Rolls battery real gravity vs real capacity

SolarMusherSolarMusher Solar Expert Posts: 173 ✭✭✭
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,


  • VicVic Solar Expert Posts: 2,993 ✭✭✭✭
    edited March 2016 #2

    Hi Erik,

    YES,  you are correct.   Rolls does spec most of their RE batteries this way  --  I have never seen any of their RE batts with anything other than 1.265 SG as the fill electrolyte.

    At least,  now,  they do spec  on the battery spec sheet,  that the Capacities and other related data is as if the SG was 1.280,  and to derate the Capacity,  and run times,  etc by reducing them by 5%,  due to the usual electrolyte SG of 1.265.

    They do this,  IMO,  because,  there has long been Capacity wars among battery manufacturers.  Some,  actually increase the SG of the electrolyte (a bad thing,  to me),  and many have slightly increased the Reference temperature from 25 C to 27 C.   And,  this is just for RE batteries.  Some batteries are pushing 1.300 SGs to get a bit more Capacity.

    The reason that my sig states that the 4KS25 "1350 AH" batteries are 1280 AH.

    Personally,  think that Surrettes are a fine battery,  and still spec/recommend them.  Am glad,  however,  that they have started noting reality,  on the data sheets.   And,  Surrette specifies everything anyone would want to know about their batteries,  on those spec sheets ...  number of plates,  plate dimensions,  separator and envelop thicknesses,  electyrolyte reserve capacity,  dry weight,  wet weight,  OH,  plus,  CCA, MCA Reserve Time ...   everything.   Try getting that info from Trojan,  or most others!

    FWIW  Good Luck,   ... Mush,  Mush!!  Have Fun,   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.
  • SolarMusherSolarMusher Solar Expert Posts: 173 ✭✭✭
    Hey Vic, hope you're fine!
    So, finally, 1.280 is just for marketing and 1.265 for real life. That's what I thought.
    Anyway, as you said it's not a problem, I prefer to have it at 1.265 for longevity and two strings give me 712Ah which is enough for my needs and better suited to my 3.2kw array.
    Did you take a look at the capacity/temperature curve? After checking it, I've set the WBjr capacity temp compensation to 0.5% per °C  which is more accurate than it was with 1% classic default setting for my temperature.
    I would like to keep this bank for 10 years and want to have it right from the begining (I hope).
    No more mushing, too old!
Sign In or Register to comment.