Battery issue? Or other?

I've had my system up for about a year now. During the summer I noted that when I drained my batterys to ~50% (trimetric reading) my voltage would be about 23.9v. So I set my inverter low voltage cut off to 23.5v. Now recently sometimes I wake up with no power becuase the voltage has reached the cut off but the trimetric says 70%. How did I loose that 20% of capacity to reach the same low voltage?

Note that I've only equilized once and only for an hour in the past year. This is an off grid place I only vist for about 5 days a month and when I return they are back up to 100% via solar charge contoller. I find that I have to run my propane generator everyday 1 to 2hrs now just to keep them up so the refridge and other stuff doesn't kill it overnight. Didn't have this issue several months ago. I turn the fridge off when I leave. My hydrometer was broke so couldn't check the SG last trip. I realize the days are shorter and more cloudy this time of year but why the 20% drop in capacity at the same discharge voltage?

XW60 CC, 440ah, 24V, magnum inverter, trimetric gauge..

Off Grid Cabin, 24V 440ah 6V GC battery bank, Xantrex MPPT60-150 CC, Magnum MS4024 inverter-charger, >1200w Solar bank

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery issue? Or other?

    Couple of things I see here which may/may not be an issue.
    First is that you're a bit shy on panel (820 Watts, yes?) for 440 Amp hours of battery. I see this:
    820 W @ 80% efficiency (best case) = 656 W / 28.4 charging Volts = 23 Amps - good enough for 440 Amp hours @ minimum 5%, but are you really getting all that power?

    This leads to the fact that as batteries age they go down in capacity. If they're not charged perfectly they go down faster (sulphation). So you may no longer have 440 Amp hours.

    Another thing is that battery Voltage is not a good indication of SOC, really. Your LVC @ 23.5 Volts is a bit high - you could set it at 22 or even 21.

    Then there's how you have the Trimetric programmed. And possible issues with temperature as well.
  • machinemanmachineman Solar Expert Posts: 129 ✭✭✭
    Re: Battery issue? Or other?
    Couple of things I see here which may/may not be an issue.
    First is that you're a bit shy on panel (820 Watts, yes?) for 440 Amp hours of battery. I see this:
    820 W @ 80% efficiency (best case) = 656 W / 28.4 charging Volts = 23 Amps - good enough for 440 Amp hours @ minimum 5%, but are you really getting all that power?

    This leads to the fact that as batteries age they go down in capacity. If they're not charged perfectly they go down faster (sulphation). So you may no longer have 440 Amp hours.

    Another thing is that battery Voltage is not a good indication of SOC, really. Your LVC @ 23.5 Volts is a bit high - you could set it at 22 or even 21.

    Then there's how you have the Trimetric programmed. And possible issues with temperature as well.

    Yes, the panels are on the low side, but they do get it up to 100% and in float mode while I'm not there. I did notice while using my inverter/charger and generator to boost them back to 100%, my XW60 CC went into float mode about 45min before the intevrter did. The XW does have temp probe and the batterys are in a closed concrete shed that rarely gets <40F in winter and >75F in the summer.

    The trimetric is programmed the same since the install.

    So LVC of 21 to 22V is OK?

    Also one time I waited about 5 months to check the water level and the caps has a lead paste build up on the underside. The water level was still above the plates and the thin grey paste wiped off. Is that normal?

    Off Grid Cabin, 24V 440ah 6V GC battery bank, Xantrex MPPT60-150 CC, Magnum MS4024 inverter-charger, >1200w Solar bank

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery issue? Or other?

    you could possibly go as low as 21v, but try lowering it in 1/2v increments until it does ok overnight. the problem is that when your frig turns on it will drop the voltage of the batteries even lower than the at rest voltage due to its heavy turn on surge.
    i'll let the guys with flas and those caps answer your concerns of normalcy, but use caution when contacting lead as it can do you harm.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery issue? Or other?
    machineman wrote: »
    Yes, the panels are on the low side, but they do get it up to 100% and in float mode while I'm not there. I did notice while using my inverter/charger and generator to boost them back to 100%, my XW60 CC went into float mode about 45min before the intevrter did. The XW does have temp probe and the batterys are in a closed concrete shed that rarely gets <40F in winter and >75F in the summer.

    The trimetric is programmed the same since the install.

    So LVC of 21 to 22V is OK?

    Also one time I waited about 5 months to check the water level and the caps has a lead paste build up on the underside. The water level was still above the plates and the thin grey paste wiped off. Is that normal?

    So you're good on keeping the battery temperature stable.
    Now, what sort of Voltage levels have you set for ABSORB and FLOAT? Normally a "24V" system will be set for 28.8 and 27.6, but if that's causing too much bubbling you can knock it back. Bit more art than science there.
    There shouldn't be any gray paste on the caps - sounds like there's a bit too much "activity" going on during charging.

    I think the Trimetric is supposed to be programmed for "useable" capacity. That is if you have 440 Amp hours of battery and want to keep DOD to a maximum 50% you program it for a 220 Amp hour bank. It then counts the electrons going in and out and gives an idea of the relative amount left available for use.

    Niel's right about knocking the low Voltage setting down half a Volt at a time 'til you get a figure that works for you.

    Pity about the disparity between the CC and Inverter readings. One good thing about Outback equipment; it will all "talk" to each other.
  • machinemanmachineman Solar Expert Posts: 129 ✭✭✭
    Re: Battery issue? Or other?

    The absorb voltage is 28.8v which was the defualt, not sure about the float voltage, I will check what the factory defualt is. I recall a thread somewahere where somebody was complaining about the XW60 CC hitting float to early.

    The trimetric is set to 440ah which is what the set-up instructions said to do. I will double check that. I have a feeling if I set it to 220ah then when at full charge it will read >100%. Will double check that also in a couple weeks when I get back there. I suspect the panels will get covered with snow this week.

    Still not sure how I lost 20% capacity over a 6month period unless the batteries are now broken into normal capacity. Or one or more of them are screwed up.

    Off Grid Cabin, 24V 440ah 6V GC battery bank, Xantrex MPPT60-150 CC, Magnum MS4024 inverter-charger, >1200w Solar bank

  • BB.BB. Super Moderators, Administrators Posts: 32,609 admin
    Re: Battery issue? Or other?

    In theory, your batteries should increase capacity some 10-20% or so in the first few months of cycling.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dagr51dagr51 Solar Expert Posts: 72 ✭✭✭✭
    Re: Battery issue? Or other?

    Do you have the "automatic reset of amp-hours" preference selected on the Tri-metric? This will reset the "Battery % Full" display as well as the "Amp-hours from full" display". If it doesn't get a chance to reset periodically, it may "drift" and not sync to the actual battery performance. See C.3. "Preferences" in the owners manual.
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: Battery issue? Or other?

    In regards to a "20% loss in capacity", as has been addressed there are many variables that could contribute to your issues. Even the temperature variance in your shed would be a significant factor of easily 10-15% or more. Are you certain that the batteries don't get any warmer in the summer, or colder in the winter? A few more degrees in each extreme would likely add up to a 20% difference between summer and winter.

    As for the low voltage cut off point I'd say you may also want to consider the size of your loads when setting this. The idea is that a large short term load will drop the voltage to a lower level relative to overall SOC (ie, you could be okay with a lower low voltage cut off), versus a smaller or steady load where you probably don't want to go all the way down to 21V since by the time such a load has pulled the voltage down that low you'll have drained the batteries quite deeply.
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