Specific Gravity Question

Re: Specific Gravity Question

dagr51 wrote: »

I have a question regarding reading specific gravity numbers. If my hydrometer reads 1.270 and the included thermometer says the temperature is 40F (as an example to make this easy), I am supposed to subtract 10 from the 1.270 number to temperature correct, which is 1.260. Is the specific gravity of the cell 1.270 or 1.260? Is the state of charge in this example 100% or 95%? This has confused me for awhile and I end up comparing the numbers to previous readings but I'm still not sure what the numbers really mean. Thanks!

Hooboy, did you ask a complicated question!

Or is it just the answer that is complicated?

The problem is that technically the specific gravity is the ratio of two densities, that of the test liquid and that of water. But since the density of both the test solution and water varies with temperature, the SG measurement should be written to include both temperatures (for example SG (20C/20C) = 1.21.) But nobody does that.

So to get a practical answer to your question, you have to ask what the purpose of the measurement is. In this case you are really trying to get the concentration of sulphuric acid (H2SO4) in the electrolyte. That concentration is not going to vary with temperature, but the SG will. That is, if the electrolyte is at 40C, the SG (40/20) will not be the same number as the SG (20/20) measured at 20C.

For that matter, the SG of water will change with temperature! So at 40C, the SG (40/20) of water will not be the same as the SG (20/20)., that is 1.00000.... On the other hand, the SG (40/40) will be the same as SG (20/20).
What the temperature correction factor does is tell you for a given temperature, T, what the SG (20/20) would be for the liquid for which you measured the SG (T/20).

In that sense, the SG of the electrolyte really is what the uncorrected hydrometer reads, but the number you want to use to check the state of the battery is the corrected number. It is the corrected (to an ideal rather than actual value) that tells you the H2SO4 concentration, and that is what you really want to know.

PS: More or less what Coot said, but with the formal notation. :-)

Re: Specific Gravity Question

dagr51 wrote: »

So it can be at a 100% charge at different readings depending upon the temperature? In my example, 1.260 at 40F is 100% charged? If I put the battery in the sun for a few hours the sg reading would go up, but the battery would still be 100% charged, correct?

I take issue with Coot on this one. (With his words, if not with his meaning)
If you take a 100% SOC battery and heat it up, the SOC will not change, but the uncorrected SG will.
The corrected SG (the equivalent SG (20/20)) will not change.
You do have to be careful not to apply the correction twice:
If somebody makes up a table of uncorrected SG versus temperature for fixed SOC values, then you should use the uncorrected SG to look up your SOC in that table.
If your Hydrometer includes automatic temperature correction, you have to either un-correct or use a different table. :-)

Coots answer of "No" is correct based on the fact that the density of the electrolyte will go DOWN not UP. To some people that would mean that the real (as measured by the hydrometer actually) SG, designated SG (T/20), will decrease. But the calculated SG (T/T) which is also very close to SG (20/20) will not have changed since the H2SO4 concentration will not have changed.

But it all comes down to the question of what you mean by "real" SG in the first place. To most people the "real" SG is the corrected value. To the purist, who would say that the SG of pure water changes with temperature, the "real" SG is the uncorrected value.

PS: For at least some SG versus SOC tables (for example those from Trojan) the reference temperature is set at 80F (27C). So my although my examples using (20/20) are still perfectly accurate, the reference given in those tables appears to be either SG (27/27) or maybe SG (27/20). <sigh> Oh the ambiguity of it all. :-(
Regardless, if you calibrate your hydrometer to read 1.277 corrected, using the batteries as shipped and then fully charged, you would then correct your other readings on the basis of the difference between the current temperature and 27C.

Re: Specific Gravity Question

dagr51 wrote: »

Okay, let me ask it a different way. Most of the time when I'm checking specific gravity, I am trying to compare the numbers in individual cells with previous readings of the same cell. I try to do it quarterly for this purpose, comparing it to a base reading taken when the batteries were new - looking for trends. Of course, these readings are taken after a full charge, then an equalization and I let them sit with no load or charge at least overnight. So, my question: When comparing previous numbers, do I compare the temperature compensated numbers or the "raw", uncompensated number?

Since your goal is to compare concentration to concentration, you should always be comparing corrected numbers to corrected numbers. Especially if the temperature of the electrolyte has changed.
Bottom line. :-)

Re: Specific Gravity Question

The OP also asked this question:

dagr51 wrote: »

Is the specific gravity of the cell 1.270 or 1.260? Is the state of charge in this example 100% or 95%?

While I understand (I think) what has been said regarding SG, I'm not sure how that relates to SOC at the present time. If the temperature corrected value is 1.260, is the SOC 95%?

Thanks

Re: Specific Gravity Question

I thought so, but wanted to confirm.

Thanks.

Re: Specific Gravity Question

dagr51 wrote: »

Thanks to all. (I think!)

As long as you are satisfied with an answer to "What should I do?" rather than an answer to "What does it all mean?", you should be just fine. :-)