Question about keeping batteries the same age.

Dean WDean W Registered Users Posts: 6
Hello all;
I'm in the process of building a battery bank that will be used mainly during times
when the power is out. Right now, I have two 6v 220ah batteries in series for 12v to
run my inverter. These two batteries are brand new, just purchased and have not been
used at all, yet.

What I would like to know is about adding batteries to the bank over a short period of
time. What I want to end up with is eight of these batteries to provide 12v 880ah,
and that will do what I need.

So, I have the first pair to start with, and next month will buy two more, and the month
after that, etc, until I have all four pairs.
What I would like to know is if it will do the battery bank any harm to add these
pairs of batteries by the month, until I have four pairs? I've heard about trying
to keep the battery bank the same age. Is there some leeway on this, as in how I plan
to do it? It's possible I may want to use the batteries purchased this month before
I acquire the next month's batteries, and the following month, etc.
The length of time between the first pair of batteries (which I now have), and the
final pair, will be about 90 days. Am I going to harm the battery bank by doing this?

Thanks very much,



  • BB.BB. Super Moderators, Administrators Posts: 29,811 admin
    Personally, I am not a big fan of lots of parallel battery strings... I would suggest 1 string is best... 2-3 can be OK... 4 or more--Only if you don't have other choices.

    Basically, there are lots of issues with trying to keep battery strings balanced, and detecting failed connections/cells in parallel strings... Although a combination of 6 volt batteries and a DC Current Clamp DMM (not very expensive, good enough for our needs from Sears meter) makes working with parallel strings/DC (and AC) current measurements much easier and safer.

    Placing parallel strings of 2x 6 volt batteries in service over 90 days is not optimal--But if you purchase the same brand/model of batteries and monitor specific gravity with a good hydrometer (and volt/amp meter)--They should give you good service.

    Just remember, at 4x 220 AH = 880 AH @ 12 volt battery bank. Even a 10% rate of charge (88 amps) is pretty much the limit (or over the limit) for many solar charge controllers... If you could go with 24 volt (2x strings of 4 batteries)--You are not at 440 AH @ 24 volts--And you can find a bunch of charge controllers that will run 45-80 amps--And you need only one ~45 amp charge controller (instead of a pair of 45/60/80 amp charge controllers) if you plan on charging at 10% or greater rate of charge (5% is OK for weekend/seasonal usage--10% or more is recommended for full time off grid).

    But--This does always get back to loads... AH or WH per day usage, maximum current (continuous/surge current ratings), etc.... That is what really defines your battery bank design (and AH vs Voltage choices).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dean WDean W Registered Users Posts: 6
    Bill, thanks for the info and Current Clamp link.

    The batteries are for a back up. I'm on utility power, but we have occasional power
    outages due to weather, and sometimes scheduled outages for maintenance. It's a small
    co-op power company.
    My house is small, and the batteries will run it for about eight hours with selected
    loads. I have a generator in case there were extended outage times, but it is pretty
    noisy. Mainly, I want to be able to be able to keep things going during the outages.

    I don't have solar panels. I plan to charge the batteries with a battery charger that
    runs on house current. It will charge at up to 12% of the 880ah of the batteries.

    I may not need to use any of the batteries until I have all eight of them purchased. That
    depends on when the next outage occurs. The batteries will all be the same make and

    I appreciate your time, Bill.


    p.s. I'm sorry if I put this in the wrong section.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Just make sure that you keep your earlier batteries at full 100% SOC charge while waiting for the rest. That will minimize any differences among them. (But you do not know how they were treated between manufacture and sale if they were not part of the same shipment to the dealer.)
    SMA SB 3000, old BP panels.
  • Dean WDean W Registered Users Posts: 6
    Thanks for the reply, Fizzy. I'll keep them charged up and hope I don't have to use them until I get the full set.

  • softdownsoftdown Solar Expert Posts: 2,754 ✭✭✭✭
    Perhaps you intentionally left something out.

    This is a highly inefficient way to supply power during an outage. Better to just use a genset as needed. The losses with this battery back up system are pretty significant. Plus the batteries will likely go bad in a few years.

    The only way this has merit is to charge the batteries with solar or wind. Unless we are missing something.
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
  • 2manytoyz2manytoyz Solar Expert Posts: 373 ✭✭✭
    Dean, you are planning to do what I've been doing for years. I have 8 golf cart batteries primarily setup as emergency power at my house. They are charged by solar panels, and if needed, my generator connected to an Iota 75A charger. Both charging methods can safely be used at the same time. I am slowly taking more and more items off the grid, and powering them via this system exclusively.

    As for adding batteries spread out over months, it will NOT be an issue. I've added batteries that were more than a year apart, and they all lasted between 5-6 years from the in-service date. My present set of batteries were purchased at the same time, and while they're doing great, there's no noticeable difference in performance from the last set.

    A huge advantage to battery power, over generator power, is that it is silent at night. If you've ever suffered through an extended outage, you know which neighbors have generators (and resources), and the neighbors that don't will be banging on their doors with extension cords asking to tap their setup.

    Since I'm in hurricane prone FL, we've had our share of outages. When a storm hits, it's typically VERY windy and raining sideways. Running a generator during those times isn't an option! In 2004, we had two back to back hurricanes, and power was out for 18 days. Not only to homes, but to restaurants, grocery stores, and gas stations. No gasoline available locally! I have 10 5-gallon gas cans that I will fill before an approaching storms hits, but solar will run my critical items long term.

    I recommend using bus bars to parallel the batteries. This will balance the charging and discharging. I periodically measure the draw from each pair of 6V golf batteries (in series to make 12V). The numbers track very closely.


  • Dean WDean W Registered Users Posts: 6
    Robert, thanks for your informative reply. Same thing as what you said, I just want to have backup power available for
    when my power is out, so things around the house continue as normal during those times. The batteries will always
    be charged and ready to use when the time comes.
    softdown wrote: »
    Perhaps you intentionally left something out..
    Like what, "Softdown"? I don't know what you mean by this. Please explain.

  • Alaska ManAlaska Man Solar Expert Posts: 252 ✭✭
    Nice set up Robert from FL. That bus bar is a great idea. Think I'll have to copy that one.

    My 2 cents.......... If you keep the batteries charged as up best you can, adding should not be an issue. It's a matter of cycles and depth of discharge. If you time it right, you may not have either occur. If that's the case the difference will be in transport, and shelf life. I would think the differences in the battery sets to be minimum at that point.
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