M25 XP Stock Alternator and AGM batteries?

GrandpaSteve

Sustaining Member
I know this must have been asked many times, but my search terms are failing me. Can I safely replace flooded batteries with AGMs using the M25 XP stock alternator? I know it is not optimal, but I don't want to do anything dangerous. My charger manual says the charger is safe for AGMs and Gels.

Thanks,
Steve
 

Tin Kicker

Member III
https://stephenstuff.files.wordpress.com/2014/05/mass-capacity.png?w=450Absolutely yes from the perspectives of the boat and charger.

From the battery side, it looks like you're already aware that the AGMs will not give the performance of the OEM deep cycle flooded lead acid (FLA) stock batteries. About the only reason other than a slight weight saving to go to AGM is that you'll never have to check the water level again. The following plot is an example of how a similar FLA will let you keep the lights on longer:
 
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Christian Williams

E381 - Los Angeles
Moderator
Blogs Author
The gel cell and the AGM batteries are specialty batteries that typically cost twice as much as a premium wet cell. However, they store very well and do not tend to sulfate or degrade as easily as wet cell. There is little chance of a hydrogen gas explosion or corrosion when using these batteries; they are the safest lead acid batteries you can use. Gel cell and some AGM batteries may require a special charging rate. If you want the best ,most versatile type, consideration should be given to the AGM battery for applications such as Marine, RV, Solar, Audio, Power Sports and Stand-By Power just to name a few.
If you don't use or operate your equipment daily, AGM batteries will hold their charge better than other types. If you must depend on top-notch battery performance, spend the extra money. Gel Cell batteries still are being sold, but AGM batteries are replacing them in most applications.
There is a some common confusion regarding AGM batteries because different manufactures call them by different names. Some of the more common names are "sealed regulated valve," "dry cell," "non-spillable," and "valve-regulated lead acid" batteries. In most cases, AGM batteries will give greater life span and greater cycle life than a wet cell battery.

From https://www.batterystuff.com/kb/articles/battery-articles/battery-basics.html
 

debonAir

Member III
All you need to know about theory:
https://batteryuniversity.com/learn/article/absorbent_glass_mat_agm

The bottom line for me is the low self-discharge and the ability to routinely go much deeper discharge (FLA go 50% or so, AGM 80%) and better cold weathering. In practice my AGM appears nearly fully charged after winter storage.

The only issue I can see wrt to your stock alternator is the float charge voltage. If you motor for long periods of time, the alternator charger will keep the AGM slightly above their happy place in warmer (i.e. sailing) weather. I hadn't even heard that before re-reading that article, and my two AGMs are 2 and 12 years old, the 12 yo works OK but doesn't last as long, but I also don't know how it was treated before I bought the boat.

I suppose one of those LiON drop-in replacements would be the ultimate boat battery.

This guy is 28 lbs and cranks 1000 amps, 100 continues, for 100AH, and has built in smart charging. Of course it costs 3x the AGM

https://www.lithiumion-batteries.com/products/12-volt-lithium-ion-batteries/12v-100ah-lithium-ion-battery.php

Compare to AGM of similar spec weighs 70 lbs
 

sharonov

Member II
Charge acceptance rate of AGM can be more than double that of flooded batteries. Let's say you have 2 group 24 flooded batteries with total capacity of 150Ah and a stock alternator rated for 51A (cold). Then the batteries will accept about 30-40A of charge current and that is well within alternator spec. However equivalent AGM batteries will try to pull over 60A and alternator just can't provide that. Will it fry right away? Probably not since the bank is not that large but one day when batteries are deeply depleted and/or additional loads are active it may overheat and cook itself. The numbers are rough estimates and failure is not guaranteed but I would be concerned.
 

GrandpaSteve

Sustaining Member
Thank you everyone for your thoughtful replies. If I do replace the flooded batteries this season I will use AGM batteries. I can then plan on upgrading the alternator and regulator next season.
 

Tin Kicker

Member III
The others did point out some benefits that I'd not thought of, so my apologies. Part of my response was from what I see people actually install are not more expensive and better batteries or equipment like what Christian wrote about, so they don't get much benefit. Basically, most people just put in what they think they can get away with budget-wise. Personally if I were to bite the financial bullet for a significant improvement, I'd go to Lithium (LiFePo4) and significantly increase the storage capacity.

As for the alternator and your original question, a bigger one will help speed how fast the battery charges, but you aren't going to put maximum load on the alternator or the regulator until at high throttle settings. The chart below shows outputs of 5 regulators (generic chart). You can see that the initial limiting factor will be the rpm until at high rpm the limiting factor will be the regulator. Just because the battery can accept a higher rate of charge does not mean the alternator and regulator can or will provide it and in the end the battery will just charge slower than possible.
 
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Christian Williams

E381 - Los Angeles
Moderator
Blogs Author
With my 100 amp Balmar with smart regulator I can return 25 amps to the battery bank in about an hour of motoring, or less, if the bank is depleted to 60 percent or so. (batteries accept charge much faster when discharged; a battery bank at 95 percent capacity would take hours to achieve 100 percent from the same regulator.)

For that reason, on say a week-long cruise, a yacht's batteries are never recharged to 100 percent. The sweet spot is to maintain somewhere around 50-80 percent charged. Then, when the engine is turned on, the alternator output is initially maybe 80 amps. You watch on the battery monitor as the output drops as the bank recharges. You watch as the charge percentage rises from, say, 50 to 75. then turn off the engine.

A rule of thumb makes such recharging easy: when the alternator output drops to 15 amps, stop charging. From then on the charging rate becomes slow and you're just wasting diesel fuel.

The justification of a more powerful alternator with external regulator is speed of charging of discharged batteries. The stock 55-amp Motorola does fine, it just takes longer. Its internal regulator cannot provide variable charge rate.

A 100-amp alternator does place additional strain on the alternator belt. But a factory single-belt system is fine, if it is aligned and the belt is new.

Such an alternator uses one horsepower for each 25 amps of output. If the bank is low, it can really lug the engine at startup.

For this reason, some fit a cut-off switch so you can start the engine and engage the alternator later, after warm-up. Or disengage it for full engine power if fighting a current or such.

A Balmar 100-amp alternator can also be detuned to, say, 80 amps. This is done if full power puts too much strain is placed on the belts or the engine.

For cruising, a powerful alternator combined with an external regulator and a Victron battery monitor means no worries about energy management.

Blog entry here.
 
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Tom Metzger

Sustaining Partner
Here is a very good discussion of AGM vs Flooded cell batteries for sailboats by Maine Sail:

https://forums.sailboatowners.com/index.php?threads/agm-batteries-making-the-choice.124973/

My personal thoughts are that there is no way to justify the added expense of AGMs on a typical boat of our size range with the exception of it having to being placed where it can't be maintained. They typically do not last longer than maintained flooded cell batteries, they can't be charged quickly because of alternator/engine limitations, and they cost much more.

That much more hair in the scuppers.
 

Pat C.

Member III
A good article I read some time ago, don't know how I came across it...perhaps someone on this site, if so I apologize. By Nigel Calder

[FONT=Verdana,Arial,Tahoma,Calibri,Geneva,sans-serif]http://www.oceannavigator.com/January-February-2011/Battery-revolution/

AGM's are nice, no battery box required, deeper discharge. But a Flooded cell will last thru more discharge cycles, at least I read that somewhere. I use golf cart 6 volts in series, lots of AH capacity, readily available. Put Hydrocaps on a lead cell and your watering requirements and gas emmissions drop to negligable levels.

[FONT=Verdana,Arial,Tahoma,Calibri,Geneva,sans-serif]http://hydrocapcorp.com/

If you do change battery types, it's first probably best to consider your boat's wiring and whether its up to the task of charging the new battery type. Thats where TPPL and Lithium ion batteries become "complicated". The article by Nigel Calder talks about that I think.[/FONT][SUB][SUP]<strike>
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Christian Williams

E381 - Los Angeles
Moderator
Blogs Author
Flooded batteries are fine. On a recent boat they were old, and fried by an ancient charger. I installed a new smart charger and new flooded batteries.

The battery guy put the old batteries on his cart, tilted it, and left a stain of acid for 500 feet up the pier, dock, and parking lot that was there for six months.

Fac is, I only had to add distilled water to my new batteries about twice a year. I kept the water nearby. I kept the hydrometer in a plastic bag, so as not to burn up its drawer. Flooded batteries don't really need all that much attention.

However, they were on my mind anyway. I checked them more frequently than was necessary. They were part of the crowded head space labelled "maintenance". They had a little alarm bell that asked, "how is the electrolyte level?"

The current boat has a new smart charger, a new $400 Smartplug wire system, a new Victron--and this time I and chose AGMs. If rolled by a wave, they don't spill! That was never going to happen anyhow.

So, for the money what did I really get?

No need to think about the battery bank. I find it was worth the money.
 
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