Specific Gravity and Temperature

samuel · January 2012

In my free time I've been taking a closer look at specific gravity in FLAs. I'm aware of acid stratification and have observed charge lag and temperature effects. I'd like to pose a question to others who measure the specific gravity of your batteries.

Is measured specific gravity higher or lower in the cold? Elaborate if you can. Please only reply if you have first hand numbers, but if you want to quote the literature please drop a web address to your source.

waynefromnscanada · January 2012

Re: Specific Gravity and Temperature

Cold electrolyte is more dense than warm, thus reads higher for any given state of charge.
I just came in from checking the SG in every one of the 18 cells in my six L-16's, then adding water before selecting EQ to get them bubbling, so as stir it up.
My hydrometer has a built in thermometer that shows the temp of the liquid, minis 2 C in this case, and it also has a scale indicating how many points should be added for hot electrolyte, or subtracted for cold. Before subtracting the 25 points indicated on that scale, my SG showed way over charged, in fact, it was right off the scale past overcharged. After making the correction for the denser, cold electrolyte, they still showed a very high state of charge, but no longer off the scale.
Just for fun, before the EQ mode had a chance to mix things up, I checked the SG again, and as expected, it showed only "fair", as the lighter, warmer water was still on top.
BTW I check SG every time I'm about to add water, and a couple of times found one cell out of wack with the others, requiring EQ to bring it back, but that was a couple of years ago and it's been fine ever since. BTW, it's been 3 1/2 months since last water was added to these 10 year old batteries.

samuel · January 2012

Re: Specific Gravity and Temperature

Thanks for the response Wayne,

My hydrometer is from freas glass and also has a temperature correction scale. I've been observing a lower SG in winter with my golf-cart batteries. While taking readings through a charge and discharge cycle I noticed that specific gravity remained relatively constant after the 'full charge' despite my drawing current from the batteries. I'm reasonably sure my mistake was not waiting a few hours after EQ'ing to measure SG (wasn't practical in my situation) - my guess is that the bubbles skewed my readings.

Cold liquid has a much greater capacity for holding gas/bubbles (think of warm versus cold soda and the amount of fizz). In summer when I do a reading after an EQ the bubbles probably dispersed very quickly, but when around 0°C the bubbles linger much much longer.

samuel · January 2012

Re: Specific Gravity and Temperature

If I want to get an accurate specific gravity reading after equalizing, how long do I have to wait before I can take a measurement?

waynefromnscanada · January 2012

Re: Specific Gravity and Temperature

Once there's no more signs of the bubbling inside the battery, you should be good to go. In fact, I've had no problems even if the battery was bubbling, because any bubbles that might be in the electrolyte come right to the top and escape inside the hydrometer, leaving only liquid around the float, so all is good.

samuel · January 2012

Re: Specific Gravity and Temperature

Once there's no more signs of the bubbling inside the battery, you should be good to go. In fact, I've had no problems even if the battery was bubbling, because any bubbles that might be in the electrolyte come right to the top and escape inside the hydrometer, leaving only liquid around the float, so all is good.

Even in the cold (30-40°F)? I can't seem to get those results with my T-105 generic equivalents from Sam's Club.

waynefromnscanada · January 2012

Re: Specific Gravity and Temperature

samuel wrote: »

Even in the cold (30-40°F)? I can't seem to get those results with my T-105 generic equivalents from Sam's Club.

Even in the cold, ZERO*F. There will be a very thin layer of concentrated electrolyte right up next to the plates of a battery that is being charged, or has just come off charge, and this may take up to 3 hours of total rest, to disperse, and that may raise the SG slightly, but only very slightly - - if at all.

Vic · January 2012

Re: Specific Gravity and Temperature

Speaking about the systems that I keep an eye on, The only bubbles that I see in my Hydrometer, are caused by the hydrometer being used too many times without RINSING. Rinsing is required after each use. Rinse several times with distilled water, and store the hydro vertically if possible -- helps the water to drain. If it has been a long time since the last use, then rinsing before use is good.

Then, the first cell that you measure the SG on should (or could be) be the cell with the highest historic SG (IMHO). This will help avoid further diluting the electrolyte in one's pilot cell, which is chosen by finding the cell with the lowest SG.

Hydrometers that are seldom, or never rinsed retain a sticky residue. This stickyness increases over time, and it retains air, that forms bubbles when the hydro fills with electrolyte. Bubbles adhering to the float of the hydro cause erronious high SG readings.

When one's Asorb voltage is reasonably high, the later portion of the Asorb stage causes significant bubbling. This bubbling helps diffuse the higher SG electrolyte at the surface of the plate.

Reaize that the bubbles that have been referred to earlier in this thread may not be the ones that I have mentioned, however, I do not see bubbles in the hydro. If the hydrometer is clean, any bubbles should immidiately pop, and the contents rise to the top of the hydro.

Normal diffusion should cause any acid concentration gradiant at the plate surface to subside in a short time.

Just my observations. Goos Luck, Vic

Specific Gravity and Temperature

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