Battery SOC, specific gravity instrumentation

[BruceM]

WJ has inspired me with his determined quest for sensors;  so here's my new quest:

Lead acid battery state of charge (SOC) can be computed on the fly with by currents in and out, with some "resets" on charge complete and sanity checking the resting voltage whenever that happens.

But if I had a way to read specific gravity from just one battery cell, I'd be much more confident-  SG is the "gold standard" for wet batteries SOC. 

I've thought about a tiny magnet in a glass tube as a float, with one of the Allegro Microsystems switched bias, analog output hall effect sensors as the pickup.  The switched bias (50KHz) types are stable over time.  The tube would need a bulb on the bottom, as there's not much depth of acid over the plates, and all I can think to do is have it sitting up out of one cap hole.

I'm hoping someone will have some better ideas on how to do a real time specific gravity measurement on a lead acid battery.

Any thoughts?

[dubbleUJay]

Bruce, here is a patent, with among other things, using a transducer to read specific gravity.
http://www.freepatentsonline.com/4329406.pdf

BUT, if you had a few of these SG-Electrodes, you'll be well on your way
http://www.findasensor.com/maninfo.php?uid=387&idc=140
and when you follow the links:
http://www.electrosense.com.au/SG-Electrode.htm

Just to give you some ideas of how to do it.

[BruceM]

The polymer battery specific gravity sensor is cool, but the company doesn't have a product yet.

I really only need one sensor, as my BBCC monitors all the individual batteries during the charge completion phase.  It's not worth the $ and hardware hassle to monitor more cells, as long as the controller knows the SG sensor battery is operating within tolerance with the other 9 batteries in series.  

I found another patent  (6356675, J Weiss, Sandia Corp.) where a prototype had been built and tested but it's not available as a product.  

http://www.sensorsmag.com/sensors/electric-magnetic/fiber-sensing-measure-battery-state-charge-1484

It uses a fluid filled tube as a light guide, with a mirror in the end. It looks like a tiny dipstick in a battery cap.  An LED shines light down and the reflected back, the light level is changed by the index of refraction which depends on Specific Gravity of the acid.  I've written him to see if I might be able to buy one.

[dubbleUJay]

That one looks like the way to go Bruce, I was under the impression that you wanted to measure each cell in your 120Vdc battery bank !!!
10(12Vbatt) x 6 cells = 60 sensors (or 66 if u have 11 batteries )

[BruceM]

It is 11 batteries total- I have one for 12V which is used to power the BBCC, cooling fan for the shunt regulators, power the low side switches, output voltage regulator, etc.    It's also the "backup" battery (match to the primary 10) to put in the string in case one goes bad prematurely.

No way I'm going to pop for 66 of those babies, I'll bet it's going to be over $100. each.  For one, sure.

[BruceM]

I did get a fast response from the inventor, very nice guy.

The fiber SOC sensor is still not a commercially available product.  I can get 10 for $1000 made up special for me (not one for $100 as I'd hoped), but I don't have that kind of money for this project.  Another problem is the current units need more depth of acid that I have.