My observations and experiences over the last few years have led me to some thoughts on AGM's and how the purchasing decisions are made, could be made or things that should be considered other than glossy marketing ads...
I see a lot of folks buy them based on misguided advice or just simply for the wrong reasons. These decisions should ideally be based on how they use the boat, plan to use the boat & how they plan charge and care for the bank..
While many folks do buy them for the right reasons, many don't. AGM's certainly have some great benefits but are very expensive and are arguably, as we've learned over the last 12+ years in the marine market place, less tolerant of abuse than wet cells.
Some scenarios that I have seen:
#1 Bought a 200-800 ah bank for their high acceptance rates then fed them with a 100 amp alternator and 40 amp shore charger. This did nothing to take advantage of this high acceptance "benefit" and the previous bank of wets could actually "take more" than the systems could produce. Acceptance Benefit = 0
#2 Bought them for the "low maintenance" then kept the boat on a mooring and ran the engine 35 hours a season. Even the worst wet cells, on the worst charger, won't boil off enough to matter on 35 engine hours per year. Low Maintenance Benefit = 0.002
#3 Bought them for low self discharge rates then left them sitting at 50% - 80% charged after each sail and killed them in three seasons. Boat was used multiple times per week so self discharge was a non issue. Self Discharge Benefit = 0
#4 Bought them for the claimed "longer life" then found out they were dead in 4 seasons anyway, when the previous wet cells lasted 7. Longer Life Benefit = 0
#5 Bought them to replace wet cells that died a premature death, then the AGM's proceeded to do the same because the owner had refused to address his/her battery practices. Claimed "long Lasting" Benefit = 0
6# Bought them because they don't give off "gas fumes" when charging. The previous bank had zero signs of corrosion and in four years had never even taken water though they were due. When I opened them up they were still full and load tested at nearly new. Gave that bank to a friend who got two more seasons out of them. The new gasless AGM's died at year five. Gassing Benefit = 0.003
#7 "The guy at West Marine said they were the best." He then proceeded to fry two stock alternators and finally had to buy a fully gourmet charging system for another 1.3k. Previous wet bank on dumb regulated alt lasted six years and cost $300.00 vs. $1000.00 + $1300.00 alternator regulator installation. $2300.00 vs. $300.00. I'll be surprised if he gets 6 years out of the Deka/WM AGM's (only on year two now). Best At Cleaning Out Your Wallet = 10 Stars
I see and work on lots of boats left on moorings, it's Maine, and the only thing I have discovered is that AGM's do not like to sit discharged as mooring boats often do. They tolerate this less so than wets or gels. I have seen far to many of expensive AGM banks die before 5 seasons. I also check date codes every time I am at my local battery & wire distributor on the core pallets. The 6 year old AGM core is a rather rare up here while the 6 & 7 year old wets are not all that uncommon on the pallet. Most of the dead AGM date codes are between 4 & 5 years at Dave's shop.
Nearly every boater I know with AGM's has bought them for their ability to take a fast charge then fed them with a diminutive alternator & charging system that took no advantage of the high acceptance rates (wrong reason #1).
I have personally seen a 100 Ah Lifeline take 85 amps for more than just a few minutes. That is a 0.85C. "C" is capacity in Ah's and 85 is the acceptance percentage of the 20 hour rating, meaning the batteries are taking 85% of "C" or the 20 hour rated capacity. Lifeline claims these battteries can take 5C or 500 amps for a 100 amp hour battery for short durations.
They also stipulate that these batteries need to be charged at a MINIMUM of 0.2C or 20% of rated Ah capacity or you can shorten the cycle life. So a 600 Ah bank of AGM's ideally needs an alternator or charger capable of supplying a MINIMUM of 120 amps. Considering the hot rating of an alternator drops quickly once hot, you'd ideally need an alternator in the 160 A range to satisfy this "minimum" 120A threshold for charging to satisfy 0.2C.
"Lifeline® batteries can tolerate in-rush current levels as high as 5C (500A for a 100Ah battery)."
Trojan for example wants to see a "C" of 10-13% or 0.10C - 0.13C for standard charging and a 0.2C (20% of capacity) for a "fast charge". On a 450 Ah bank that is 90 amps acceptance for a Trojan wet cell on "fast charge". Most sailors with small aux engines don't have more than 90-100 amps of alternator, or the belt to drive it. If you charged a bank of Lifeline's at .85 C you need a 380 amp alternator to take full advantage of the batteries initial bulk acceptance. They tend to settle in at about 40-55%, depending upon condition, until they hit about 80-85% and start accepting considerably less current. I have yet to see many boaters truly take advantage of the actual acceptance rates on AGM banks because you'd need a HUGE alternator, or two, or a huge charger to do so. Small sailboat AUX engines just can't do this well without $$$$$$ modifications.
Even a 400 Ah bank would be accepting 160A at 40%. Most boaters can't even come close to providing the batteries 160A let alone the 200+ amps they can actually take..
I replaced four T105's on a local sailboat that had lasted 7 years with a 90 amp dumb regulated alternator. This boat resided on a mooring with no solar or wind. The Lifeline bank cost over $1400.00 and was flat dead going into the spring of what was to be their fifth season. At the same time we upgraded the batteries a full gourmet charging system with 150 amp alt, dual pulleys, MC-612 Balmar regulator, temp sensing etc. etc. on and on was installed. He even bought a maintenance charger that was recommended by Justin at Lifeline tech support for the off season where they were stored in his 55 degree basement and cycled on and off the charger to keep them at 100% SOC.
Total cumulative motor run time over the four previous seasons was just over 400 hours. The bank had never been discharged below 60% SOC during these four seasons as monitored by Link 20. They were grave yard dead well before they should have been and EVERYTHING was done better than he'd done with his simple and inexpensive wet cell bank..
Lifelines suggestion and response, "not out of the ordinary, try equalizing". He's back to 6V wet cells again, expensive experiment. This is one I really feel terrible about because at the time I had bought the AGM marketing hook line and sinker and really pushed this friend/customer towards this set up. I have wracked my brain as to what cased this but there is no answer. Everything was done by the book. He had less than 200 cycles and only got four years out of them. Of course back then we were told they could be cycled to 80% depth of discharge. This went into my calculations on "cost". Of course today these same manufacturers recommending not cycling any deeper than 50% depth of discharge. Somehow we mysteriously lost 20% of the cycling capacity we were sold initially which makes the cost increase even more steep..
A few years ago I was chatting with John Harries of Morgan's Cloud about AGM issues. He has since written extensively on this issue. It is a very good read and mirrors some of what I have seen:
AGM Battery Test Part 1
Lifeline AGM Batteries Test Part 2
There is also my buddy who is the head systems tech at a very well respected boat yard here in Maine. He is an ABYC marine electrician, like me, NEMA certified etc. etc. on and on and on. Probably one of the better marine guys I know in terms of knowledge. His own bank of AGM's in his own boat, fully decked out, lasted three seasons. He's gone to gel.
Don't get me wrong, there ARE benefits to AGM batteries, and if you can truly take advantage of these benefits they can definitely be worth it.
My point here is that most folks I see don't truly take advantage of the benefits. I just want to promote some real world critical thinking, beyond the marketing hype (which as related to the REAL WORLD and marine market is TOTALLY BOGUS), before making a decision where you may spend $1000.00+++. In many cases that $1000.00+ may not even be all that necessary.
What have I learned?
If you go AGM try and keep them fully charged as best you can. If on a mooring get solar or wind to augment your alternator because and alternator alone, no matter how fancy, will not get them back to 100% SOC on a regular basis. If you can afford a $1300.00 bank you should really try to take care of them in terms of charging. In my opinion/experience, and based on the "claims", these banks should have all easily gone 7+ years. while a few do, mostly dock sailed boats, the reports I see and hear in talking with customers and other marine systems specialists, are nowhere near an average life of 7-10 years like the manufacturers initially claimed.
One issue I suspect contributes to shorter AGM battery life is a lesser amount of electrolyte. Liquid electrolytes ability to help keep the battery cooler may be one explanation for the longer life of the inexpensive deep cycle wet cell when compared to AGM.. Heat kills batteries. In my experience, as one who often shoots batteries with an infrared temp gun, (geek) is that AGM batteries always tend to run hotter when accepting a charge. Perhaps this is because they accept higher charger currents or the fact that they may not dissipate heat as well as a wet battery? Either way they tend to run hotter and we know that heat kills batteries..
I had personally planned to go to AGM. I am still with wets because I have been able to live vicariously though others AGM trials and tribulations. If I do go gourmet in batteries it would very likely be GEL not AGM though the Odyssey AGM's are intriguing but I am waiting to see some real world longevity before jumping in...
If AGM's are left topped up, like if you are on a dock regularly, they can or should last as long as wets. Unfortunately I have yet to hear of very many success stories where AGM's clearly out lasted good quality deep cycle wets, which for the roughly 3X cost premium, they certainly should. The original claims were many, many more cycles than wets and this has just not borne fruit in the real world of boats.... In a "lab" maybe but certainly not in the real world. Try telling a customer who spent nearly 2k on a bank of batteries and another 2k on charger and alternator that his 3 year old bank of AGM's is dead.. The look on their face is painful to see.
Interestingly I have only topped up our bank of wet cells three times now in the last five seasons, we are in our sixth year now, took me all of three minutes each time. Our wet cell bank cost us $210.00. The same bank in Lifelines would be about $1000.00. This is a $790.00 savings for 6 minutes work. That is really, really good pay to me...
I find this to be an interesting quote by Trojan Battery. Remember when AGM's first came out the claims were BETTER cycle life than wets..
Originally Posted by Trojan Battery
Generally, gel and AGM batteries have about 20% less capacity, cost about two times more, and have a shorter cycle life than comparable flooded lead acid batteries. However, Gel and AGM batteries do not need watering, are safer (no acid spilling out), can be placed in a variety of positions, have a slower self-discharge characteristic, and are more efficient in charging and discharging than flooded batteries (see table below). Gel batteries are more suitable for deep cycling applications whereas AGM batteries are more for light cycling and engine-starting applications.
I am NOT saying don't buy AGM's at all. I am simply trying to get people to do some critical thinking about why they are buying them, and to do this thinking honestly.
For me the benefits of AGM rank in this order:
Higher acceptance -
This is a HUGE benefit IF you can take advantage of it, many AGM equipped sailors I know physically can't take advantage of this feature. If you have the charging capability it is a GREAT feature...
Low self discharge -
This is great for boats in hot climates or on moorings. Wets can self discharge at up to 13+% per month in warm weather however I know few boaters who go a month without using the boat at all. Even in a worst case scenario, 13%, in hot weather leaves a full bank still at 87% SOC after a month. Still this can be a good benefit in certain use situations. Self discharge has never been an issue for me so would not be weighed in my own benefit analysis. If you're buying AGM batteries specifically for the low self discharge a small solar panel will eliminate the self discharge issue and cost considerably less than the price difference between a bank of AGM and Wet.
Many boaters I know with AGM's already have either solar or wind making the self discharge benefit mostly irrelevant. Heck with AGM's on a mooring sailed boat wind or solar is pretty much a pre-requisite.. The majority of boaters in the US are on docks, not moorings, so again self discharge would be of little benefit to them. When off cruising you are using the banks daily and self discharge, is again, a non issue. Most boaters have a phantom load such as a bilge pump, stereo memory or LPG detector etc. so there are already loads sucking the battery down that require periodic charging anyway which would take care of any self discharge at the same time. If you don't have solar or wind, are on a mooring, in a hot climate and rarely use your vessel this can certainly be a benefit though adding a small solar panel to wets will be FAR less costly.
I find this to be a tad over blown. I have yet to work on a vessel where it took me more than 20-30 minutes to add water to batts or check them and those are the long ones with major access issues. My own batts at 6 years old have taken water just three times now and it took all of about 3 minutes each time maybe four minutes at the outset if I got ADHD..
If maintenance is the sole claimed reason for an "upgrade", I know a boater who did this for that exact reason, it really is a lot of money to spend. He could have paid a pro to do the maintenance and still pocketed $400.00 to $500.00. Wet cell maintenance gets a little over blown IMHO and experience plus for very little money one can buy hydrocaps if really concerned.
Beyond that manufacturers such as Lifeline initially sold these batteries as 100% maintenance free. Once the issues of short life began occurring Lifeline began recommending a conditioning/equalization charge. An 8 hour equalization at 15.5 volts (temp compensated), as often as once monthly, is a LOT more time consuming than checking and adding some distilled water to wet batts.
Our batteries are dumb regulated and reside on a solar charge controller yet they took a very small amount of water just three times since new six years ago. I honestly don't even consider that "maintenance". I checked them this spring and with my screw driver it took me all of 1 minute to glance at the water levels.
Even if I gave the water filling a ten minute labor allowance every year, at my $70.00 hourly rate, that is $11.60 worth of labor time. I would have to add water to my batteries 61 times over their life to equal the price premium on my bank to make the move to AGM based on maintenance alone.
Lifeline AGM batteries, as noted, are not maintenance free. The time it takes to properly "condition" them is EIGHT hours at 15.5 volts, temp compensated. You also need a charger than can equalize/condition and you should always be present when doing so. The proper maintenance for Lifelines takes a lot longer than what it takes me to check my electrolyte. Some AGM's truly are "maintenace free", you can not equalize or condition charge them, Lifelines for example are not but they may last longer because of it..
This quote is from Justin Godber at Lifeline battery on what to expect out of Lifelines based on 4 scenarios.
Lay on side -
Originally Posted by Justin G. At Lifeline (excerpted from Morgan's Cloud web site)
Put broadly, there are four ways that will yield different lifetimes based on daily 50% deep cycles:
1- Fully charge after each discharge. Estimated life: 6-9 Years.
2- Fully Recharge at least once a week and equalize once a month. Estimated life: 4-6 Years.
3- Only recharge to 85% and equalize once a month. Estimated life: 2-4 years.
4- Only charge to 85% and never equalize. Estimated life: 1 year.
This is usually much more of an issue on smaller vessels than large and folks with smaller vessels often won't spring for AGM's anyway. For about $40.00 in materials one can build a battery platform on just about any vessel. I have done them on nearly every boat I have owned for far less than the premium price upgrade to an AGM battery. You still have to find a way to secure them so you would possibly be looking at some epoxy work anyway.
I am not trying to dissuade anyone, because there certainly can be a benefit for many users. I just happen to see a lot of boaters not taking advantage of the actual reasons why they bought them in the first place, except for a couple minutes saved per year on maintenance and the occasional battery on its side.
Food for thought..