Checking hydrometer accuracy with charged battery voltage

[HiTech]

Is it true that:

Specific Gravity = Cell Open Circuit Voltage - 0.845?

Could one average the specific gravity readings across cells in a battery, and use this formula to check the accuracy of the voltmeter and/or hydrometer (based on which one is calibrated better)?

Jim

[mena661]

HiTech wrote:
So I love the song, but can I call a friend? I need a clue on the video. With luck I won't blow any thing up (this time).

Jim

LOL! The song was meant to emphasize MEX being on point as usual. 🙂

[HiTech]

So I love the song, but can I call a friend? I need a clue on the video. With luck I won't blow any thing up (this time).

Jim

[mena661]

MEXICOWANDERER wrote:
Hi Tech, do it the old fashioned bass-ackwards way. Get your hands on a flask of guaranteed brand new electrolyte that certifiably has the OEM density posted on the container.

Then go from there...

http://www.youtube.com/watch?v=4m48GqaOz90

[HiTech]

Hmmm would a battery shop have this? Maybe buy a hydrometer from them if it agrees with their acid.

Jim

[MEXICOWANDERER]

Hi Tech, do it the old fashioned bass-ackwards way. Get your hands on a flask of guaranteed brand new electrolyte that certifiably has the OEM density posted on the container.

Then go from there...

[HiTech]

Lol 3 kids in college and tuition bills almost due.

My idea in the other post about getting the relation of SG to voltage was to try a cheapie and see if it agrees.

Jim

[mena661]

You could buy a Freas and a few others, note the differences and post those here. 🙂

[HiTech]

It seems like a lot of the hydrometers available could easily be .1 or more off just fom posts here. If I could find a $15 one that was half way accurate until I get a Freas I'd be happy but it seems really hard to get info on accuracy or repeatability of the moderately priced units. Especially anything I can find locally.

Jim

[MEXICOWANDERER]

I do not see how. The longer a cell sits just after charge cessation up to an hour the higher the gravity climbs because the lag and over run tendency of chemical / electrical relationships. The electrolyte could cool and gain density but the other war round is hard to envision.

Then again I am assuming the measuring instrument is reasonably accurate. A .02 error to me is unacceptable. But an OEM releasing 1.300 FLA to the public would be even more disturbing. Oh well, that's life.

[HiTech]

Mex it was not a resting reading if i read BFLs post correctly so I guess it would drop a bit?

Jim

[MEXICOWANDERER]

I adjust 1.280 SG down to 1.265 for average temps near 100. I have never heard of OEM using 1.300 except for polar applications.

At 25C 1.300 SG will eat a battery 20% faster than 1.280 acid.

Maybe I read this wrong.

[HiTech]

mena661 wrote:

HiTech wrote:
12.78v predicted. Pretty close given likely error in hydrometer and volt meter. Even more so if it had settled slightly at 24 hours.

I know you didn't ask but I use a Victron battery monitor for voltage (does other stuff), it uses a shunt (like the Trimetric), and a Freas hydrometer which I understand to be pretty accurate.

I'm using three shunt meters as well - a watts up and two turnigies. They are not quite as good because there is 3 inches of wire between the battery and shunt vs the cool on post shunt a lot of you use, but both seem pretty accurate. I desoldered the shunt on one of my watts up with too much transient current. I am thinking of getting a larger one and remote mounting it at the battery post. They have directions on how to do this on their web site.

Jim

[HiTech]

Boundary conditions were of course a great test. At a SG of 1.000 volts are 0 so of course this is way off as you take it out of normal operating ranges. Good approach.

I wonder if there is a better formula that actually correlates specific gravity to voltage. I assume it must be a curve rather than a linear relationship.

The paper shows charge *pretty* linear down to 2 molar acid, then it avalanches.

Ok so it's a log function.

Voltage= voltage at 1 molar acid concentration (say 11.7v) + 26mV*ln(molarity of acid)

At normal temp and for a battery where it is 11.7v being discharged down to a molarity of 1.

Jim

[HiTech]

Ok more reading lol. I only got are far as organic chemistry. I considered it just a specialty of physics which pissed of chemists no end.

Yes I believe Freas to be the best commonly available.

Jim

On edit: ktmrfs add "battcon" to google searches for deep papers. I have found it helpful.