Battery SG question...

hillbilly Solar Expert Posts: 334 ✭✭
Thanks again to all for sharing such a wealth of knowledge, and more importantly experience. I have yet another question regarding batteries:
today being EQ day, I checked the SG of all cells of my Surrette S460's (two strings of four). I was suprised that there seemed to be a wide range of numbers, granted I am using a fairly cheap Hydrometer, and I am aware of how touchy they can be in general to get a good reading. I usually just sort of take the numbers with a grain of salt, and don't trust them implicitly to judge the state or health of my batteries... but... I was getting reading of as low as 1.225-1.230 from some sells and others were nearly 1.270!
Most seemed to be right around 1.250 plus or minus a bit, with the cells in the middle batteries averaging a bit higher, and the cells at the four corner batteries reading a bit lower (this seems to always be the case, particularly at this time of thier monthly right before the EQ cylcle).
I had always heard that a major difference in SG readings between cells of the same battery were a bad sign, but how about that big of a difference between cells of the same bank? The biggest difference in any one battery was from about 1.225+/- to about 1.250+/-, but again some of the cells in the center of the pack had readings of almost 1.270.
I am wondering if this could be a sign of poor battery performance (not sharing the load well?), or if this could be a sign of battery sulfation, or??? I should mention that the last two and a half weeks have been unusual in terms of more power used and very little to no sun input. I have topped the batteries up with the generator (via a VFX3524), but that does not give them the nice long float trickle charge, and I have never trusted that the batteries really get fully charged with just the generator. Probably pretty damn close, but not the same as good ol sunshine.
Any suggestions are greatly appreciated, thanks in advance


  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Battery SG question...

    my suggestions: (but I'm not an authority)

    1) run genset in AM to bulk up batteries, let solar top them off. Daily, till they can get to 100% on their own.

    2) don't get trapped into "deficit charging"
    The longer they dwell at less than 100%, the faster the damage accrues, and the faster they die. (sulphation)
    At least run the charge up every 48 hours, if the solar can't keep up (wintertime).

    What's easier/cheaper: dozen gallons of fuel, or a dozen batteries?

    Letting the batteries sit low for a week, takes months off their lifetime.

    3) ASAP, (run the generator if needed) as soon as batteries have gone into float, manually force an EQ cycle (only if flooded cell batteries) if you don't have enough solar, run the generator for the couple of hours it takes.
    Then after EQ, top off water levels, maybe swirl the added water in a bit, and measure SG. that will give a truer reading of health, after a EQ cycle. You may have to do 2 cycles in 3 days, if some cells are way off. the notes you keep will let you know if the EQ is improving the low cells.

    Just don't kill your batteries because the calender says you are not due for EQ for 2 more weeks.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • hillbilly
    hillbilly Solar Expert Posts: 334 ✭✭
    Re: Battery SG question...

    Mike thanks for the ideas:

    RE#1 and 2, that's been my plan of action for a while now, what I meant by my last comment was that the last two weeks we've had next to no sun for "the finishing touch", thus all I have had was just the genny :( When using the VFX, it goes from absorb to silent (supposedly to save on fuel), then finally to float... I am realizing that although very helpfull, I don't think that it's quite the same as having that bit of sun to finish the job off with a nice low trickle charge that lasts for hours.

    #3, sounds like that might help out, I have done that once or twice when my trimetric was saying that the batteries still had another 5% left to go but the inverter switched into silent mode. the other thing that I have taken to doing is to up the absorb time up to 2.5 hours.

    Lucky for me there is plenty of sun today, and I am in mid EQ as I write this... just wondering if these numbers were indicating sulfation issues, or something else...
    thanks again
  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    Re: Battery SG question...


    Surrette suggest watering before or halfway through the eq cycle, watered cells will have dilute acid near the top of the cells if the bubbling action of eq doesn't get to mix the water throuout the cell column. I've noticed this myself.

    The best investment in flooded acid batteries is a good hydrometer, that you can trust. $50 investment to protect your $10k worth of batteries? No question which way to go. I check SG's before, during and after any genset charge and never trust the electronic meters for SOC/DOD.

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Battery SG question...

    I might add two points.

    The first is get yourself a good temp compensating hydrometer so that you really know whats what.

    Second, is it possible that the batteries have significant temp differences between the inner ones and the outer ones? For example are the outer ones subjected to cold breeze while the inner ones are protected? Battery temp has a significant effect on SG reading. Here is a handy chart to compensate.
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Battery SG question...


    Here's a link to useful information on testing, charging, and maintaining Rolls / Surrette batteries:

    Jim / crewzer
  • hillbilly
    hillbilly Solar Expert Posts: 334 ✭✭
    Re: Battery SG question...

    Thanks, I am usuing a temp compensated Hydrometer. That is a good point however about the batteries in the center being undoubtedly warmer. they are in an insulated, but also ventilated box outside, so when it's cold the batteries on the outside would certainly not be as warm.
    I am a believer in using a Hydrometer (and any other meter that I can) to try and keep up on the health of my batteries. I have had a better (well, more expensive anyways) that broke, this one that I have now is a temporary one. I will say this for it, although it is a cheaper unit, it does have one big+ which is that the number scale is much larger and easier to read than those I've had in the past. I wish I could remeber the name of the last one that I had, which came recomended by the local shop... problem was that for me it wasn't really any more acurate, as I was always squinting at it and tilting it carefefully back and forth to try and decide what number I was actually reading... very little physical difference to tell them appart... anyone have recomendations on a specific hydrometer I am all ears, would love to have a back up if nothing else.
    In checking the SG's a moment ago... looks like they are getting a bit more consistant, but still some big differences overall. Two of the batteries in the center of the pack are giving readings from 1.28-1.3 while on the outside the two positive end batteries are reading about 1.255-1.265, and the two on the negative end are reading from 1.245-1.26 or so. At least there is a bit less varience within each individual battery, other than one of the positive and one of the negative end batteries they were all pretty similar readings within each individual. I've turned the charge down a bit, as it's been two hours at 32V (w/temp compensation)... thought that I would let it run at 31V for another half hour to an hour and watch the numbers... hopefully the'll settle down a bit.
    Thanks again