AGM Batteries.

[Dave1258]

Is anyone using these batteries, supposed to be the 'Dog's Danglies' in the world of D.C Batteries, trying to find out more info, and justify hefty price tag???


Regards Dave

<hr width=100% size=1> A man should have two things in life, a boat and a wife willing to let him have one.

 

[cliff]

Flooded or Wet Cells are the most common lead-acid battery-type in use today. They offer the most size and design options and are built for many different uses. In the marine business, they usually are not sealed so the user can replenish any electrolyte the battery vented while charging the battery. Typically, the cells can be access via small ~1/2" holes in the top casing of the battery.

The plastic container used for flooded cells will have one or more cells molded into it. Each cell will feature a grid of lead plates along with an electrolyte based on sulphuric acid. Since the grid is not supported except at the edges, flooded lead-acid batteries are mechanically the weakest batteries.

Since the container is not sealed, great care has to be taken to ensure that the electrolyte does not come into contact with you (burns!) or seawater (chlorine gas!). The water needs of flooded cells can be reduced via the use of Hydrocaps, which facilitate the recombination of Oxygen and Hydrogen during the charging process.

Gel Cells use a thickening agent like fumed silica to immobilize the electrolyte. Thus, if the battery container cracks or is breached, the cell will continue to function. Furthermore, the thickening agent prevents stratification by preventing the movement of electrolyte.

As Gel cells are sealed and cannot be re-filled with electrolyte, controlling the rate of charge is very important or the battery will be ruined in short order. Furthermore, gel cells use slightly lower charging voltages than flooded cells and thus the set-points for charging equipment have to be adjusted.

Absorbed Glass Mat (AGM) batteries are the latest step in the evolution of lead-acid batteries. Instead of using a gel, an AGM uses a fiberglass like separator to hold the electrolyte in place. The physical bond between the separator fibers, the lead plates, and the container make AGMs spill-proof and the most vibration and impact resistant lead-acid batteries available today. Even better, AGMs use almost the same voltage set-points as flooded cells and thus can be used as drop-in replacements for flooded cells.

Basically, an AGM can do anything a Gel-cell can, only better. However, since they are also sealed, charging has to be controlled carefully or they too can be ruined in short order.

So basically there are advantages and disadvantages for each type of lead acid battery. Personally I prefer wet cell as they take a faster charge, can be topped up and are relatively cheap.

AGM and Gel batteries are sealed for life, cannot be topped up, need careful charging and unless you need to place them deep down in the bilges do not seem, to me at least, to be cost effective.

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[jrt]

I'm very nervous of posting anything after such a learned treatise. The only additional info that I can offer is that AGM batteries can be operated in any attitude, ie upside down if needed, they can take twice as many discharge / recharge cycles as a flooded cell battery and are more robust. Almost fit and forget.


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[cliff]

It is the "almost" that worries me and they do not like fast charging. maybe ok in a mobo running its engine(s) all day but in a yacht I like a fast charge and engine off.
Also when topping my wet batteries up I can see if a cell is using more water than others etc so have some idea when of if the battery is liable to go tits up on me. Call me old fashioned but I am not a believer in maintenance free anything.

The quality of being able to work upside down is admirable but do I need that feature? - do I sail my boat upside down? - not yet. No, I am afraid until the price comes down I'll stay with my wet cell batteries that take a fair bit of stick, especially when SHMO fires up the microwave ot the vacuum cleaner while shmo in training dries her hair and also wants the fan heater on. The invertor has a fit.

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[Talbot]

if you want fast charge rate, and a better quality battery, which can deep cycle and also start your engine then the carbon fibre reinforced version may meet your needs see <A target="_blank" HREF=http://www.elecsol.com/html/vehiboat.html>here</A>

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[Althorne]

Thanks Talbot very timely as I'm looking to revamp my electrics.
Sorry I missed you last weekend.

<hr width=100% size=1><A target="_blank" HREF=http://colvicownersclub.mysite.wanadoo-members.co.uk>http://colvicownersclub.mysite.wanadoo-members.co.uk</A>

 

[Talbot]

They are a tad more expensive than normal deep cycle batteries, but also last a lot longer.

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[cliff]

Looks interesting and I like the idea of being able / needing to top up the cells, but as I have just replaced all the batteries it will be another 4~5 years (maybe more) before I am considering batteries again and who knows what may be around then. There have been serious advances in liquid sodium batteries - maybe those will be available to joe public by that time

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[bbilly]

Re: AGM Batteries - dipende

I think it depends on what the battery is for. Starting or keeping the beer cold?

<hr width=100% size=1>"If you're too open minded your brains fall out"

 

[Geordie]

A slight caveat to the oft quoted statement that sealed-gel batteries can be used in any position. If they are installed end down, then gravity will squeeze the lower plates. Not a position to be recommended. I use this type of battery because I am lazy. IMHO the extra cost is worth it. I have two that have been in regular use for 10 years (however as engine start therefore not used in their deep cycle mode) and are still serviceable.

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[Two_Hapence]

Hi,

You don't say what type of boat you have or what the charging capability is. If you have a big heavy weight insensitive monhull with a big engine and a hefty alternator then ordinary wet cell batteries will probably be the way to go. Spend your money on a smart regulator or a heaftier alternator or both. If you have a light boat and little charging capability then Gel or AGM batteries are probably better as you have a bigger useable reserve per AH purchased.

You have to be a little bit careful about estimating your total current draw and anticipated reserve as flooded cell batteries tend to resent being depleted below 50% of their capacity and draining them down to 25% of their capacity or lower seriously diminishes the longevity of the battery.

Gel and AGM batteries tend to last longer if they are deeply cycled and have a few other avatages. They are usually lighter - (I have 2 100AH AGM batteries and they weigh less than a single 135AH deep cycle flooded battery. If you have to lift them the lower weight is a big advantage. Moreso as you get older! They are smaller and will take a fair amount of abuse. They don't self-discharge as quickly - a big advantage if you have to leave the boat for a while.

Don't expect to be able to use them to start a hefty diesel - though they will start a smallish outboard.

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[cliff]

Neither AGM nor Gel are really any use to me - won't run the inverter - The invertor draws (or can frequently draw) MORE amps than starting the Diesel.
I need a balance between high current draw, fast recharge and life expectancy and as neither AGM nor Gel fit the first criterion it is another reason to stick with wet cell. (nearly forgot - the batteries are fitted in moulded compartments so size is also an issue)
I have already spent my money on both a heftier alternator and a "smart charger/ regulator" not to mention a 60amp constant, 120A peak shore power driven charger for the odd occasion I need a "quicky overnight"

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[Ships_Cat]

We went through the NiCd versus wet lead acid 8 years ago and went wet lead acid (Trojan and seem the same as new still) for the reasons you give plus we can cross connect them to start the engine if the cranking battery fails - as it has on one occasion.

Today, out of the current choices, I would still go wet lead acid because of their robustness and I don't think there is any good reason not to, unless one does not want to spend time checking electrolyte levels or else one needs the weight saving in a race boat or in a little boat or a cat that cannot carry loads - in both those cases there will be no big electrical loads anyway because the boat cannot carry the weight of the equipment.

John

<hr width=100% size=1>I am the cat but I am only 6.

 

[pvb]

Fast charge rate...

Surely the AGM battery has the fastest charge rate of all the batteries in common usage? The low internal resistance of the AGM means that, as long as voltage is properly regulated, there's no limit to charge current. They will certainly recharge faster than ordinary flooded lead/acid batteries.

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[Ships_Cat]

Re: Fast charge rate...

It might be a bit academic though because on a boat (putting aside those with small battery banks) the maximum charge rate is normally limited by the maximum sized alternator you can handle. So if one has 500-600 AHr of batteries then if wet the maximum useful alternator size for the bulk charge is around 100-125 Amps. Are you going to fit a 200 Amp monster alternator to cope with and utilise the higher charge rate of a similarly sized bank of AGM batteries? I suspect not.

For smaller boats (or those with little weight carrying capacity eg cats and race boats) then they may only have 100-150 AHr of batteries and AGM may be useful. But again, are you going to replace the 30-40 Amp alternator typical on smaller boat engines (and which will supply the bulk charge of 100-150 AHr battery) with a bigger one, or fit a second bigger alternator, to provide a higher charge rate? One might, but I suspect generally not. In which case you are back to similar charge times as for wet batteries.

Then in both cases the maximum charge rate can only be supplied during the bulk stage of the charge, which is typically less than half of the charge time from start of charge to the start of the float charge. So again the opportunity to realise the higher charge rates on a boat are limited.

Am not knocking AGM batteries, they certainly have a place, but especially for pleasure boats with larger electrical demands I personally don't see the point as things now are with respect to cost, overcharge/overvoltage vulnerabilities, etc.

John

<hr width=100% size=1>I am the cat but I am only 6.

 

[cliff]

Re: Fast charge rate...

"Surely the AGM battery has the fastest charge rate of all the batteries in common usage? The low internal resistance of the AGM means that, as long as voltage is properly regulated, there's no limit to charge current. They will certainly recharge faster than ordinary flooded lead/acid batteries. "


Based on what I have read about AGM and Gel batteries - NO, simply because they are sealed and therefore cannot be charged at as high a voltage (current) as wet cells. - High voltages / current cause gassing of the electrolyte (loss of water) and as these batteries are sealed any loss of electrolyte cannot be replaced. They are apparently not suitable for heavy discharge currents - eg engine starting.

While they may have lower internal resistance the problem of gassing at high charge (and discharge) currents still exists albeit at an even lower voltage then with wet / flooded cell batteries.

If one believes the sales blurb they are the answer to all problems but when one researches the technical aspects of them they have severe limitations in some applications.

I will stay with the older, proven technology for the meantime.

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[pvb]

You need to do a bit more research...

Well, I don't know what you've read, Cliff, but you seem not to understand AGM batteries, and so your statements may mislead other forum users who are considering using AGMs.

Contrary to your claims, AGM batteries do take a faster charge than ordinary wet lead-acid batteries. As long as the charge voltage is properly regulated (as it should be for any battery) there's no problem with gassing. And AGM batteries also deliver better starting current than similar-sized wet batteries.

Anyone wanting to check these facts can look at the comparison of battery types, or the <A target="_blank" HREF=http://www.merlinequipment.com/prod_range.cfm?cat_id=72>Merlin Equipment info on Lifelines.

There are a lot of myths about batteries, but AGM battery technology is actually pretty good. They're not appropriate for everyone, and they're not cheap, but in the right application, where fast charging, high current delivery and long life are desirable, AGMs take some beating.


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[bedouin]

Re: You need to do a bit more research...

On first inspection I couldn't see any reference on either sight to figures comparing the charging rate of AGM vs Flooded wet cell.

What do you mean by "charging voltage correctly regulated"? Do you mean simply a constant voltage or does the voltage need to be reduced once the battery nears full charge?

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[Dave1258]

Re: You need to do a bit more research...

I've been looking and came across this.

<A target="_blank" HREF=http://www.dcbattery.com/agmtech.html>http://www.dcbattery.com/agmtech.html</A>

<hr width=100% size=1> A man should have two things in life, a boat and a wife willing to let him have one.

 

[Ships_Cat]

Re: You need to do a bit more research...

I have seen charging figures ranging from 33% of C20 capacity (from a manufacturer) to 75% (from a non professional). Seems to be little information around but I get the impression that generally you can charge them as fast as you like during the bulk charge as long as they don't overheat and they are not overcharged. Would be interested in any authorative information too.

They charge through a 3 (or more) step regulator (bulk, absorbtion, float) as a wet battery but different absorption voltage and may be damaged if that is exceeded (unlike wet cells which you can plonk high voltages on with little or no damage - in fact does them good some say for equalisation).

John

<hr width=100% size=1>I am the cat but I am only 6.