Battery Maintenance for Unoccupied Off-grid houses

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Tulumtam
Tulumtam Solar Expert Posts: 37 ✭✭
I do maintenance for a couple of off-grid houses that are only occupied 2-3 months out of the year. The two systems:

1. 546 a/h FLA batteries @ 48v, 6-225w modules + 3-215w modules, Outback VFX3648, Outback MX-60. This system was installed 4 years ago.

2. 1305 a/h FLA batteries @ 24v, 10-200w modules, 2 Outback FX2524T, Outback MX-80. This system was installed 3 months ago.

In each case, the homeowner occupies each house 3-4 time per year for approximately 3 weeks each visit. No loads need to be running while the houses are unoccupied. I have suggested switching to AGMs, but both homeowners hope to either spend more time here in the future and/or rent out their houses and assume more constant occupation at some point.

What would be the best charge controller settings to maintain the batteries without overcharging and excessive water loss? I can change the settings the day before the owners arrive and change them again after they leave. As it is, I am stopping by once a month to check water levels, equalizing twice a year.

All thoughts and suggestions appreciated!

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,493 admin
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    Re: Battery Maintenance for Unoccupied Off-grid houses

    If you can, check the battery manufacturer's specifications for float charging voltage... Around 13.2 to 13.6 volts seems to be common recommendations.

    If there is no load on the battery banks, make sure you don't have the timer/bulk absorb voltages set too high (maybe closer to 14.2 vs 14.5-14.8 volts).

    Equalization, if you can measure the specific gravity and it is ~0.030 or greater difference from low to high cell, then do the equalization (excessive equalization is hard on battery banks).

    And watch water usage--if you have to add water every two months (or a bit less), they are probably being charged pretty well (during "float", probably less water usage would be a good thing). No water usage (especially during cycling, probably indicates undercharging).

    AGM's are nice batteries, but I am not sure they would last any longer--Basically just saving the checking/filling of water levels and cleaning battery tops/connections. That may be worth it to you and them, or not--But you are looking at 2x or so more for an AGM bank that probably will not last as long as a good flooded cell battery bank with proper maintenance.

    The systems sound like they could use more solar array--but, frankly, if the loads are not heavy and only (presently) used a few months out of the year, then it may not be worth adding more to the solar array at this time (unless they need to use a backup generator too much while they are there).

    Normally many of us here would recommend ~5-13% rate of charge from solar (with 5% being a pretty good "hard minimum" amount of charging current).

    So, taking your two systems and their present battery banks with a 5% minimum charge, the recommended minimum rating size:
    • 546 AH * 58 volts charging * 1/0.77 panel+charger derating * 0.05 minimum rate of charge = 2,056 Watt Minimum array (1,995 watt array currently)
    • 1305AH * 29 volts charging * 1/0.77 panel+charger derating * 0.05 minimum rate of charge = 2,457 Watt Minimum array (2,000 watt array currently)
    And, if you wanted a "hefty" sized solar array vs battery bank capacity, you could double the arrays recommended above (~10% rate of charge).

    In the end, if they are happy with their system capacity when they are there, and the batteries are lasting their expected life (3-5 years for less expensive batteries and 8-15 years for high end batteries)--It sounds like you are doing everything right and should continue as you have been.

    -Bill

    PS: Do you have Remote Battery Temperature Sensors? Those can be a help too--especially if the charge controller and the battery bank are not in the same room/at the same temperatures.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
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    Re: Battery Maintenance for Unoccupied Off-grid houses
    Tulumtam wrote: »
    ...
    In each case, the homeowner occupies each house 3-4 time per year for approximately 3 weeks each visit. No loads need to be running while the houses are unoccupied. I have suggested switching to AGMs, but both homeowners hope to either spend more time here in the future and/or rent out their houses and assume more constant occupation at some point. .....

    Renters & Battery banks don't mix. Renters will likely over use the power, and then whine about the lights and toaster don't work.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.
    BB. Super Moderators, Administrators Posts: 33,493 admin
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    Re: Battery Maintenance for Unoccupied Off-grid houses

    If they where going to rent these out--I would suggest a Battery Monitor with alarm contacts (Xantrex or Victron both have models with programmable contact output) that started a electronic horn/flashing blue light/etc. when the batteries hit 50% or 20% (your choice) state of charge.

    The alarm contact plus the ability to read (estimated state of charge in XX%) from the display will make it much easier to explain to renters/children to turn off power and/or start the genset when the state of charge of the battery bank is below 50% state of charge.

    My suggestion on quick how to use off grid power:
    • Above 75% state of charge, everything OK
    • If below 75% state of charge in morning, start genset if cloudy weather and charge until >80% state of charge, charge until >80% state of charge (charging >90% state of charge on genset is usually a waste of fuel).
    • If below 50% state of charge, turn off loads/start genset and run until >80% state of charge
    • If below 20% state of charge, turn off loads/start genset, call for help--Something is seriously wrong.
    In general, discharging and staying below 75% state of charge for days/weeks/months will sulfate the battery bank.

    Cycling below 50% state of charge will reduce battery cycle life (but cycling only a few months a year--less of an issue--batteries will probably age out before cycling out for most people--500-1,000 cycles means 5-10 year cycle life if assuming 100 cycles a year).

    Cycling below 20% state of charge will run the risk of permanently killing a cell/battery/group of batteries. Below 20% State of Charge runs the risk of "reverse charging" weak cells--which is usually instant death (or death within a few weeks).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Battery Maintenance for Unoccupied Off-grid houses

    i would advise you to disable any automatic eqs and do that if needed when you do your maintenance check.

    often times battery manufacturers will give a range for the float charge and if the batteries are mostly on float then go to the bottom end of the float range. do verify that it is the proper voltage at the battery and not just what the controller shows as v drops can fool the controller to thinking it is a higher voltage. if you still feel the batteries are boiling away their water needlessly you can cut back on the absorb charge time, but be careful you don't windup going too far the other way and start undercharging the batteries.