Battery Charge State

[FourPoints]

I know that voltage is usually a poor measurement of charge state, but I'm not really questioning the state of my batteries. I know they are near 99 or 100% charge as a result of my wonderful little Genasun 3 stage controller, which I disconnected Friday night in preperation for Irene when I took down my solar panel. I left almost everything turned off, except the knot log and bilge pump (they are on the same circuit until I install the new electrcial panel). The GPS is also on that circuit, but was turned off.

I checked the battery voltage last night and it was reading 13.1v (and I had just minutes before manually kicked off the bilge pump as there was about an inch of water in the bilge to pump out, not enough to trip the auto switch). There has been zero charge input to the batteries in 4 days, and actually had a (very slight) draw on them the whole time, plus the running of the bilge a few moments before so they were pretty well in a resting state, minus the bilge pump draw.

According to the following graphic which was posted here by someone recently (sorry, saved the link but forgot who posted it), 12.6v is generally considered 100% charge, and each 0.1v drop very roughly equates to a 10% drop in charge stage (at least in the 50% and above levels)...

http://forums.sbo.sailboatowners.com/attachment.php?attachmentid=43205&d=1314730099

Following that aproximation, my batteries are showing 150% charge... Now I know they aren't actually at 150% charge, but I was wondering why they would still be above 13v after 4 days?

 

[Bill Roosa]

Try this
http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

I don't have a monitor and have not needed it once I realized that voltage is directly related to SOC IF you know the current flow.
I'm having trouble believing that after 4 days of very low draw you are seeing 13.1 v. Your volt meter probably needs calibrated. Test the back side of the volt meter with a multi meter and compair. The little plastic screw on the front of the meter (right under the needle pivot) is used to adjust it

 

[FourPoints]

I don't have a dedicated volt meter hardwired to the boat (althought I will have one on the new elec panel when it gets installed), I turned on the GPS (Garmin 540s) to get the reading, it has a digital meter built in. I do also have a multimeter that I keep in my tool bag, but I didn't pull it out to check at the batteries directly.

As for calibration, when the charge controller is indicating it is at float stage, the GPS voltage reading matches to within 0.2v of what the manual says the controller should be outputting, and I have attributed the difference to the temperature compensation of the controller dropping the output voltage, as it has been pretty warm lately, and the GPS voltmeter normally showed the voltage lower than the 13.8v float level, and on cooler days it did show 13.8v...so I belive it to be pretty accurate.

 

[mitiempo]

What kind of batteries do you have? 13.8 is pretty high for float voltage unless they are gel batteries.

The chart I posted in the linked thread was for flooded batteries.

 

[FourPoints]

regular flooded lead acid

charger specs:
http://www.genasun.com/gv-4.shtml

 

[mitiempo]

Have you measured the specific gravity of the cells? That could be interesting.

The chart I posted was from Trojan and other brands may be slightly different, but I have never seen a flooded battery at a resting voltage as high as yours.

 

[FourPoints]

Have you measured the specific gravity of the cells? That could be interesting.

no, would have to buy a proper hydrometer... one of the batteries has a built in hydrometer, but it on'y has 4 balls to indicate charge state in 25% increments...

 

[mitiempo]

All I can think of is that the specific gravity may be higher than normal in your batteries.

Here's a chart showing specific gravity vs charge % from Rolls batteries. And here's the link to the page it came from. http://www.surrette.com/content/mot...d65c08&phpMyAdmin=3jSJ-jdC5E7b53DHgV8TGvpSCF6

 

[HEINZIR]

My old Garmin GPS III+ has a volt meter on it as well. It always read a bit low when compared to my Fluke 77. I called Garmin to see if there was any way to calibrate or correct this; they said no. Because yours was accurate at one voltage does not mean it is accurate at others. It may have a non-linear response, or, as the saying goes, "even a broken clock is right...."

Verify your voltage readings with a "real" meter.

 

[All U Get]

Hi guys,
Our new boat has a complex battery system and an Ample Monitor that tells me volts, amps, temperature, and charge state of all three battery banks. The manual is two three-ring binders and several schematics to explain it all. After drowning in information I got a "Quick and Dirty" from Main Sail that made more sense. He used a real good Multi-meter at the batteries and explained Peukert's law so I could get a grasp. Since then I don't fret if the monitor reads 92% or 100% charged (the battery temp makes those numbers fluctuate.) If my DC use starts to draw the banks down too fast it means the batteries are getting bad or I should have bigger banks. After two months I'm finally understanding a little of what my battery monitor is telling me. Hope this helps.
All U Get

 

[monkeybars]

never read any data that saying 12.6 is a full charge.everything i've ever read stated 12.85 was considered 100%.my current batterys read 12.95-13.00 after resting 24hrs when fully charged.its probably more important to learn the "norm" for your batteries and not worry about specific numbers.apparently due to many different variables these number can vary quite a bit for many different reasons.p.s I test with a decent Fluke multi meter (450.00) that is calibrated every six months for aircraft and faa work.

 

[mitiempo]

As I posted it will vary with the brand of battery. But I work on marine electrical systems daily and have never seen a battery quite that high (13.1) after sitting for four days and just handling a small load. Ask the manufacturer of your batteries what they suggest the resting voltage should be.