the series type connection.
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  • Crotalus
    Crotalus Solar Expert Posts: 26 ✭
    #5
    Re: Parrallel Batteries and charging

    I found this to be interesting on battery connections.

    http://autonopedia.org/renewable_energy/Energy_Storage/Batteries_In_Parallel.html
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Re: Parrallel Batteries and charging

    That looks like it was from the Smart Gauge site.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #7
    Re: Parrallel Batteries and charging
    BB. wrote: »
    That looks like it was from the Smart Gauge site.

    -Bill
    Yeah, nothing is exclusive on the internet, they did leave out the complicated setup , Please donate. Everyone should have a site like that. 99% of whats out there is all copy and pasted, plagiarized, passed on BS rules of thumb.
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  • YehoshuaAgapao
    YehoshuaAgapao Solar Expert Posts: 280 ✭✭
    #8
    Re: Parrallel Batteries and charging

    Looks like if you parallel your strings on bus bars, you won't have to worry about any of that complicated wiring. Positive Inverter cable -> busbar (center) -> battery strings -> busbar -> negative inverter cable (connected in center of busbar). Illustrated in http://www.sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf - Page 18. I'm doing the same thing, but using the PD busbars in the PDP as I only have two battery strings - this requires separate inverter cable sets, but that just means more wire bandwidth. My battery strings are within 0.2A of each other (measuring following advice because I'm having battery capacity problems, most likely due to sulfation).
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  • Crotalus
    Crotalus Solar Expert Posts: 26 ✭
    #9
    Re: Parrallel Batteries and charging
    BB. wrote: »
    That looks like it was from the Smart Gauge site.

    -Bill
    You appear to be correct on that. I have not seen the the Smart Gauge site before. Is the document accurate?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #10
    Re: Parrallel Batteries and charging

    The Smart Gauge site is accurate--The other site is just a subset. I guess it is accurate as far as it goes.

    There was one site that was scraping our posts and assigning random User ID/Information and reposting. I found the IP address they appeared to be using to scrape our site and blocked it. About all we can do.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #11
    Re: Parrallel Batteries and charging
    Crotalus wrote: »
    You appear to be correct on that. I have not seen the the Smart Gauge site before. Is the document accurate?
    In theory it's accurate until one battery in the line decides that it doesn't want to cooperate with the rest of them. It's hard enough to get 3 cells in a 12 V battery to remain consistent 100 % of the time in Charge / Discharge cycles with each other. Someday we'll see a BMS ( Battery Management System ) that will save a lot of headaches of dealing with Batteries.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #12
    Re: Parrallel Batteries and charging

    Just to be clear--The one string (ideal) to 3 parallel battery/battery strings (suggested maximum) is my personal suggestion. Others have confirmed that filling 4x or more battery cells is a pain and better if it can be avoided (and automatic watering systems can have their own issues). Also, diagnosing failed cells/cabling in large parallel systems is not always fun either. And there is the cost of extra fusing/breaker per parallel string (ideally) if you want to protect against the odd short circuit (not cheap).

    Our host has said before there are many others with 3 or more parallel strings and they were very happy with their installations and have not seen a negative impact on battery life in parallel connected banks.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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