Pros and cons of 1 big battery vs several small ones?

TerrangTerrang Registered Users Posts: 12
I am planning my battery bank and i am wondering if it is better to have 1 big battery or a few smaller ones?
I was given prices for 2x [email protected] ah batteries and also 1 [email protected] ah battery.
Both prices are pretty much the same. Both batteries are Gel batteries and come with 7-10 year working life.
What are the pros and cons of 1 battery vs many?

Thanks

Comments

  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    One big one weighs more, may require more muscle or different technique to move. One big one would eliminate the potential parallel battery issue if they are all 12 volt batteries.


    But there are better solutions ... ... are they really GEL cell batteries or AGM? Those are very different but appear similar. GEL's are usually the least desirable for RE use, the most fragile lead acid type when it comes to use and care. An AGM is different from what most of us refer to as GEL. And most 12 volt batteries of any type are not usually true deep cycle batteries. You will find AGM and FLA true deep cycle batteries in 6 volt sizes. Two in series to make 12 volts.
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • BB.BB. Super Moderators, Administrators Posts: 29,725 admin
    I personally recommend that people have one series string of batteries as "ideal", with 2 to 3 parallel strings of batteries as "OK". Some people run more than 3 parallel strings (cannot get larger batteries, cannot afford them, etc.) and can do OK with that.

    However, Lead acid batteries tend not to share current very well. And fewer parallel strings usually is better, and less work for you (fewer electrical connections, fewer cells to check water levels--if flooded cell, etc.).

    For paralleling batteries, check this website:

    http://www.smartgauge.co.uk/batt_con.html

    Also, I would suggest that two x 6 volts @ 200 AH "golf cart" type batteries in series are better than two x 12 volt @ 100 AH batteries in parallel. You have 1/2 the number of cells to check (6 cells vs 12 cells for 2x parallel batteries--Plus you can take your volt meter and do a quick check on each 6 volt battery's voltage (charging, discharging, resting). It is much easier to find problems this way. Measuring across 2x 12 volt batteries in parallel--Even if one battery has a "weak cell", you could not find that with just a volt meter.

    If you are going to be paralleling battery strings, and just very handy for general debugging--See if you can justify a DC Current Clamp DMM (digital multi-meter). This one from Sears is ~$60 USD and is "good enough" for our needs. A DC Current Clamp meter is very easy to use (and very safe)--You just clamp the cable for each battery string and measure the charging (or discharging) current. You are looking for "differences" (one string taking too much charging current or supplying very little load current--A couple of examples).

    Now--The next major question is the use of GEL batteries in a solar power system. In North America, most GEL batteries are not very good for solar power. The major issue is that when charging, you can only charge them at the 20 Hour Rate (C20) or 5% rate of charge (i.e., a 100 AH battery charging at 5 amps). For off grid / solar power systems, it is usually much better to charge the batteries are around a 10% to 13% or so rate of charge--Simply, because the sun is not up in the sky long enough to fully recharge a well discharge battery bank at 5% rate of charge....

    For example, a 50% charged battery would take ~10-14 hours to fully recharge at a 5% rate (2-3 days of solar charging). At 10%, it would only take ~5-9 hours--A little more than 1 to 2 typical days.

    If you are up north, then in the winter you may not even get more than a couple of hours per day charging.

    And similar with a genset--5% maximum rate of charge vs 13% (or sometimes higher) with flooded cell (or AGM).

    GEL batteries, if they are charged too fast, develop permanent gas pockets in the GEL and ruin the battery's capacity.

    However, there may be some European (primarily) GEL batteries that can take higher charge rates--As always, check the battery manuals for exact charging instructions.

    Lastly, if this is your first set of batteries--Getting a good quality hydrometer and a flooded cell battery bank will give you a lot more accurate data on your state of charge and how well you manage the battery systems. If you have sealed batteries (GEL, AGM, etc.)--Getting a Battery Monitor may be a good idea (although battery monitors can have their own issues).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TerrangTerrang Registered Users Posts: 12
    Thanks for the advice. I will get some pricing on FLA 6V batteries and see what i can get.
  • kansaskansas Solar Expert Posts: 100 ✭✭
    One important difference in Gel/AGM versus FLA is the need to vent the potentially explosive hydrogen gas created when charging an FLA battery.
    Two 140 watt Kyocera panels, wired in parallel; Ironridge top of pole mount; two 6 volt, 242 AH US batteries, wired in series; Morningstar ProStar 30 charge controller and SureSine 300 inverter; Trimetric 2025-A meter; IOTA DLS-45 charger, Honda EG3500X generator; Aermotor 702 water pumping windmill.
  • BB.BB. Super Moderators, Administrators Posts: 29,725 admin
    You have to be careful about the "Venting Issues" with GEL and AGM... if they are over charged and/or the catalyst fails, they will vent hydrogen gas/Oxygen gas/Electrolyte too.

    Then venting is typically at end of life/failed charge controller/thermal runaway.

    So--I would not not put an AGM battery bank next to my Picasso paintings.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TerrangTerrang Registered Users Posts: 12
    I decided to go smaller and buy 4 6V200ah batteries for my new system i am designing. They are FLA batteries.
  • softdownsoftdown Solar Expert Posts: 2,695 ✭✭✭✭
    Good choice Terrang. Gels are likely the worst solar batteries available. Must also be charged at a much lower voltage rate.
    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
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,842 ✭✭✭✭✭
    BB. wrote: »
    You have to be careful about the "Venting Issues" with GEL and AGM... if they are over charged and/or the catalyst fails, they will vent hydrogen gas/Oxygen gas/Electrolyte too.

    Then venting is typically at end of life/failed charge controller/thermal runaway.

    So--I would not not put an AGM battery bank next to my Picasso paintings.

    -Bill


    I just have prints near my flooded battery. I can't see well enough for paintings.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

Sign In or Register to comment.