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  #11  
Old 05-31-2012
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Re: Voltage puzzle

"The only true method of determining the charge state is to use a hydrometer."
Consider this, Gary. If that were so, then there would be no way to determine the charge state of AGM, gel, and sealed wet cells.
So, how do you think battery makers, distributors, and warranty services are done on all these other batteries? Right, with digital voltmeters, and internal resistance testers, aka electronic "load" testers.
The main purpose of a hydrometer is to burn acid holes in fabric, so folks can sell you new clothing and upholstery. It has some other minor uses, but that's the main one these days.
The electronic tools are easiily misused, but you can't blame the tools for poor workers.
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Old 05-31-2012
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Re: Voltage puzzle

denverd0n, Hellosailor, and MaineSail are absolutely correct.

Accurate open circuit voltage measurement, done correctly, is an accurate method of determining the state of charge (SOC) of all lead-acid batteries....flooded, AGMs, gelled, etc. Much, much better than attempting to use a hydrometer, for the reasons cited by MaineSail.

For the OP: 12.9 or even higher OC resting voltage is very common with AGMs and gels, but not with flooded batteries. Your battery appears to be a flooded "no maintence" type, so I'd guess that your voltage measurement device is inaccurate. Or, just maybe, there's some charging device still connected, like a solar panel?

Note that the SOC is not the same as capacity. Batteries typically begin to lose their rated capacity practically from the date of manufacture, and they continue to lose capacity throughout their useful lifetimes.

The best way to measure capacity is with a controlled, purely restive 20-hour load test, with the load being 1/20th the ampere-hour (AH) rating of the battery and the load test continuing until the battery reaches 10.5VDC. This is not possible for most users, lacking the time and/or equipment necessary.

The industry standard for measuring battery capacity these days is with a device which measures internal battery conductance and resistance, such as one of the Midtronics series. These cost upwards of $600 and more, so are not available to most users.

For measurement of SOC, however, IMHO you'd be hard-pressed to find a better way than an OC voltage measurement directly at the battery terminals with a calibrated voltmeter after a suitably long resting period.

BTW, this includes the fashionable-these-days "battery monitors", which are only accurate if they have been properly installed and calibrated -- and recalibrated as the battery ages.

Bill

Last edited by btrayfors; 05-31-2012 at 11:09 AM.
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Re: Voltage puzzle

bill, my impression was that the $50 load testers (electronic, not pile, and typically good for 1000-1200 cca ratings at that price) wre also a good way to measure real capacity. While they are "just" in theory measuring how much of the batteries rated CCA capacity is left, isn't that the same thing? If you're expecting a battery to put out xx amps into a starting load, and it only has half that capacity let...? Same same?

I thought they were measuring internal conductance and the price had just ramped down because they were a little less sophisticated, lower power rated, and some electronics had dropped in price.
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Old 05-31-2012
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Re: Voltage puzzle

Quote:
Originally Posted by hellosailor View Post
bill, my impression was that the $50 load testers (electronic, not pile, and typically good for 1000-1200 cca ratings at that price) wre also a good way to measure real capacity. While they are "just" in theory measuring how much of the batteries rated CCA capacity is left, isn't that the same thing? If you're expecting a battery to put out xx amps into a starting load, and it only has half that capacity let...? Same same?

I thought they were measuring internal conductance and the price had just ramped down because they were a little less sophisticated, lower power rated, and some electronics had dropped in price.
Not sure which new meters you're referring to. Can you be specific about makes/models? Have you tested or used any?

The good ones still cost a bundle. My MDX-650 still costs $650-$750 new. The cheaper ones that I've seen all have some limitations.

But, hey...it's always possible. The Chinese have a "conductance tester" that sells for $10 :-)

Bill
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Old 05-31-2012
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Re: Voltage puzzle

Thanks Maine?
I was hoping you would weigh in on this one.
Yes these are brand new batteries.
I can't think of anything to put real load on them. The boat has very little to turn on.
Maybe all the cabin lights together would do something.
The engine panel on for 24 hours brought it down to 60%
I will test it every time I get on the boat just to learn what to expect.
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Re: Voltage puzzle

david-
If you have no heavy loads to use, disable the engine and crank the starter a bit, use the starter as a load. It should be good for a 15 second run, ten minutes of rest, another 15 second run, that way there's no danger of overheating it but you'll certainly knock off any surface charge on the batteries. And of course, make Real Damn Sure the raw water inlet is closed when you do that, so you can't hydrolock the engine.
If for some reason that's impractical, get any high-wattage auto headlamp or bulb i.e. 65-100 watts, and some jumper wires or alligators. Connect to batteries and leave it on for 5 minutes, check again at 10 minutes. If you don't mind dumpster diving, you can often find a dual-beam headlight behind any auto parts store, with the low beam burned out and the high beam (what you want) perfectly good.
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Old 05-31-2012
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Re: Voltage puzzle

I thought the OP was talking about standard, lead-acid, marine batteries. I have a very expensive battery monitor on the boat. And, when I had a totally dead house battery (1 of 4, 6-volt golf-cart batteries in a series parallel rig) that battery continued to provide readings that would have been consistent with a fully charged battery. Even with a light load, it still provided good readings. The hydrometer, however, quickly revealed one cell was sub-par and failed when under a heavy load.

Granted, you cannot test gel batteries with a hydrometer, but the vast majority of folks that I know don't use them. I guess some folks like sealed wet-cell batteries, but I'm not among them. Kinda' like the "no maintenance" car batteries that tend to die much faster than the traditional batteries that allow you to add water. As for getting battery acid on your clothing--I think I may have done that about 10 years ago, but it may have been longer than that.

I also have a very high-quality digital multi-meter here in the office, one that I use for measuring minute changes in line outputs of audio systems. It was very expensive when I purchased it a decade ago, and I don't take it near saltwater--too darned expensive. I also have an old-fashioned load tester from back in the 70s when I owned a service station. It still works, but I rarely use it anymore.

I guess I'm a bit old fashioned--I'll stick with the hydrometer. I've never had one that failed, and you cannot beat the price.

Cheers,

Gary
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Re: Voltage puzzle

Gary, if it is ten years old, guaranteed it has drited out of calibration. As the Mad hatter said "But it was the best of butter!" and even the best components shift as they age.

If you had one bad cell, that should have been reflected in a significant change in the voltage measured from that battery, and a calibrated meter should have picked it up.

I know, "should". If the turkey baster works for you, great. I found that calibrating my meters was cheaper than replacing anything the acid splashed onto, acid loves me but I have different feelings about it.
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