Lead-Acid Battery Bank Frequently Asked Questions

    800RE EnergyCell 48V 672Ah AGM Battery System
    Protect your energy storage investment! This article answers some of the most lead-acid important battery bank related questions.



    1) How often do I need to charge or "top off" my lead-acid batteries?


    Maintaining your batteries is the key ingredient to ensuring that your battery bank reaches its designed life expectancy. What you may not know is that this typically does not happen on a partial state of charge on most batteries.  So, yes, your PV system may be charging during the day, but that does not mean that your bank is getting a full charge. The rule of thumb here is to ensure your bank is getting a full charge at least once a week.


    2) How do I know if my battery bank is getting a full charge?


    It is recommended that you install some type of battery monitor. This equipment helps you keep tabs on the state of charge of your battery bank, every day at a glance.  Many charge controllers and inverters are already equipped with this functionality, but if they are not there are third party devices that can fit the bill.


    3) What if my battery bank is not getting at least one full charge per week? 


    There are a few things you can do when you discover that your bank is not getting at least one full charge per week.  You can proactively decrease the amount of energy you are consuming by using some of your loads less.  If you find one or more of your loads is an energy hog you may consider replacing them with energy efficient models to help reduce your energy consumption. 


    If those two options do not seem reasonable, consider adding a backup generator to your system.  The generator can help to pick up the slack and help maintain your battery bank by assisting the PV system in ensuring the battery bank is getting a full charge.  You may even consider expanding your PV system to make up for the lack of energy that is able to be harnessed and stored in the system.


    4) If I connect my batteries to the inverter and/or charge controller, can I just turn the system on?


    Default factory settings can harm your batteries.


    Many inverters and charge controllers come with a factory default setting for voltage, battery type, and capacity.  Since this is almost always different per each individual system, carefully planning out your own settings is very important.  Consult those manuals!


    Failure to properly set your battery bank to meet the demands of your PV system could result in overcharging or undercharging of batteries and over cycling of the bank.  Each leading to a shortened life for your battery bank.


    5) My battery bank has served me well for some years now.  Can I replace my existing system with any type of new battery?


    Unfortunately It’s not that simple. The settings on the inverter and/or charge controller must be considered first.  Before connecting the new battery bank first adjust the settings on your equipment.  These adjustments are made according to the type, voltage and capacity of the new batteries to be installed. The settings on your solar electronics are always done according to the type of battery installed.


    Skipping over this critical step may shorten the lifetime of your new battery bank. You expect the bank to last 5-7 years, but may be disappointed when you get only two.



    6) Can I mix new batteries into my old bank?


    In a word, “don’t."


    Many of our customers who currently have a battery bank later desire to expand their capacity or in some cases replace batteries that have failed for one reason or another.


    Mixing different batteries is never a good idea. First of all, mixing batteries which are different in type, size, brand or voltage will not even work. Worse still, it can lead to battery bank failure.


    If you were to mix a new battery to an old string or a new string to an older bank, the new batteries would reach their state of charge sooner than the older batteries.  This could lead to them boiling.  In terms of longevity, the new batteries will tend to cycle more often than the older batteries.  More cycles equate to a shorter life span for those batteries.  Typically this will not cause the older batteries to die first, but instead reduce the lifespan of the new one you have just installed. 


    However, let’s say that we are working with identical batteries (exact type, size, brand and voltage), you can mix them up when the age difference between the old and new batteries is not more than 6 months. When adding new batteries to a battery bank that has been used for approximately 6 months or less, your new batteries will tend to adopt the age of your current battery bank.


    7) When are batteries eligible for the ITC?


    In short, for energy storage to qualify for the federal ITC the battery bank must be charged by the PV system onsite. If routinely less than 100% of the energy used to charge the battery bank comes from the grid and not the solar this will prorate the ITC incentive.  In other words, "if 90% of the storage charging energy is derived from solar panels, then the storage is only eligible for 90% of the ITC."  Particular attention must be paid to the energy used to charge the batteries in the first year.  "If the solar charging energy remains about 75% buts falls below the percentage established in the first year, then a proportional amount of the tax credit claimed in the first year must be recaptured."

    For a more in depth analysis of energy storage and the ITC see Susan Kraemer's article in Renewable Energy World HERE.

    4 years 3 months ago
    Written by
    Adam Diehl
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    Distributed Energy Storage
    lead acid battery
    battery backup
    off grid