SG versus voltage

maokmaok Registered Users Posts: 20
Hi everybody,

I hade today morning SG at 1,225 what means 75%,
and open voltage at 12,24V what means around 55%.

do you think is it OK?
I equalized it a week before.

batteries are one month old rolls s-530




  • BB.BB. admin Super Moderators, Administrators Posts: 29,634 admin
    Re: SG versus voltage

    From an earlier battery discussion thread:

    Well mixed electrolyte is important--so if you notice all cells not coming back to "full charge" level of s.g., then there is a good chance that "mixing" (equalization) will bring the s.g. back up to your logged readings (keep these as your "good batteries" / "fully equalized" numbers.

    The amount of variation you are seeing is not that great (less than 0.030 between all cells). And having "high" s.g. is not always a good thing either.

    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 Geral 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
    Also on that page is the formula between cell resting voltage and specific gravity:
    Specific gravity = single-cell open-circuit voltage - 0.845 (example: 2.13v – 0.845 = 1.285)
    Single-cell open circuit voltage = specific gravity + 0.845.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • maokmaok Registered Users Posts: 20
    Re: SG versus voltage

    Thanks for answer,

    I am starting to be a little bit sick of it.

    Also I found bigger AC-DC/55A/, but when I started to charge, my batteries was takeing only 24A. not 55A. and it was only bulk charge yet. so It should take 55A.
    I dont understand it.
    could the battery be sulfated more, that I thought?
    what do you think?

  • BB.BB. admin Super Moderators, Administrators Posts: 29,634 admin
    Re: SG versus voltage

    24 amps at what voltage? What brand/model of battery charger?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hillbillyhillbilly ✭✭ Solar Expert Posts: 334 ✭✭
    Re: SG versus voltage

    I can understand your frustration, sometimes trying to piece all the information together to understand the health of your batteries can be confusing. Really hard to say from here what the health of your batteries are, but here are a couple of things to consider:

    If your batteries are only 1 month old, they likely are still being "worn in" and thus your SG levels should still be gradually rising to slightly higher levels from regular recharging. Surrette batteries do have a breaking in procedure that they recommend, of course I didn't find this out until two years after owning my own set of Surrette batteries. I'd go get their solar user manual if you don't have it yet: you need to register to have them email it to you, but worth the effort to get a lot of helpful tips.

    I think that it's also good for piece of mind to not get too crazy about the numbers (SG, Voltage, etc) matching up EXACTLY, as there are many factors that can affect them. In theory the SG levels *should* be the truest measure of the batteries state of charge, but even then it is not an exact or perfect measurement. To me the SG levels tell a lot more by looking at how they *perform* over time; in other words comparing the levels to other similar situations in terms of both charging and discharging. With only one month of ownership, you don't have a lot historical information to compare.

    As for that charger, more info would be good. What makes you say that the batteries were still in bulk charging mode? Perhaps they were getting close enough to being "charged" that the current was backing off?

    I don't know what other off grid folks think, but I've gradually come around to the realization that I can only pin my SOC level down "for certain" to within about a 10% range from the SG levels and probably only within about 15% or so from looking at voltage levels. I can narrow that down my SOC level to perhaps a 5% range (at best!) if I start factoring in EVERY possible piece of "data" that I can think of: SG levels + Voltage, when the batts were EQ'd last, how long they have been in absorb, how much the charging current has dropped, how deep the last discharge was, the temperature, watching the voltage change at different charging currents, the alignment of the stars and phase of the moon.... etc. Try not to go too crazy in figuring this all out precisely, as battery charging and discharging is difficult to measure in exact precise numbers. Do keep asking questions and reading as much info as you can, eventually it does start to make a lot more sense... well most of the time anyways.
    Good luck,
  • maokmaok Registered Users Posts: 20
    Re: SG versus voltage


    thanks for the reply. about the charger, now I noticed , that I hade little current setting on it. so it will be better I hope.

    Hillbilly, I read your text about obsession about measurment, numbers and informations about battery state. I liked it, because last month I didnt think about anything else than charging. I started to be really frustrated, because of not understunding of what is going on.. I am going to less looking on hydro&volt&ampermeters, because it started to be a dominant of my life and it does me tired of it.

    so I think , that I have to be more pationed and I have to wait a bit to see, how it will envelop itself.

    And thanks for knowlidge, that the numbers of battery state need not to be always the same. I will reduce my measurments at one quarter, because I did measurments almost every hour..

    so thanks to you all


    show must go on..

  • mbzebrombzebro Registered Users Posts: 18
    Re: SG versus voltage
    maok wrote: »
    I hade today morning SG at 1,225 what means 75%,
    and open voltage at 12,24V what means around 55%.
    Sorry, I know this wont help you a lot, but I dont think you can really judge the health of batteries from these "open circuit voltage vs. SG" formulas without calibrated instruments and taking EVERYTHING into account that has an influence on SG readings and voltages.

    Do you compensate temperature in your SG readings? For example, If your batteries have a temperature of only 5 or 10 celsius, the real SG would be about 1.215 instead of 1.225.

    How are the electrolyte levels? If they are 1cm lower than originally the real SG could be about 0.01 less also.
    Is your hydrometer accurate? For example, my dealers hydrometer is showing 0.01 higher than mine (which is supposed to be a good one) in the same cell.

    Is your volt meter calibrated?

    The difference in SOC (55% vs. 75%) could be easily explained by a few of the above factors adding up.
  • maokmaok Registered Users Posts: 20
    Re: SG versus voltage

    yes, you are right. in fact, I dont know if my instruments for measuring are good calibrated. for example the voltmeters accuracy is 3%, what is quite a lot by 13V.
    and hydrometer also is not very accurate. it is cheap one. and temperatre.. uufff...
    I think I should a bit more to set me free from it.
    by thees all measurments and stressing , I cant really enjoy the energy from the batteries.
    so , like you said, by measuring , I should leave a place for unaccuracy of instruments and to be more relaxed with it.


  • RCinFLARCinFLA ✭✭✭ Solar Expert Posts: 1,280 ✭✭✭
    Re: SG versus voltage

    Most common error is poor voltmeter accuracy. To do SG + 0.845 volt per cell (which is pretty accurate down to about 40% SOC) you need a DVM with better then 0.5% accuracy (10 mV per cell accuracy). A +/-3% Radio Shack DVM won't cut it.

    The SG + 0.845 per cell is a no-load equilibrium (rested) voltage. This voltage is only dependent on electrolyte SG (unless there are other contaminents in electrolytes or high internal battery current leakage between plates) Any loads changes the number since having battery current flowing by default requires kinetic driving of electrolyte and plate ion material movement that takes some voltage off the yielded battery terminal voltage.
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