battery behavior

pedro65
pedro65 Registered Users Posts: 21 ✭
in Off Grid Solar & Battery Systems #1
My saga began with a new off grid house with an underpowered set of panels and terminally undercharged s460 batteries. 4 of the 16 batteries were completely shot (a two year old system by the way) and the rest recoverable to some level at least. I added 4 new batteries, at that time about a year after the bank went into service.

Since that time I have added another kw of solar production (now about 3kw) and an FN DC. The system has been running well, and I have been able to equalize nicely and getting good spgr readings.

I am having a hard time understanding the behavior of the new batteries though. The older batteries cycle through sp gr as expected (perhaps running down faster than they ought, but not surprising giving the near terminal undercharge conditions they were kept in). The new batteries seem to hold their sp gr in an almost supernatural form. If the bank is down to 1.25 overnight, the new batteries are reading 1.65. Now I understand that the older cells aren't going to hold charge well, and that they should be bleeding off charge from the new ones. But I can't really make sense of this current behavior.

What am I missing?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: battery behavior

    Possibly the wiring. it could be arranged in such a way that the old batteries are doing most of the work.
    Can you give some specifics about the arrangement?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: battery behavior
    pedro65 wrote: »
    The new batteries seem to hold their sp gr in an almost supernatural form. If the bank is down to 1.25 overnight, the new batteries are reading 1.65

    1.65 is not a possible (or at least not normal) specific gravity...

    12, 24, or 48 volt bank? The new batteries are in one string or one per parallel string or what?

    I know I keep pushing the DC Current Clamp Meter--Maybe I should start a web store that cells them? ;)

    Using it, you can figure out if which strings are charging and discharging... And help you isolate a weak cell/battery/connection/cable.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: battery behavior

    My brain "auto-corrected" that to 1.265. :p Maybe that's wrong. "1.65" certainly is.

    From his sig he has a 48 Volt system, so two strings of eight. Probably one string of old and one of new (as opposed to mixing within the strings).
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: battery behavior

    Well duh... If you want to read the whole post and find out what his battery bank is that way... :blush:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • pedro65
    pedro65 Registered Users Posts: 21 ✭
    #6
    Re: battery behavior

    yup coot, I did the same thing....skipped the two.

    The new batteries are mixed in among the older ones, some in the middle of the string, some on the ends. Doesn't seem to matter much where they are though. It is two strings of eight.....with the new ones mixed in.

    It always could be the wiring, but seems unlikely that all the wiring is equally hosed up in such a way that only the new batteries are getting the advantage.

    My assumption is that the old batteries are just discharging much faster since they are sulfated....but I am not sure how sulfated and healthy batteries behave together?

    Thankfully it is a place I use on the weekends, and I have enough solar that I can get by with the batteries as-is, but I am curious as to whether this is expected behavior or not.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: battery behavior
    pedro65 wrote: »
    My assumption is that the old batteries are just discharging much faster since they are sulfated....but I am not sure how sulfated and healthy batteries behave together?

    Thankfully it is a place I use on the weekends, and I have enough solar that I can get by with the batteries as-is, but I am curious as to whether this is expected behavior or not.

    That explains it. Mixed in with the old is like constructing a string of batteries where some are 'X' Amp hours and some are 'Y'. The internal resistance will be different. It would actually be better to have the new ones in one string and the old ones in another. Better yet is separating the two strings with a 'A' / 'B' switch and not connecting them at any time. Best of all would be all new batteries, of course.

    This is a good example of why mixing old and new is bad.
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,022 ✭✭✭✭
    #8
    Re: battery behavior

    ''Best of all would be all new batteries, of course.''

    It's always fun to spend other people's money;). I'm accused of that all the time. Some friends won't take me shopping for project materials any more. I have caddilac tastes and they have a Chevy budget. (it's easy to tell I haven't car shopped in years!)

    Ralph
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