Battery "State of Charge" meter vs hydrometer (long winded techie rambling)

[rv_safetyman]

This subject has come up several times over the last couple of years.  The most recent thread is probably:  http://www.busconversions.com/bbs/index.php?topic=3018.15.

I thought I would bring it up again to seek some opinions from the "techies". 

The background is that battery state of charge is almost impossible to determine with a voltmeter (unless the battery bank has not been charged/discharged for several hours).  The solution is to buy a fancy integrating meter (keeps track of the amperage flow in and out of the batteries) or a simple hydrometer.

I opted for the TriMetric battery monitor from Bogart Engineering (http://www.bogartengineering.com/) based on some discussions with gumpy.  I really like the unit. There are other similar products on the market.

I thought I would check the calibration with a good hydrometer.  The problem is that there does not seem to be agreement on hydrometer scale vs battery state of charge.  The two sources I played with are:  http://www.trojanbattery.com/Tech-Support/BatteryMaintenance/Testing.aspx and https://www.pacificpowerbatteries.com/aboutbatts/Deep%20Cycle%20Battery%20FAQ/dcfaq3.html.  I looked at other sources and it seemed that there was no agreement. 

I plotted the data from the two sources listed and the data points seem to be fairly linear for both sources, but the slopes were different.  In the meaningful range (50% to 100%) the difference was not huge and was probably about the same difference as one would get from different hydrometers.  However, it is still troubling that there is not agreement in the industry. 

So to the experts:  does anyone have a well accepted chart for hydrometers?  I chose to favor the Trojan data, since they are among the leaders in the battery industry.

The results of my calibration effort were equally confusing.  My readings for meter readings  were as follows: 100% 1.275, 65% 1.246, and 51% 1.209 (% values are from the TriMetric and the hydrometer readings are corrected for 80 degrees).  My three point of data are much less linear than the tables.  It would appear that the settings I entered into the TriMetric give me a conservative reading.  When my meter reads 51%, it would appear that my battery bank is about 65%. 

Bottom line, I did set the parameters on the conservative side, so it would appear that the TriMetric is reasonable accurate.

Comments are welcome.

Note:  I just read what I wrote and it is obvious that I am a data freak/engineer whose mind is fried from driving 900 miles in the last two days in the worst wind/rain conditions I have ever encountered.  Oh well, I am going to post this anyway - maybe it will be meaningful to someone

Jim

[buswarrior]

Molson Canadian?

Good for unfrying a long day....

and the numbers all look better, too!

happy coaching!
buswarrior

[Barn Owl]

This does not answer your question, but one of the most accurate ways to test your battery (and antifreeze) is with a refractometer.

This is what I use and love it:

http://www.misco.com/products/7084VP%2B.html

There are some that are less expensive out there but I do not know how well they work. Another benefit that they feature: "Automatic temperature compensation provides immediate, accurate, direct readings without the need to measure temperature or apply a correction"

[H3Jim]

Jim, its the kind of stuff I'd be interested in seeing, so you are on track even if a little fried.  Glad you  made it safely!  

I ended up using Blue sky solar charge controller with shunt to measure all inpout and output.  I'm very pleased with the unit, but have no other objective data to compare it to.  In fact, my batteries are sealed AGM's so I can't check specific gravity, and voltage as you point out is very unreliable if the batteries have not been sitting unchanged for several hours.  Very difficult if you are using the coach.

Thanks for the post.

[niles500]

Jim - hydrometer = good for checking individual cells / load test = good for testing individual batts - HTH

[Dallas]

I have to agree with Laryn.. The refractometer is much more accurate than any of the floating ball hydrometers that I have.

The hydrometers don't even come close to agreeing with each other but when I checked my refractometer against a friends refractometer, they both read the same, even though they were from different manufacturers.

I consider the use of one completely necessary for long battery life and health, along with periodic cleaning of cases and filling with only pure distilled water.

My last set of group 31 house batteries lasted for almost 9 years before 3 of them became bad enough to replace. The others are still starting different vehicles, or were as of July '07.

I would love to have a trimetric battery monitor but it can only be as accurate as the parameters fed into it, and without accurate specific gravity information, the readings from any battery monitoring system will be wrong and may end up hurting the batteries more than they help.

Good Luck,

Dallas

[Nick Badame Refrig/ACC]

Hi Jim,

I think your true answer will always be different no matter how many different ways you test....sorry!

The more important piece of information that you want to achieve is that the specific gravity to be consistant

from one cell to another, and from one battery to another. [1 reason we want to buy batterys all from the same lot]

The more equal they are the better the charge state, and the longer the charge state of the bank.

Good Luck
Nick-

[edroelle]

Jim,

As you probably know, part of the accuracy of your meter is dependent on setting the battery bank capacity correctly.  Xantrex has a procedure on page 8 of this document

http://www.xantrex.com/web/id/76/docserve.asp

I think this would compensate for "aged" batteries and address your question in a different way instead of specific gravity.

Ed Roelle
Flint, MI

[Sojourner]

Jim....John Palmer is a man who every RV manufacturer calls on to correct their 12v problem from A to Z when he go up to Elkhart IN during summer time & back to Lakeland shop in FL during winter.
I have had a few chats with John at Elkhart Camp ground a few years ago. I myself am well studied about wet cell battery since 1968 at Delco Remy school. So after talking to John, I learned that he knows everything you ever want to know.
A quote from one manufacturing company "John is well known for his impeccable work and high rate of customer satisfaction. He specializes in coach electrical work, batteries, solar systems, inverters and chargers." unquote.

Phone # in this link:
http://www.jolynenterprises.com/Inverters.htm

FWIW

Sojourn for Christ, Jerry

[rv_safetyman]

Wow, great input.  I can't believe that folks would even read this thread

Buswarrior:  Remember it is now Molson/Coors/Miller

Barn Owl:  About the time I think that I have all the tools (toys?) someone comes up with a new one that I have to buy.  That looks like a great tool since it also does antifreeze as well.  Thanks for the input.

H3Jim:  You are the SOB that got me started down this path in the first place when we talked about it during our stay at your place on our maiden voyage.

Niles500:  I was glad to see that the readings for all of the cells (8 batteries X 3 cells) were pretty even after 2 years of service and at least two accidental very deep discharges.

Ed:  my main goal was to see if the settings I used were correct.  The main value is battery capacity (amp hours) was my main concern.  I am using Exide golf cart batteries and you can find differing values for capacity.  I used a reasonably conservative value and that seems to have worked.

Jerry, Nick, Dallas, thanks for the input.  BTW, I talked with John a bit at one of Jack's parties and he is quite knowledgeable.

Assuming that I crack out and buy the Misco unit, that should cut my tolerance quite a bit on the instrument side, but there is still the issue on the industry not having agreement on SOC vs. specific gravity.

I am at a point where I feel comfortable with the gauge and calibration and am now probably at the "splitting hairs" stage (no comments about my follically challenged condition ), but that is the kind of thing that old engineers do.

Jim

[Rick Brown]

....I thought I would bring it up again to seek some opinions from the "techies"....
Speaking to you now is an electrical engineer with electric vehicle and off grid experience.
....The background is that battery state of charge is almost impossible to determine with a voltmeter...

From a practical standpoint a volt meter is the most useful tool in the kit.

I have a line powered 4 - 40 Volt digital meter (alliedelec.com #527-0050) continuously monitoring the Bus 'bus' voltage which I can supply from the house battery, the starting battery, both, or nothing.

When the Bus is siting in my drive unused both batteries are connected to the bus in parallel and in parallel with a 3 Amp 25.5 Volt (my Bus is a 24 Volt system) ac line powered power supply.  Thus a quick glance at the meter tells me battery health.  When fully charged the reading will be 25.5 +/- about 0.4 Volts depending on temperature.  A different reading may mean the batterys are not yet charged or you have a problem.

When I'm driving I should see somewhat more than 27 Volts.

When off grid and using only the house battery the voltage will decline from about 25.5 Volts down to about 24 Volts, which is as low as I'll allow it to go for battery lifetime considerations.

I don't care what % of charge remains because I don't know how many Amp hours are in the fully charged battery to start with.  Every time you cycle the battery it looses some if its full charge capacity.  In my particular case a new house batery will drop from 25.5 to 24 Volts over a weeks time.  An old battery will make that plunge in a couple of days.

I recommend you place a volt meter someplace where you can take a glance at it every time you walk by.  You will soon learn the ebb and flow of your battery system without having to fuss around with messy, hard to interpret hydrometers.

-RickBrown in Reno, NV

[gus]

Jim,

Your last line says it all!!

At least this stuff will keep you data freaks busy until you actually have a bus problem!!
Grin

[HB of CJ]

WOW!!!  Thank you everyone.  I just learned something new that needed to be learned.  Love this board!! 

[H3Jim]

Jim, Glad I could help!! 

[pvcces]

For those of you that might be interested, I have found that when battery temperatures drop, battery internal resistance increases pretty dramatically. In using our L16 house bank while at freezing, just using the inverter to make a cup of coffee will bring the voltage down to 11.5. If they are at 70 degrees at the same state of charge, the voltage will be right near 12.0.

When charging, even with the temperature compensation, the top 10% of the charge is quite slow at freezing, while the top 10% of the charge is quite fast at 60 degrees. The difference that 20 or 30 degrees makes is quite striking.

For an alternative energy plant to be successful, some means is needed to keep the battery temperatures near their optimum of 80 degrees or so. Subfreezing battery temperatures can put the system pretty much out of commission. Avoiding the fumes from charging while keeping the batteries warm can be an issue.

For what it's worth.

Tom Caffrey