Real gravity vs real capacity

rooneysoccerrooneysoccer Registered Users Posts: 1
Hi all,
Recently installed a brand new bank of 16x Rolls S-480 375Ah @ 20hr and SG 1.280 @ 25°c as stated on their spec sheet.
On the battery sticker, it appears that these batteries were filled with 1.265 SG electrolyte.
After charging and EQ, it seems obvious that the SG will never reach 1.280, 1.265 seems to be their real maximum.
I was wondering what should be the real capacity @ 20hr of these batteries to properly set my battery monitors (WBjr/BMK). On the S-480 spec sheet, Rolls asks to lower this capacity by 5% to match a SG of 1.265. So to me, real capacity is 356Ah/20hr if SG is 1.265.
I don't understand why Rolls claims for 375Ah if they cannot reach 1.280, why not simply write 356Ah at 1.265/25°C?
Could someone confirm from experience if I'm right or wrong.
Thanks for any input,


  • EstragonEstragon Registered Users Posts: 3,128 ✭✭✭✭✭
    Lower SG electrolyte WILL result in a lower capacity.

    My understanding is differing concentrations are used for different applications. There's a theoretical trade-off between capacity and longevity, for example. In a motive application, power to weight ratio might be more important than longevity. In a stationery deep cycle application like renewable energy, longer life and less suseptibility to sulfation might be preferable.

    I agree the battery should be sold based on actual capacity with the actual electrolyte used though.

    In terms of the monitor, I'd set it to the lower value. 5% is kind of a rounding error for most monitors. Factors like battery temperature and charge efficiency can put them off by more.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mcgivormcgivor Solar Expert Posts: 2,512 ✭✭✭✭✭
    When ordering was the proposed use mentioned, perhaps lower SG for RE application. Have you contacted them to clarify? 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • BB.BB. Super Moderators, Administrators Posts: 28,320 admin
    I ran across this page on why different types of batteries have different starting s.g. fills... Is pretty interesting:
    Specific Gravity vs Applications
    1.285 Heavily cycled batteries such as for forklifts (traction).
    1.260 Automotive (SLI)
    1.250 UPS – Standby with high momentary discharge current requirement.
    1.215 General applications such as power utility and telephone.

    As mentioned earlier, the specific gravity (spgr.) of a fully charged industrial battery, or traction battery, is generally 1.285, depending on the manufacturer and type. Some manufacturers use specific gravities as high as 1.320 in an attempt to gain additional Ah capacity, but at the cost of a shorter cycle life.


    Higher Gravity = vs Lower Gravity =
    More capacity / Less capacity
    Shorter life / Longer life
    Higher momentary discharge rates / Lower momentary discharge rates
    Less adaptable to "floating: operation / More adaptable to "floating" operation
    More standing loss / Less standing loss
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MrM1MrM1 Registered Users Posts: 311 ✭✭✭
    Bill,  I wonder if that is why I can get at least some/most of my cells to reach 1.300 SG when I do an EQ before cells stop rising.  (In those cases the lowest cells usually stop around 1.285 to 1.290ish).   My L16's are actually purposed for Floor Scrubbing machines and not RE.   That would seem to possibly fall into your first category,  "heavy cycled"

    And this highest SG of 1.300 that i can reach has always made me wonder if that should be my target for "Fully charged" ... or in an RE situation should i shoot for the Trojan spec of 1.277.  To get those hi SG numbers in my RE setup I have to use 2.61vpc for Vabs to get there in the amount of day light available.
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • BB.BB. Super Moderators, Administrators Posts: 28,320 admin
    You have to make sure that you knock all of the bubbles off of the float--A gassing battery is almost like a soft drink and the gasses may be collecting on the float and giving you "false" high readings.

    And, I would suggest that you aim at >90% state of charge for "daily" (or once or twice a week) for normal charging. And the EQ/charge until SG stops rising the once a month (or when variations between cells is ~0.015 to 0.030 (or more).

    And there is the SG temperature correction (EQing batteries can get them warm/hot).

    And, when you are done, logging the temperature corrected SG to a log book (batteries/cells numbered to keep track of readings) is the new "fully charged" reading. As batteries age, it is possible for cells to diverge from "matching" sg levels.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,199 ✭✭✭✭✭
    Prayer can be useful also :) Slow breathing, meditation, along with regular exercise will help!

     I still have a bunch of clients doing this but thankfully most of the new ones can skip all of this and just not be concerned about anything more than checking if the sun is out. 

    The op never mentioned the EQ voltage he was using to get the SG up? Pretty critical ! Forgive me if I missed it!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • VicVic Solar Expert Posts: 2,938 ✭✭✭✭
    edited June 2018 #8


    Yes,  almost all Surrette Solar batteries (like the S-480),   use a fill electrolyte of 1.265 SG.   This will reduce the published nominal Capacity by 5%.

    If you do not have it,  yet,  here is a Link to the Surrette Battery Manual:

    With new-ish Flooded batteries,   the actual Capacity will build as the batteries are cycled,   often needing 50,   or more cycles,   to about 80% SOC,   or lower,   to count as a cycle.

    As has been noted,   these Capacity numbers are not absolutes.

    Would suggest you use  the 95% if 375 Ah times the number of parallel strings as a starting point for the WbJr setup value,   BUT,   do realize that  the SOC &  Remaining Ah values shown from the Wb,   are just approximations,   NOT absolutes (as all of these similar readings on any battery monitoring device).   IMO.

    The actual capacity of our FLA banks  is equal to the capacity of the lowest cell  ...   although with several parallel strings,   this is not true   ...

    FWIW,    Vic

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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