shelf life of rarely used batteries?

SystemSystem Posts: 2,511 admin
Let's say I keep a small bank of batteries charged off of solar panels... also, these batteries are only used in power outages.

My understanding is that it is common to replace these expensive batteries every 5 years on a typical system.

...but what if these batteries are rarely put to use? What if they are merely kept charged until needed a few times per year?

How long would one expect such batteries to last under these conditions?



  • BB.BB. Super Moderators, Administrators Posts: 30,962 admin
    Re: shelf life of rarely used batteries?

    It really depends on the batteries... Some are designed for float service and other are designed for constant cycling.

    Interestingly, typical deep cycle lead acid batteries actually last longer with some occasional deeper cycling.

    AGM are typically used in UPS systems because they are sealed and have very high amperage output for their size. Plus they are very efficient (charging/discharging--around 90-98% efficient). However, the standard AGM used in a small UPS system would probably only last 1-2 years.

    Deep Cycle batteries, on the other hand can be designed for 20+ year life with heavy use (forklift/traction batteries). However they are designed to be cycled and may not last as long in pure "float" service. Also, they need more service (cleaning, adding distilled water, venting hydrogen gas); and are less efficient (~0.80 to 0.90 efficient). Plus they have higher self discharge (upwards of 1% to 2% per day for older batteries). And, for these batteries, they are large and heavy to output their rated capacity.

    There are other battery chemistries out there that may last longer/be smaller/lighter/etc... But they will not be cheap.

    And, your choice of chargers/charging algorithms can affect battery life too... For UPS type applications, you should look for a charge controller that has at least 3 stages (be careful--marketing keeps redefining number of stages)--Float voltage setting is just enough voltage to keep the battery charged (reduce sulfation) and not so high as to cause problems (using too much water, positive plate corrosion, etc.).

    Your best bet--define your power needs first (amps, watts, hours of operation, space, maintenance, costs, power sources --- grid, generator, solar, grid tied, hybrid off grid, UPS function for computers, does your utility allow net metering/grid tied solar, etc.)... Then start a paper design of the system. You will probably end up with several variations and need to make your choice from there (costs, functionality, ongoing costs/maintenance, etc.).

    Sorry for so much hand waving--Your question is actually quite complex and will involve lots of trade-offs.

    -Bill "why yes, I am an engineer--why do you ask?" B. :roll:;)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: shelf life of rarely used batteries?

    as bb stated it does depend on the battery for some are better than others even among the same types. there are no guarantees there won't be a premature failure on any battery no matter how well cared for. as to agms they are very good batteries and could easily go far beyond their warranty period, but it depends too on how it is charged for floating it with too high a voltage may shave some life from it. some pwm controllers do not have a float setting, but rather continue to shrink the current at the full absorb voltage point or as some may refer to it as the bulk limit voltage. many of those ups batteries probably are not being charged via 3 stage charging thus keeping the voltage quite high. for the record i have an agm going on its 8th year.8):D

    no matter what type of battery you have, if you are floating it most of the time i would recommend a periodic drain to maybe 70% or 80% soc (full is 100% soc). this could be weekly or monthly, but that shows if the system is still operating properly and would show any problems that may arise before that power is actually needed.
  • RCinFLARCinFLA Solar Expert Posts: 1,324 ✭✭✭
    Re: shelf life of rarely used batteries?

    Niel is correct. Long term float will build up too much lead dioxide on the positive plate and progressive slow grid oxidation. Both result in higher internal resistance. Lead dioxide is what makes a positive plate but it does not have good conductivity so too much is a bad thing.

    Also the negative plates require more voltage then typical float voltage to prevent self discharge and build up of lead sulfate. Float voltage is a compromise between oxidation of positive plate and self discharge build up of negative plate.

    Some discharge at a 15-20% rate and recharge will help relieve these detrimental effect however a lead acid battery is marked for finite life from point where electrolyte in poured in.
  • System2System2 Posts: 6,290 admin
    Re: shelf life of rarely used batteries?

    What if I have a few batteries that I do not charge at all ?

    Meaning, I find batteries for a bargain and want to buy a few more than I really need.

    Is there a way to store unused, new batteries without degrading them?

    Also, is there a type of battery that lends itself to unused, long-shelf life?
  • BB.BB. Super Moderators, Administrators Posts: 30,962 admin
    Re: shelf life of rarely used batteries?

    For Lead Acid type batteries--No, not really. All of them have problems with storage... They start slowly degrading from the moment of manufacture.

    Flooded cell need charging probably once per month. AGM you might get away recharging every 3-4 months (or use a float charger to keep charged). If you don't keep them charged (or want to buy batteries that have been sitting for months or longer without charging and a quick check shows them well below ~75% state of charge), you should pass them up. Lead Acid Batteries, once fully sulfated, are usually only good for their scrap value.

    Even "dry charged" batteries (as shipped from factory) will only last 18 months or so maximum, before you should fill and use them...

    For emergency use, Lithium primary cells (non-rechargeable CR123, AA, etc.) are good for about 10 years of storage. Even good quality alkaline batteries are good for 5+ years.

    Storing batteries in a cool/dry place should increase their life. For every 10C (18F) reduction in temperature, the storage life of a battery should double... Conversely storing in a hot building (over ~25C/77F) will reduce the storage life.

    We used to store flashlight batteries in the refrigerator--but frankly, we stopped (decades ago) because we could not see any difference.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: shelf life of rarely used batteries?

    Flooded ni-cad is not harmed if stored discharged, or never fully recharged. Its also much more tolerant of cold temps, but as Bill said "they will not be cheap".
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • rplarryrplarry Solar Expert Posts: 203 ✭✭✭✭✭
    Re: shelf life of rarely used batteries?

    If you are looking for something just to get you through a power outage, maybe a small genrator like a Honda 1000 or even a 2000 would work better for you. They store for long periods of time, and if youy drain the carburetor they are very reliable. You only have to worry about storing gasoline long term.
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