Measure SoC of lead acid battery

[kcs]

I want to measure a charge level of 12V lead acid battery. It seems the only way to do it is to measure battery's voltage in open circuit and compare to a given table/graph. There are few problem though:
 - I want to measure battery that is not in open circuit (battery is powering uC and I want to know how much time I have left before it dies)
 - Graphs/tables that show how voltage is related to SoC are all a bit different (so which one should I take as a true reference)?

I did an experiment. 12V 1.2Ah battery was charged and left for few weeks. In open circuit DMM reads 13.04V. When 76mA load is connected, voltage quickly drops from 12.90V to 12.68V, at that point decrease in voltage becomes very slow. Based on this data, should I assume that battery is 100% charged?

Also, I am curious how SoC is measured with professional devices like Midtronics?

[Neverther]

Other stuff:
BCM recording current in and out with known battery characteristics (modern crap you need service tool for to tell the battery has been changed).
Weight/density of the electrolyte (some batteries have an eye for builtin indicator on one cell).


Connecting load to the battery takes the surfacecharge off and gives out the true voltage after recovery.
Measuring while loading tells you how good the battery is giving amperage (sulfated or damaged plates/increased internal resistance, etc).
Those tools are mostly for bigger batteries.

Make a voltage chart for the battery by experiments with the current drawn. If you got something like logging multimeter+resistor (or accucel-6 or the real ones/obvious fakes, you can use the discharge function and serial comm on PC for recording the data). Map/plot a curve for data and use if in the uC with voltage measurement.
Every battery has different chart as the geometry is different. Some can supply more current relative to their size than others.
Characterize your battery.

And note that normal lead-acids start to die if you deep discharge them often.

[michaelym]

You can tell when the charging is done when the charge current gets close to "0", actual value is dependent on the battery chemistry.

[kcs]

I will be using 12V 7.2Ah batteries of different brands. Making a voltage chart for each battery is not really possible in my case. What is the next best option? Maybe there any smart IC that can do a precise SoC (closed circuit) measurement? I have only found BQ34Z110, but it's a bit of an overkill.

[KD0CAC John]

There is any number of devices to measure specific gravity , then measure each cell and keep the data on a chart , do this on a monthly basis .
It works on all FLA batteries .

[kcs]

Give me an example of such device.

[KD0CAC John]

Google works sometimes
https://www.google.com/search?q=specific+gravity+test+of+battery&espv=210&es_sm=91&tbm=isch&tbo=u&source=univ&sa=X&ei=TcY-U5KbH6Oj2QX8_4CYAg&ved=0CFYQsAQ&biw=1215&bih=666

[kcs]

Sometimes it does...

How would you use this
device with this

battery? I would really like to know...

[David Hess]

If I had no better way, I would use a measurement of the internal resistance as the battery discharges to estimate the state of charge.  Temperature should also be taken into account.

[KD0CAC John]

There are many ways to skin a cat , deciding which way depends ?
If that battery is all your worried about then not spending much is probably an issue ?
But it you use batteries much and want to save money , then buying some equipment for the job can be in order .
But you leave out details and make it difficult to come up with an answer that you will accept ?
All batteries need 2 ways to measure there usefulness , volts loaded and volts & amps under load .
To stick to my original idea , using an optical specific gravity test tool , that battery has 2 layers of caps , the 1st you see on the top uncovers the 6 individual caps , then it could be called a 3rd cap , it is a anti-spill plug under the 2nd cap .
Use a syringe to recover a few drops of electrolyte and apply to optical tester .
On another angle , getting one of these or something similar - http://www.westmountainradio.com/product_info.php?products_id=cba4 

[G7PSK]

10.5 volts under load for maximum discharge levels is what is recommended by suppliers of deep discharge batteries for wheel chairs.
That device is a hydrometer best tool for wet battery checks.

[kcs]

David Hess,
Thanks for suggestion. I see two challenges implementing your idea:
 - To build a circuit that measures internal resistance (the one listed on 3rd page does not look noob friendly: http://m.eet.com/media/1131277/13405-33105di.pdf)
 - To get a formula that could give an approximate SoC based on internal resistance and temperature (temperature will vary between -20 to +30)

KD0CAC John,
If I have missed to tell some important details, just let me know and I will try to add them.
For my project I need to know when my battery is dying. I am looking for a simple yet not expensive solution (<=$50) that I could build.
I believe using a gravity tool is big overkill in my case. It will be an untended box (approximately 1 km away from me) that will gather some environmental data and send it back to me wirelessly. So I can hardly imagine how using a gravity tool in my case would work.

[KD0CAC John]

Same thing over again , provide the details under which the test is to be carried out .
Adding factors after the answers and saying the one answering is wrong , is ridiculous .
As you 1st posed the question , my answer would have been the most accurate and cheapest .

[kcs]

Same thing over again , provide the details under which the test is to be carried out .

Specify what extra details do you need.
As I have said, the whole thing will be unattended, so your suggestion will not be suitable.
Also, only non-intrusive method  should be used.

Adding factors after the answers and saying the one answering is wrong , is ridiculous .

I am not saying it is wrong, I am saying it is not suitable in my case.

[David Hess]

David Hess,
Thanks for suggestion. I see two challenges implementing your idea:
 - To build a circuit that measures internal resistance (the one listed on 3rd page does not look noob friendly: http://m.eet.com/media/1131277/13405-33105di.pdf)
 - To get a formula that could give an approximate SoC based on internal resistance and temperature (temperature will vary between -20 to +30)

The circuit you linked which was designed by Jim Williams pretty much covers the idea.  The change in battery voltage is measured when a known current is added to the already existing load on the battery.  In your case this would be done periodically when the battery state needs to be checked and not continuously.  There is a roughly fixed contribution to the resistance measurement from the leads and connections but it is relatively constant compared to the internal resistance of the battery cells themselves.  Comparing the measurements from a new battery in the charged and discharged state should give an idea of how much the measurement will change as it discharges.

As far as calibration, I would just determine the general formula empirically through testing.  This is not the sort of information the battery manufacturers are likely to have and even if they did, I would verify it anyway.

Temperature complicates matters because it directly affects the internal resistance of the battery.

As far as implementation, I would either use an integrating type of analog to digital converter (delta-sigma, integrating, or voltage to frequency) to make high resolution voltage measurements and a simple switched current sink or a synchronous modulator and demodulator design like you linked but only operated when a measurement is to be made.  I am assuming your nominal circuit power draw is relatively constant but if it is not, maybe the state of charge measurements need to be synchronized with your normal load so they do not interfere.

[kcs]

Thanks!