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Deep cycle battery full state of charge

Discussion in 'Power Electronics' started by brianwjohnston, Jan 31, 2010.

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  1. brianwjohnston


    Jan 27, 2010

    I am a newby to this forum and have a limited knowledge of electrochemistry.

    I recently installed a small battery bank of two Exide hybrid marine group 31 deep cycle batteries 205 reserve minites,115 AH each(will be upgrading to three in 2 weeks).

    The purpose of these batteries is to be used as household backup system for power failures.

    The batteries are wired parallel and hooked up to a Xantrex 1875(1500W) modified sine wave inverter.

    Charger is Statpower Truecharge 20 capable of charging up to 400 AH bank, automatic bulk, absorption and float, with a manual 15.6 V desulfation (mine will only be 345AH).

    I just installed and charged the battery bank, and applied a 20A load (about 300 W AC from inverter).and noticed that the voltage only started at about 12.22 VDC (which I believe is already in a state of about 40% of discharge).

    My intention is to not allow the batteries to go below 50% DOD (12.06 VDC), unless of an emergency.

    I allowed the bank to drop to 11.6 VDC (80 DOD), and recharged.

    I have done this for several consecutive days and I cannot for the life of me not understand why the bank does not start out at 12.6 to 12.7 (full charge).

    On my previous bank of group 27 batteries, the voltage after (allowing the batteries to settle down from the float charge at 13.34 VDC) was always close to 12.6+ (for nearly 10 years).

    The specific gravity in 5 of the 6 cells is 1.270 or better, and one cell is around 1.255.

    Would it be a reasonable assessment that desulfation at 15.6 volts for 6 hours or so help to get the initial voltage up to 12.7 volts.

    The float charge is 13.34 volts, the electrolyte levels are all above the plates.

    The mfg dates on the batteries are May and June 2009, and charged only only now, can this be the culprit?

    I hope I can get some assistance with my problem on this forum section, if I goofed and posted in the incorrect section, please let me know and I will transfer to the appropriate section.

    Best regards,

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I would try to get information from the manufacturer. They almost certainly have some publication which would assist you.

    This may also help you. Check out the graph of voltage vs charge level for various discharge rates. These will also vary with temperature.

    According to that, C/10 discharge will give you approx 12.5V for a 100% charged battery. Your rate looks like C/12 (ish) so I would say 12.22 is in the ballpark (but according to this graph, indicates about 60 to 70% charge). I assume (from reading this document) that the temperature they are using is about 78F.

    If you say the batteries are already 40% discharged, then the figures match very closely.
    Last edited: Feb 1, 2010
  3. brianwjohnston


    Jan 27, 2010
    Deep cycle full charge

    Thanks for reply,

    My real problem with this battery bank is that at 77 degrees farenheit I need to know the starting voltage.

    I am conversant with the peukert effect (non linear and the Peukert coefficient), but the self proclaimed expert at Canadian Tire states that full charge is 12.2 volts (this in reality is about already 40% DOD).

    I believe that the full charge state should be 12.65 to 12.8 VDC.

    When I apply a load of 20 Amps on battery side, the banks drops from 13.34 volts to 12.20volts, but I am of the opinion that after the residual charge is expunged, the remaining voltage would be full charge at 12.65 to 12.8 VDC.

    I max out at 2 hours from a two battery bank wired in parallel from 12.20 volts down to 11.57 volts (80% DOD).

    The batteries were at a Canadian Tire outlet in Quebec, with a manufacture date of May and June 2009, with the <charge before> stickers removed.

    I got in touch with Exide (private label manufacturer of Canadian Tire Nautilus brand) and have agreed to exchange the two batteries already purchased (about a few weeks ago, along with the third I ordered at the same time),

    As well they assure me they will have a new wet date of around January of February 2010.

    Would you know if a deep cycle battery sitting around for 9 month off charge could cause this dramatic drop in voltage ( I would expect a DOD of 80% from a 2 bank battery set) to be at lease 4.5 - 5.5 hours, not the two hours I am now getting.

    Any thoughts,

    Best regards,

  4. Resqueline


    Jul 31, 2009
    Getting only 40Ah from a 110Ah battery that has had the <charge before> stickers removed and which have likely been sitting for 9 months certainly tells me the full story; it's sulphated.. Even if it's a so-called deep-cycle battery I'm positive it gets as damaged below 50% as a starting battery does. Time is a factor in the process.
    Self discharge is highly temperature dependent. At room temp's it shouldn't sit for more than 3-4 months between each charge, but if put in the deep freezer it would easily manage two years between each charge.

    My previous information & experience is also that the fully charged resting voltage is around 12.75V - but the link steve provided claims that this is unreliable and that it should be loaded somewhat to get a good indication. Maybe that's what your CTspe is talking about (without actually knowing what he's talking about)..

    Would you like me to move this thread somewhere it would get more attention btw.?
  5. brianwjohnston


    Jan 27, 2010
    By all means, I really need help.

    I am by no means an electrochemist.

    BTW, I die desulfate at 15.6 VDC for 6 hours twice in one week, with almost no improvement.

    Incidentely, the good guys at Exide are taking care of the Canadian Tire "Expert" real well.

    Thanks again,

  6. shrtrnd


    Jan 15, 2010
    I agree it's probably your batteries themselves. Your expected change-out with replacements (if THEY"RE good), should solve the problem.
    9 months on the store shelf was not good, but manufacturing problems with the plates and/or electrolyte are always a possibility. It really sounds like in this case though, the batteries sat on the shelf too long before you bought them. Most people would not notice a problem, as the batteries would appear to work fine in normal applications, it's just that they wouldn't last for the expected number of years.
    When you get your replacements, make sure your charging voltage/current is not excessive. I would VERIFY your charging system output is what it's supposed to be. It's probably ok, but ASSUMING something, doesn't always make it so.
    Good luck.
    Last edited: Feb 11, 2010
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