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When do batteries need to be replaced

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  • When do batteries need to be replaced

    We are on a 48 volt PV system using 2 strings of Interstate L16 batteries in use since 2006, off grid in Colorado.
    Most years we have not used the system for several months during winter.
    The batteries have not run dry, have been equalized regularly with a large genset and the sun.
    Is there a definitive test to determine whether the batteries can supply the rather modest power demands we have, including running a 220volt well pump and refrigerator. They do not function as new, drawdown is quick and cloudy days see lower voltage readings, however with care we are able to maintain over 48 volts reading

  • #2
    There are two major tests you can perform, one simple, and the other too difficult to do correctly but easily approximated. Both are more useful if you have historical data to compare against.

    1. Check the Specific Gravity (SG) of each and every cell. There should be only minor variations from cell to cell and the overall SG should be close to that specified for the batteries when new. A significant reduction of SG when fully charged is an indication of sulfation, which reduces battery capacity over time and ties up some of the sulfate ions from the sulfuric acid.
    Cell-to-cell differences could just be a sign that an Equalization charge process should be undertaken, then recheck SG. If any of the batteries is really bad off, it would be best to replace them all. Otherwise you will just ruin the new battery sooner and not prolong the life of the bank by much. If you have a few bad batteries and cannot afford to replace the whole bank right now, you may be able to select the four best and make up a single 48V string while you save up money.
    2. Do a crude load test by connecting a known load of about 20-40 amps to the batteries and seeing how long it takes for the voltage to become unusable.
    Officially, according to industry and most manufacturers specifications a lead acid battery has reached end of life when it can only deliver 80% of its original design power at the 20 hour rate.
    You can clearly get along with worse than that, but once you have passed the 80% line the battery gets worse faster.

    You have done very well to still be using your 2006 batteries, but the fact that you are even noticing a quicker drawdown means that they are ready to be replaced now.
    Last edited by inetdog; 09-29-2016, 11:40 PM. Reason: 2008 --> 2006
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      10 years on a set of average batteries (2006). They are past their lifespan. Plan on a new set soon,
      Powerfab top of pole PV mount (2) | 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

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      • #4
        Replace your batteries when the grief of dealing with the old ones exceeds the cost of buying new ones.
        Hang in there if you can, the industry is really trying to come up with better batteries and hopefully will be soon. (Check out something called a "Liquid Metal Battery" on youtube)
        However so far, there are some really better batteries available, but they are also really, really expensive.
        BSEE, R11, NABCEP, Chevy BoltEV, >3000kW installed

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        • #5
          my suggestion, get an 8 pack of golf cart batteries and run them to death in 2 or 3 years. In that time, we'll know if a new battery chemistry will be viable and you can buy it.

          GC batteries are cheap ($100 ea) - way less than L-16's In 2 years, we;ll know if something better has arrived, or you just get another set of L-16's ( or stick with frequent replacement GC batteries)
          Powerfab top of pole PV mount (2) | 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

          Comment

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