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Old 07-10-2009, 15:12   #46
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"Virtually all AGM batteries are sealed - no vents. "
Not quite. All AGM batteries will be BOTH sealed and vented. The vents will normally be "spring-loaded" so that once the inside pressure has risen, typically from overcharging, the excess pressure can blow out through the vent--rather than causing the battery casing to explode.
Even inexpensive NiCad and NiMh batteries often have small vents for this same purpose. They're all one-way valves, designed to let off pressure in order to prevent explosions.

Since equalization would generate lots of gas and lots of pressure, one might guess that an AGM battery designed to be equalized would require a much stronger case, and a correspondingly higher-pressure safety vent, than an AGM battery not designed to be equalized. Since Lifeline batteries are among the most expensive--it may be that simple. AGMs may or may not benefit from equalizing, but you can bet they'll be more expensive if they are designed to take the extra pressure from it. Since the main problem with sulfation is that the compounds precipitate and literally FALL DOWN off the plates, and the glass mat in an AGM battery physically prevents that kind of movement, there's good reason to suspect that an AGM battery simply can't suffer from conventional sulfation.
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Old 07-10-2009, 16:47   #47
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Originally Posted by GordMay View Post
However, if the path is as low as 0.04 Ohms*, you might expect get a discharge of about 3,000 Amps.

Distilled Water has a resistance of about 1,000,000 Ohms per meter; whereas Sea Water has a resistance of bout 0.1 Ohms per meter.

However, Conductivity is measured in MS/cm2 which is Micro-Siemans* per Square Centimeter. Resistivety is measured in Meg-Ohm/cm2 which is Million Ohms per Square Centimeter. These terms were adopted because Ohms of resistance per foot of distance was too easily understood.
[...]
Someone help me !!!
Oh Gord, I got you now ;-)

First, it's a Siemens, not Siemen nor Siemans. Yes, this is the German scientist and related to the Siemens brand of electric/electronic products.

Next, MS/cm2 would be MegaSiemens per square centimeter, not Micro-Siemens. that's a 10^12 difference. uS(micro) -> mS(milli) -> S -> kS (kilo)-> MS(mega) -> GS(Giga).

According to the wiki, avg. seawater is 4.8 S.m^-1. If we take the two battery poles to be 25 cm apart, the conductivity in S becomes 4.8 / 0.25^-1 = 1.2S. If we convert that to resistance, we get 1/1.2 = 0.83 Ohm. At 12V battery we get 12/0.83 = 14A

So, no sensational explosions, sorry...

ciao!
Nick.
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Old 07-10-2009, 16:57   #48
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AGM's are not vented of course. Vented batteries are the old tech, like the Trojan T-105's, also called flooded batteries.

AGM's are VRLA (valve-regulated lead-acid) batteries, What they have is a safety pressure relief valve. Think of it like the safety valve on a pressure cooker.

I know HelloSailor has the right thing in mind but he used the wrong word because vented means an open cell that can "breathe" in & out, while the AGM's are sealed: nothing can go in, but over-pressure can get out.

The wiki says it all: VRLA battery - Wikipedia, the free encyclopedia

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Old 07-10-2009, 18:25   #49
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Semantics, Nick. It may not be an open-air-constant-flow vent, but it still is a vent. The fact that it is a valved vent, doesn't affect the purpose of the hole in the case on the outside of the valve.

VRLA is only one of the two leading acronyms for them.
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Old 07-10-2009, 18:52   #50
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Unfortunately, AGM's are not all that they are cracked up to be, and neither are Gel cells, which is why dollar for dollar the regular flooded wet cell battery is still the best deal in town.

I recently had a guy tell me that his onboard 'house' battery (Sear's Die Hard Marine) was the greatest thing, and had never let him down. His charging meter showed the battery as "fully charged", and on the automatic setting would shut off charging the battery. I put a multimeter on the battery, and it was reading 12.25v with no load (i.e. main breaker shut of). 12.25v is an almost dead battery...about 25% charged. I just had to laugh. Whatever, dude.
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Old 07-10-2009, 19:14   #51
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"12.25v is an almost dead battery...about 25% charged"

Typical battery voltage is 12.6-12.7 for a fully charged brand new battery, dropping 0.1 volt per 10% of charge/capacity loss.

So 12.6 minus 12.25 equals a drop of 0.35V, a battery that at 65% capacity/charge. 55% for the rare ones that start at 12.7 volts.

And that, of course, presumes your voltmeter is dead accurate. Typically on the 20V scale, the rightmost digit can be expected to be 1-3 digits off from what is real, unless you've got a brand new calibrated meter, and even those are often subject to float.
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Old 07-10-2009, 19:23   #52
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Seems to me there is no difference in flooding danger between liquid LA battery and SLA batteries...its the terminals that are the issue...

What kind of battery box is going to protect the terminals of ANY battery from flooding ?

I'd really like to know !

Cheers

Alan
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Old 07-10-2009, 19:24   #53
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sorry...wrong thread !
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Old 07-10-2009, 21:42   #54
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Originally Posted by hellosailor View Post
"12.25v is an almost dead battery...about 25% charged"

Typical battery voltage is 12.6-12.7 for a fully charged brand new battery, dropping 0.1 volt per 10% of charge/capacity loss.

So 12.6 minus 12.25 equals a drop of 0.35V, a battery that at 65% capacity/charge. 55% for the rare ones that start at 12.7 volts.

And that, of course, presumes your voltmeter is dead accurate. Typically on the 20V scale, the rightmost digit can be expected to be 1-3 digits off from what is real, unless you've got a brand new calibrated meter, and even those are often subject to float.
12.7 is fully charged (100%). 12.2 is 50%. 12.0 is 25%. 11.9 is 0%.

So I took a little poetic license to make the story better.....so sue me. Regardless, the meter on the charger was still claiming 'fully charged' (100%) and shutting off the charger when the battery was actually at 50% charge, which is technically '0' considering that your battery should always stay above 80% charge, and absolutely never drop below 50% charge.
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Old 07-10-2009, 21:59   #55
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I don't mind poetic license, but your story only proves...The guy might have been right. What your meter said, versus what his charging system said, is meaningless without some calibration standard. And says nothing about the quality of AGM.

I think we all realize that AGM costs more per amp/hour. Until you factor in the cost of the first shirt, jacket, or pants with an acid burn from the wet cell. Or the extra 20% of engine or genset run time to recharge a wet cell. Or things like that.
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Old 08-10-2009, 04:25   #56
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Originally Posted by s/v Jedi View Post
Oh Gord, I got you now ...
... According to the wiki, avg. seawater is 4.8 S.m^-1. If we take the two battery poles to be 25 cm apart, the conductivity in S becomes 4.8 / 0.25^-1 = 1.2S. If we convert that to resistance, we get 1/1.2 = 0.83 Ohm. At 12V battery we get 12/0.83 = 14A ...
It shouldn't be difficult to "get" this math-challenged dolt, who doesn’t really get this seawater conductivity stuff.

Could you direct me to the Wikki?

How do we convert a linear measurement (electrodes 25cm apart) to a volumetric measurement (cubic/cm or m)?


According to the chart on page 9, the specific conductivity of “average”* (±S = 35, or 3.5% salt ?) seawater is 0.0532 Ohms per centimetre (? 18.7969 mhos/cm ?).
http://www.ocean.washington.edu/cour...otes/CHPT3.pdf
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Old 08-10-2009, 08:34   #57
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It shouldn't be difficult to "get" this math-challenged dolt, who doesn’t really get this seawater conductivity stuff.
Well, I'm not a math wizard either compared to some study-friends but we did take 6 different math classes and I managed just above average. I amazed myself that I even managed Discrete Math because when I first looked at that, it went beyond my imagination (prove that 1 equals 1 etc.)

Quote:
Could you direct me to the Wikki?
I found my conductivity number here: Electrical conductivity - Wikipedia, the free encyclopedia

and the Siemens unit (uS, MS etc.) here: http://en.wikipedia.org/wiki/Siemens_(unit)

Quote:
How do we convert a linear measurement (electrodes 25cm apart) to a volumetric measurement (cubic/cm or m)?
You don't. Conductivity of a liquid is, by definition, between two electrodes. A bigger volume only matters when absolute volume is very low. It does matter for maximum current carrying capability (not start boiling away with more volume and same current).

Quote:
According to the chart on page 9, the specific conductivity of “average”* (±S = 35, or 3.5% salt ?) seawater is 0.0532 Ohms per centimetre (? 18.7969 mhos/cm ?).
I use the notation "^-1" where you put your "?" It is "to the power of -1".
The notation of ohm^-1 or cm^-1 means you take the reciprocal value of that number (Multiplicative inverse - Wikipedia, the free encyclopedia). The easy way of doing that is to divide one by that number.

The value/formula I came up with:

1/R = 4.8 x 1/D where R=resistance in Ohm and D=distance in meters
gives R = 1 / (4.8 x 1/D) equals 1 / (4.8 x 1/0.25) equals 1 / (4.8 x 4) equals 1/19.2 equals 0.052 Ohm. At 12V this is 230A

Earlier in the thread I did 4.8 / 4 while I noted S.m^-1 (and that "." is multiplication) which was stupid and because my mind works in "resistance" instead of conductivity which is the opposite ;-)

And the one you found:

1/R = 0.0532 x 1/(D/100)
gives R = 1 / (0.0532 x 1/(D/100)) equals 1 / (0.0532 x 1/(0.25/100)) equals 1 / (0.0532 x 1/0.0025) equals 1 / (0.0532 x 400) equals 1/21.28 equals 0.047 Ohm gives 255A at 12V

I should have converted to 1/R = 5.32 x 1/D but that's too easy ;-)

All we need now is someone to take a multimeter to the sea and measure resistance with the probes 25 cm apart in the water. I can't do that here because it's rainy season and the upper level of the water is brackish so much higher resistance.

So, even if resistance is that low, all we end up with is an empty battery.

cheers,
Nick.
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Old 08-10-2009, 09:24   #58
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I don't mind poetic license, but your story only proves...The guy might have been right. What your meter said, versus what his charging system said, is meaningless without some calibration standard. And says nothing about the quality of AGM.

I think we all realize that AGM costs more per amp/hour. Until you factor in the cost of the first shirt, jacket, or pants with an acid burn from the wet cell. Or the extra 20% of engine or genset run time to recharge a wet cell. Or things like that.
Uh, no. The point was that the 'meter' on the charger he was trusting to keep the battery charged was a piece-o-s***. The battery was, for the intents of a properly-managed 12v battery bank, 'dead' at <50% charge, meaning that it could no longer be discharged without permanently damaging the battery. And the moral of the story is that there is no substitute for a quality multimeter, or, if you're a fanatic, an electrolyte tester.

'Calibration standard'? Pl-ease. It's simple really -- slap a decent handheld multimeter on a resting battery. If it's reading 12.2v and you believe it's 'fully charged' ...there's a problem, and it isn't the multimeter. A fully charged and rested 12v battery should read between 12.6v - 12.8v at 100% charge on a good handheld multimeter, period.
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Old 08-10-2009, 09:35   #59
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Well, I'm not a math wizard either compared to some study-friends but we did take 6 different math classes and I managed just above average. I amazed myself that I even managed Discrete Math because when I first looked at that, it went beyond my imagination (prove that 1 equals 1 etc.)

I found my conductivity number here: Electrical conductivity - Wikipedia, the free encyclopedia

and the Siemens unit (uS, MS etc.) here: http://en.wikipedia.org/wiki/Siemens_(unit)

You don't. Conductivity of a liquid is, by definition, between two electrodes. A bigger volume only matters when absolute volume is very low. It does matter for maximum current carrying capability (not start boiling away with more volume and same current).

I use the notation "^-1" where you put your "?" It is "to the power of -1".
The notation of ohm^-1 or cm^-1 means you take the reciprocal value of that number (Multiplicative inverse - Wikipedia, the free encyclopedia). The easy way of doing that is to divide one by that number.

The value/formula I came up with:

1/R = 4.8 x 1/D where R=resistance in Ohm and D=distance in meters
gives R = 1 / (4.8 x 1/D) equals 1 / (4.8 x 1/0.25) equals 1 / (4.8 x 4) equals 1/19.2 equals 0.052 Ohm. At 12V this is 230A

Earlier in the thread I did 4.8 / 4 while I noted S.m^-1 (and that "." is multiplication) which was stupid and because my mind works in "resistance" instead of conductivity which is the opposite ;-)

And the one you found:

1/R = 0.0532 x 1/(D/100)
gives R = 1 / (0.0532 x 1/(D/100)) equals 1 / (0.0532 x 1/(0.25/100)) equals 1 / (0.0532 x 1/0.0025) equals 1 / (0.0532 x 400) equals 1/21.28 equals 0.047 Ohm gives 255A at 12V

I should have converted to 1/R = 5.32 x 1/D but that's too easy ;-)

All we need now is someone to take a multimeter to the sea and measure resistance with the probes 25 cm apart in the water. I can't do that here because it's rainy season and the upper level of the water is brackish so much higher resistance.

So, even if resistance is that low, all we end up with is an empty battery.

cheers,
Nick.
The theory behind covered plastic or wooden battery boxes is that in the event of water egress it hopefully protects the battery bank long enough that the bilge pump or other drains can get rid of the water, with the batteries suffering minimal contact with the water limited to what can enter thru the cable holes in the box before the water level recedes. Absent enclosed battery boxes, your boat's electrical system is entirely at Poseidon's whims.

It's a moot point to discuss the conductivity of seawater other than.... suffice it to say that when you mix seawater with your battery bank, bad things are likely to happen, the least of which is replacing the batteries.
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Old 08-10-2009, 11:52   #60
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NotSure,

If you really think your batteries inside your boxes will not become submerged , you need to re-think that. The boxes are not watertight and will fill up in a flash.

Someone, maybe you, also thinks that putting a battery with the posts facing up makes a difference because the posts will be above water. That one really needs rethinking also. If you put most batteries on their small side, one post will be further up from the "floor" than when it's positioned "right side up" and so that position on it's side is the better one in that respect too. Not that it matters much when the water comes up high enough because the bilge pumps don't cope. If the bilge pumps cope, the position doesn't matter anyway.

Positioning AGM's on their side is a big positive when you want to stack them up, something that isn't possible with vented/flooded batteries.

And then there's many batteries that are meant to be on their side, or with the terminals on the side, like the big Sonnenschein 2V gel cells or the deep cycle Odysseys.

cheers,
Nick.
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