Question about measuring specific gravity

PlowmanPlowman Solar Expert Posts: 203 ✭✭✭✭✭
I know that when measuring voltage to determine state of charge the batteries need to rest for a few hours.

I thought measuring specific gravity could be done at any time, but I just came across advice to the contrary, that SG should be also be measured when the batteries have rested.

My system is charging and discharging almost 24/7. How important is resting the batteries before measuring SG?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Question about measuring specific gravity

    It isn't, once you understand the nuances.

    Certain batteries such as tall case designs will not give an accurate SG reading some times because the electrolyte won't be properly mixed just after charging; it takes time for it to "settle out". Standard batteries like your T105's may show this to a lesser degree. Also the temperature will be different right after or during charging vs. when resting.

    But for the most part we have active systems and we really can't turn off the battery connections and wait half a day to see what the 'true' SG is. Thus you have to learn how to interpret your own system's readings "on the fly". Having a fully-charged baseline reading is a good start. Knowing how much temperature affects your readings is good too. It is after all a comparative thing, so if you take the readings under similar conditions you see the change and know if it is enough or not. It also helps to have weak eyesight so you don't get fixated on that third decimal place. ;)
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Question about measuring specific gravity
    Plowman wrote: »
    I thought measuring specific gravity could be done at any time, but I just came across advice to the contrary, that SG should be also be measured when the batteries have rested.

    Where did you find that advice? It's new information to me and I would like to read more about it.
    Certain batteries such as tall case designs will not give an accurate SG reading some times because the electrolyte won't be properly mixed just after charging; it takes time for it to "settle out".
    <snip>
    Also the temperature will be different right after or during charging vs. when resting.

    Your point about temperature is well taken. The "settling out" is not as well taken... I can't think of any reason why the electrolyte would "settle out". Gassing is what stirs the electrolyte and that stops right away after charging is complete. Are you suggesting that the "settling out" is a process of diffusion?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • PlowmanPlowman Solar Expert Posts: 203 ✭✭✭✭✭
    Re: Question about measuring specific gravity

    Thanks, that's kind of what I figured. I fill and empty the hydrometer 3 times before I take a reading to try to get the electrolyte as mixed up as possible. I'm using a Hydro-Volt hydrometer which has temperature compensation built in, though I wonder sometimes how (if) it works.

    When I'm expecting my batteries to be charged (full day of sun, minimal loads since I've been at work all day), the hydrometer does show a 100% state of charge. So I guess taking readings of SG without resting the batts is working....

    The charge controller's voltage and amps can also give an idea as to SOC, correct? If it's floating, the batteries are obviously at 100% (right?). How about when it's still at 14.8V but amps are very low? This is usually the case when I have something small plugged in (cell phone or netbook). My understanding is that it won't stay at float if I have a load.

    I've checked the SG when the voltage from the CC is at 14.8V but the amperage is very low (<1), and gotten a reading of 100%.
  • CDN_VTCDN_VT Solar Expert Posts: 492 ✭✭✭
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Question about measuring specific gravity
    vtmaps wrote: »
    Your point about temperature is well taken. The "settling out" is not as well taken... I can't think of any reason why the electrolyte would "settle out". Gassing is what stirs the electrolyte and that stops right away after charging is complete. Are you suggesting that the "settling out" is a process of diffusion?

    --vtMaps

    It depends on how you want to read it: immediately after charging when mixing is still going on or after this process has finished. "Settled down" might be a better term, as you're looking for the sample to representative of the whole. If you suck up electrolyte while it's still bubbling you can pick up tiny gas bubbles that will float the float and make the reading artificially high.
  • PlowmanPlowman Solar Expert Posts: 203 ✭✭✭✭✭
    Re: Question about measuring specific gravity
    CDN_VT wrote: »
    Would be nice to try one of these

    VT
    That's what I'm using. I really like it compared to the cheap glass hydrometer I was using, it gives consistent readings that are easy to see and doesn't dribble nearly as much.

    I do wonder how accurate the built-in temperature compensation is, though.
  • PlowmanPlowman Solar Expert Posts: 203 ✭✭✭✭✭
    Re: Question about measuring specific gravity
    vtmaps wrote: »
    Where did you find that advice? It's new information to me and I would like to read more about it.
    Of course now I can't find it, been googling around a lot this morning and forgot where I saw it.

    This is the closest I can find, but it doesn't say the batteries need to be rested, just all loads and charging sources removed, which of course I never do when measuring SG.
    Keep in mind that both voltage and SG readings will be more accurate if taken under a no-load condition, with all charging sources and loads disconnected from the battery.

    http://www.trojanbatteryre.com/Tech_Support/BatteryMaintenanceOffGrid.html

    EDITED:

    I think this is where I read it.
    Battery Testing can be done in more than one way. The most accurate method is measurement of specific gravity and battery voltage. To measure specific gravity buy a temperature compensating hydrometer, to measure voltage use a digital D.C. Voltmeter. A quality load tester may be a good purchase if you need to test sealed batteries.

    For any of these methods, you must first fully charge the battery and then remove the surface charge. If the battery has been sitting at least several hours (I prefer at least 12 hours) you may begin testing. To remove surface charge the battery must be discharged for several minutes. Using a headlight (high beam) will do the trick. After turning off the light you are ready to test the battery.

    http://www.batterystuff.com/kb/articles/battery-articles/battery-basics.html#6
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Question about measuring specific gravity

    That would sure be inconvenient. "Got to shut the system down for half a day to check SG". I can hear the screams now. :p
  • CDN_VTCDN_VT Solar Expert Posts: 492 ✭✭✭
    Re: Question about measuring specific gravity
    Plowman wrote: »
    That's what I'm using. I really like it compared to the cheap glass hydrometer I was using, it gives consistent readings that are easy to see and doesn't dribble nearly as much.

    I do wonder how accurate the built-in temperature compensation is, though.

    Well I bought two for my setups. I compared them against my 1960 imperial eastman that has always been mine & looked after and against a cheaper glass & bulb style. Both of the Hydro volts read the same , they read so close that i think it's my eyes trying to see a difference. ONE HUGE FACTOR is that the Hydro volt CAN be read with just glasses and not glasses plus a magnifying lens as well.
    I picked mine up in USA but a backwoods store since NAWS didn't list them.
    My EYES like these ..


    I have always rinsed them out with distilled water , same water that I use for topping up the battery . For cleaning the Glass bulb type , I soak up bleach and let sit for a bit then use water to flush out any yellow acid slime . My Imperial glass looks new , would like to have new rubbers for it.

    VT
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