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 > Some thoughts on AGM batteries...

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time2roll

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Posted: 12/16/05 12:28pm Link  |  Print  |  Notify Moderator

n7bsn wrote:

First, if you want real accurate answers, and not a mixture of good and bad advise, don't ask here.

It's hard telling if you AGMs would be good for RV use, since they appear to be designed for a comparatively short discharge cycle (15 minutes vrs the longer times RV/Marine AGMs are designed for)


All I hear is that AGM can be discharged deeper with less wear than a wet cell. Ironic to see these two statements together.


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RoyC

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Posted: 12/16/05 12:28pm Link  |  Print  |  Notify Moderator

Quote:

I have 24 of these...



AGM (Absorbed Glass Mat)


I'd be carefull with statements like that. You could be arrested for possession with intent to distribute.


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Big Rig Tech Too

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Posted: 12/16/05 03:00pm Link  |  Print  |  Notify Moderator

Absorbed Glass Mat
In the absorbed glass mat (AGM) battery a boron silicate glass mat is saturated to about 95% with an acid/water solution. Because the mats are not totally saturated the batteries do not leak as a flooded battery would if tipped or broken. Freezing is less likely in AGM batteries due to the lack of excess water. Another added benefit is the ability to use standard charging methods. An AGM battery can also sit for an extended length of time without needing recharging.

Maintenance
The best way to have a battery last is to check and maintain it regularly. On a wet or low maintenance battery the water level should be checked frequently. The water level must be kept at an optimal level for the battery to be at its most efficient. Only distilled water or electrolyte should be added to the battery. Add this prior to charging the battery. Caps should be left on the cells while charging to conserve water in the battery. Maintenance Free and Absorbed Glass Mat batteries are sealed and do not require water

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Posted: 12/16/05 05:27pm Link  |  Print  |  Notify Moderator

smkettner wrote:

All I hear is that AGM can be discharged deeper with less wear than a wet cell. Ironic to see these two statements together.


That's something that I've ALWAYS heard and seems to be the commonality between different opinions. That fact coupled with...

Big Rig Tech Too wrote:

Because the mats are not totally saturated the batteries do not leak as a flooded battery would if tipped or broken. Freezing is less likely in AGM batteries due to the lack of excess water. Another added benefit is the ability to use standard charging methods. An AGM battery can also sit for an extended length of time without needing recharging.


... makes AGM seem like the PERFECT battery for RV use. Deep drain ability (without battery damage), can't freeze (in most poorly insulated basements), can't spill (if they somehow tip over in the basement), and can sit out a winter season without recharging. Win, win, win, win situation. I guess the reason EVERYONE doesn't use them is because of initial cost, but because I got these at such a reasonable cost, why not use them?

Anyway, I kinda had my mind made up, and reading the text here (and from MANY Google searches), just strengthened my thoughts.

For anyone wondering about the moderator edit on my original post, he thought my "loose offer" to get of some of them on eBay was an attempt to sneak in a "For Sale" post. It was not. Carry on.


Tony Berry
Lafayette, LA
http://LifeOnTheRoad.org

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Posted: 12/16/05 05:30pm Link  |  Print  |  Notify Moderator

RoyC wrote:

I'd be carefull with statements like that. You could be arrested for possession with intent to distribute.


You have no idea how true that actually turned out to be.

n7bsn

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Posted: 12/16/05 06:16pm Link  |  Print  |  Notify Moderator

smkettner wrote:

..
All I hear is that AGM can be discharged deeper with less wear than a wet cell. Ironic to see these two statements together.


You are aware that a two different AGM batteries can be designed for entirely different uses? Like use as in a UPS (which is what these batteries appear to be) as opposed to power for a boat or RV?


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Posted: 12/16/05 09:45pm Link  |  Print  |  Notify Moderator

As a long time designer of batteries (particularly AGM) and holding 5 patents in this technology, I am always tickled by the mystery surrounding this subject. BIG RIG TECH TOO is right on target on this subject. The agm technology was invented in 1965 but was used mostly on military stuff due to cost until about 10 years ago. It uses a recombinant oxygen cycle. That is why the glass separators are about 95% full, so the O2 produced at the positive plates during overcharge can pass thru and recombine on the negative plates and reform H2o instead of escaping as gas. This technology uses the same materials as a flooded cell, except the recipies for active materials are slightly different, and the cells are under compression to facilitate recombination. The main characteristics of agm are ability to suffer more deep discharges than flooded cells, lower internal impedance that produces substantially higher currents if needed, ability to accept very high charge currents without damage, lighter, works in any position, no venting (except during overcharge) means almost no post corrosion. The max charge voltage is 2.40 volts/cell at 77F. This is 14.40 volts at 77F. (Flooded cells require up to 2.60 volts at the same temp.) This voltage must be compensated +/- a couple of millivolts/F. Fortunately the alternator has temperature compensation that makes this adjustment automatically. Now in RV/marine use when a diode isolator is used, the diodes normally have a forward drop and this reduces your total charge voltage. Generally this means that charging on the alternator alone will never fully charge an agm battery. Good reason to plug in to shore power once in a while to top it off, but don't leave it permanently plugged in because if you overcharge and lose water you will be up the creek. Incidently an agm battery can be charged at the same max amps as its rated discharge. In fact the battery works better when slammed with high charging currents (100+ amps if you are below 14.4 volts), because this causes slight internal heating which promotes formation of a particular crystal structure on the positive paste (Beta PbO2). They like it like candy. There is a difference between deep cycle and SLI(starting,lighting,ignition) batteries in agm just like flooded batteries. Mainly plate thickness, acid gravity, and paste density (porosity). Unlike starting batteries there is no significant difference between a rv or marine deep cycle battery. The rv agm is true deep cycle while the agm marine sacrifices a little life for thinner plates at lower density to achieve higher amps for starting a big engine. This egg nog is making me long winded. Does this happen to anyone else?

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Posted: 12/16/05 10:19pm Link  |  Print  |  Notify Moderator

If you're really sleepy read this. After performing thousands of autopsies on failed batteries (some people have no life), you might be interested to know what the failure mechanisms are on most deep cycle batteries. It's the same as Dunlop's disease where when you eat too much, your stomach done lopped over your belt! The culprit is usually the positive plates. When charged they are PbO2, but when you discharge the cell the positive paste goes to PbSO4 which is 40% bigger volumetrically than the PbO2. This stretches the metal grid (the conductor that transports the electrons out of the cell). Now unfortunately the grid is made of lead, a metal which has a very poor electrical conductivity but is the only economical conductor that will survive in sulfuric acid solutions. To make matters worse, lead has only about 1000 psi of tensile strength even in its best alloys. This is about as strong as your ex-wife's pancakes. So when the positive paste (attatched to the grid)expands by 40%, it stretches the lead grid in the x-y direction. Now when you recharge and the PbSO4 in the positive paste reconverts back to PbO2 and shrinks, that stubborn lead grid now possesses an extremely high compression strength and doesn't budge. So the grids curve in the z axis (ever notice the bulge on the ends of a wore out battery?). This separates the plates over time and causes internal resistance to rise. What is even worse is that the paste pellets that lie within the grid spaces become cracked during shrinkage and no longer touch the grid metal and that pellet can never be recharged again. It stays lead sulfate (PbSO4) and the battery is said to be "sulfated". This is bad and is like having your black tank clogged up, can't live with it. The capacity of the battery gets lower and lower. This problem is much worse on thin grid cells like starting batteries and is why they are only good for about 10 deep cycles before they are trash. Keep this in mind when you are deep discharging your rv batteries, it will happen to them eventually if you overdo it. I wouldn't go past 80% discharge, about 11.5 volts. Of course the voltage can only be accurately checked if several hours have passed since the last charge or discharge to allow the acid to equilibrate. What a big help this is right? Well actually if you see the voltage go down to 11.5 while you are under load, that is a good place to quit. Get back on charge as soon as possible. Why? Because when you fully discharge a lead-acid battery, you drive the gravity of the acid down to almost water. Lead dissolves in water and basic solutions, but not in acid solutions. So if you let it sit dead, then the lead in the grids and paste will start to dissolve in the water (it only takes a couple of hours). And when you start recharging, the gravity starts to rise and the water slowly becomes acid again. Now the lead precipitates out of solution (remember it only dissolves in water or basic solutions), and to everyone's surprise it doesn't go back to where it came from. It usually forms a dendrite which is a tiny short between the + and - plates. Drat. What a disaster. Back to the store.

time2roll

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Posted: 12/16/05 11:18pm Link  |  Print  |  Notify Moderator

n7bsn wrote:

smkettner wrote:

..
All I hear is that AGM can be discharged deeper with less wear than a wet cell. Ironic to see these two statements together.


You are aware that a two different AGM batteries can be designed for entirely different uses? Like use as in a UPS (which is what these batteries appear to be) as opposed to power for a boat or RV?


Absolutely, And I would not touch the pictured battery for RV use.

cruising7388

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Posted: 12/17/05 12:04am Link  |  Print  |  Notify Moderator

Icky wrote:

As a long time designer of batteries (particularly AGM) and holding 5 patents in this technology, I am always tickled by the mystery surrounding this subject. BIG RIG TECH TOO is right on target on this subject. The agm technology was invented in 1965 but was used mostly on military stuff due to cost until about 10 years ago. It uses a recombinant oxygen cycle. That is why the glass separators are about 95% full, so the O2 produced at the positive plates during overcharge can pass thru and recombine on the negative plates and reform H2o instead of escaping as gas. This technology uses the same materials as a flooded cell, except the recipies for active materials are slightly different, and the cells are under compression to facilitate recombination. The main characteristics of agm are ability to suffer more deep discharges than flooded cells, lower internal impedance that produces substantially higher currents if needed, ability to accept very high charge currents without damage, lighter, works in any position, no venting (except during overcharge) means almost no post corrosion. The max charge voltage is 2.40 volts/cell at 77F. This is 14.40 volts at 77F. (Flooded cells require up to 2.60 volts at the same temp.) This voltage must be compensated +/- a couple of millivolts/F. Fortunately the alternator has temperature compensation that makes this adjustment automatically. Now in RV/marine use when a diode isolator is used, the diodes normally have a forward drop and this reduces your total charge voltage. Generally this means that charging on the alternator alone will never fully charge an agm battery. Good reason to plug in to shore power once in a while to top it off, but don't leave it permanently plugged in because if you overcharge and lose water you will be up the creek. Incidently an agm battery can be charged at the same max amps as its rated discharge. In fact the battery works better when slammed with high charging currents (100+ amps if you are below 14.4 volts), because this causes slight internal heating which promotes formation of a particular crystal structure on the positive paste (Beta PbO2). They like it like candy. There is a difference between deep cycle and SLI(starting,lighting,ignition) batteries in agm just like flooded batteries. Mainly plate thickness, acid gravity, and paste density (porosity). Unlike starting batteries there is no significant difference between a rv or marine deep cycle battery. The rv agm is true deep cycle while the agm marine sacrifices a little life for thinner plates at lower density to achieve higher amps for starting a big engine. This egg nog is making me long winded. Does this happen to anyone else?



Your post regarding batteries (both) is great reading with a wealth of information I haven't seen published. Thanks.

A couple of points you made re alternators and diodes could use some clarifying. You indicate that alternators have temperature compensation for making adjustments. With the rare exception of units that actually have a temperature sensor to modulate field current, this isn't true. The reason that most alternators drop output under heavy supply conditions is that the rectifying diodes heats up to the point that voltage drop increases across the diode matrix. Also, the alternator winding resistance increases as the winding temperature increases. You're right, it will drop alternator output, but the cure is about as bad as the disease. Overheating the alternator to provide voltage changes is hardly my idea of "compensation".

You also mention that isolator diodes result in diminished battery charging voltage resulting in undercharging. Yes and no. In older installations, without an alternator sensing tab, this was indeed a problem. But in computer controlled vehicles with external alternator regulation, often the field excitation is regulated by the computer which is simultaneously monitoring battery terminal voltage and will compensate for any voltage drop between the alternator and the batteries, including isolating diodes and cable losses. That's the good news. The bad news is that some installations have the alternator feeding appliances upstream of the isolators and consequently, these devices are looking at an alternator output of approximately .7 volts higher than what is supplied to devices looking directly at the battery terminals.

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