Do AGMs have more usable capacity than regular Flooded?

[George_SD]

I can't seem to find a more or less definitive answer on this.

Most places list many benefits of AGM, but don't really say that they have greater capacity (ability to provide electricity at a rate acceptable for electronic and electric devices on a boat). A few casual mentions here and there do state that AGMs can "output" more useful energy. FireFly specifically says that their batteries have 80-90% depth of discharge, but does it mean that it will be outputting voltage levels that would be usable or will it be too low to really be useful for things like fridge, chart plotter, etc?

I've read many times that for flooded recommended state of discharge is about 35% of total capacity, some say for AGM's up to 50% (with Firefly claiming 80-90%). But if that's achieved at voltage levels that are not really acceptable to any device, then what's the point?


I'm quite confused and can't figure out what would give me the longest runtime from a banks that I can fit.

Right now I have 2 flooded deep cycle (house and engine). I have severe space restrictions and the most I can fit is: 2x6V golf cart batteries and 2x G27 size batteries. So I'm trying to figure out if going with AGM is worth the cost in terms of giving me more usable battery run-time.

getting 10% more capacity at 50% higher cost is not worth it. Getting 30% more capacity for 50% higher cost is worth it to me...

[tkeithlu]

There are too many variables in play to allow an unequivical answer.

For the major advantages: "While regular lead acid batteries need a topping charge every six months to prevent the buildup of sulfation, AGM batteries are less prone to sulfation and can sit in storage for longer before a charge becomes necessary. The battery stands up well to low temperatures and has a low self-discharge."

Use the Amp hour rating of the individual battery you are considering. Past threads on the forum have come to a consensus recommendation of the Duracell batteries, with the best price being Sam's Club.

[dougweibel]

Firefly batteries are not "regular" AGM. They use a carbon foam chemistry/construction that has very different properties than traditional AGM batteries. My secondhand understanding is that Firefly does fairly well deliver on their specifications, but they are expensive and have had severe availability issues in the past.

[kenbo]

AGMs are lead acid batteries and have voltage vs SOC curves very similar to flooded batteries. Google Lifeline battery manual, they have voltage decay curves for several discharge rates. Might help you make a decision.

[akopac]

I was in the buy new bank boat. Had flooded. Did research. My opinion/conclusion.
I was able to reduce my house bank from 5 t31 batteries to 4 and increase usable battery capacity from ~170 amphrs to ~410 amphrs by going to fireflys. Life expectancy they say is 6-10 years. Charge at up to 250 amps. I had to to turn down my alternator regulator output to be nice to the alternator.
I was looking at lithium too. But they are twice the price and I would have had to change out alternator and solar controllers as well. I think by the time the fireflys need replacing the next best battery will be here, be it a lithium derivatives , solid state or otherwise.
Price is relative.
I won’t say no maintenance as you occasionally need to to top them off, but with 400w’s solar, led lights, and good fridge insulation I don’t watch the Emeter anymore.
Ocean planet said I could have done it with 3 t31’s. I agree.

[Stu Jackson]

Originally Posted by George_SD

>>>>>>>>>>>>
I have severe space restrictions and the most I can fit is: 2x6V golf cart batteries and 2x G27 size batteries.<<<<<<<<<<<
>>>>>>>>>>>>>>>>
Getting 30% more capacity for 50% higher cost is worth it to me...

Have you considered relocating your existing battery location(s)?


X ah per battery is still X ah per battery. A 110 ah AGM has the same storage capacity as a 110 ah FLA.


AGMs can charge faster because they have a higher acceptance.


The shortcoming of AGMs is that they suffer serious degradation from PSOC - they NEED to be FULLY recharged regularly, even though WHEN fully recharged they will hold it for a long time.


Your boat, your choice.

[CarlF]

Flooded, AGM's and Firefly's will all see their voltage decrease at about the same rate as they are discharged. If you have equipment that needs a certain voltage, you will be limited as to how much you can discharge them.

Also realize that the voltage shown will depend on how many amps are being drawn by the boat. Voltage shown on a battery meter will often be .2V lower with a load than with no load.

Lithium Iron batteries can also be discharged 80%+. Their voltage does not fall significantly until they are almost completely discharged nor show a lower voltage under normal loads.

[Scubaseas]

There is a very good discussion of this at Attainable Adventure Cruising https://www.morganscloud.com/ but it will cost money well spent.

It really depends on how much you discharge versus how you recharge and if you can equalize or not.

What do you have for charging?

AH/space max is Lithium. There is not a huge difference in AH per sq foot of space between Gel, AGM, flooded or carbon foam. There is a big difference in how long the batteries may last if well used. If cost is the main drover a good quality flooded is hard to beat especially if you can equalize them. Carbon Foam (Firefly) may be a solution if you believe their life cycle/dod charts.

[George_SD]

I can't really relocate the banks - this will shift the weight distribution and require a very significant amount of work.

I was considering firefly's as they appear to give second best density after lithium.

Jeff Cote here says that wile cruising and recharging not having the ability to really push and recharge fully (but to be able to get to at least 80%)

https://www.youtube.com/watch?v=SIVS0WD7Xq8

FLA usable rate is about 35%
AGM usable rate is about 55%
FireFly usable rate is about 65%

So from this video:
FLA - cheapest, but lower density
AGM - best density for the money
Firefly - highest density, but at a cost


Fireflys also require high rate charging from time to time to maintain capacity, which means that I would have to upgrade both - shore charger and alternator.

I think at this point I'm down to continuing with Flooded or going with regular AGM. My boat is mostly sitting on a shore power/charger in a slip. I will be taking it out for up-to 10 day trips where my only source of power is the alternator off the engine.

After reading Maine Sail's post stating how AGMs also need fairly high charge rates in order to be effective and not die prematurely, it seems that regardless of the differences between Fireflies, Lithium, regular AGM and flooded - unless I'm ready to significantly upgrade my shore charging system and alternator - FLA is my best choice. It appears to be not as simple as just swapping batteries and paying a bit more for them.

Oh well....

[George_SD]

Originally Posted by Scubaseas

What do you have for charging?

AH/space max is Lithium. There is not a huge difference in AH per sq foot of space between Gel, AGM, flooded or carbon foam. There is a big difference in how long the batteries may last if well used. If cost is the main drover a good quality flooded is hard to beat especially if you can equalize them. Carbon Foam (Firefly) may be a solution if you believe their life cycle/dod charts.

Stock 20A Xantrex TrueCharge and whatever stock alternator came with the boat.

As as mentioned in my post above - going with anything but FLA at this time is going to be price prohibitive as full upgrade of the charging system would be needed and I don't have the that kind of money for this project at the moment.

[s/v Jedi]

Lifeline batteries can do 80% dod as well and even flooded lead acid can do that. This does reduce cycle lifespan dramatically, for all of them.
Big differences are in cycle life, self discharge and Firefly adds to that (and this is the big one... although controversial) that their batteries are not damaged by not fully charging them.

[Scubaseas]

Your charging systems should be designed with your battery bank size in mind. Your battery bank should be designed with your amp usage in mind as they're very much interrelated. If your daily usage far exceeds your charging ability you'll either be charging all the time or replacing battery banks frequently compared to a well designed system.

[Pete7]

Originally Posted by George_SD

After reading Maine Sail's post stating how AGMs also need fairly high charge rates in order to be effective and not die prematurely, it seems that regardless of the differences between Fireflies, Lithium, regular AGM and flooded - unless I'm ready to significantly upgrade my shore charging system and alternator - FLA is my best choice. It appears to be not as simple as just swapping batteries and paying a bit more for them.

Oh well....

Good choice. I watched some of Jeffs other you tube presentations at some sort of boatshow while back and the impression I got was that his aim was to sell high end batteries and systems that his company just happens to sell, rather than a cost effective solutions.

Are you weekend and holiday sailing with the boat in a marina? if so why can't solar or shore power keep your batteries completely topped up giving you far more than the 35% you think you have.

[George_SD]

Yes, I'm just a weekend user for now. This summer we plan on doing a 10 day trip to Catalina Island as well as a number of 2-3 day trips. All other time it is connected to shore power in the marina. They will be topped off and ready for day sailing for sure, but the multi-day use may deplete them.

I'm thinking that doing a slightly larger FLA bank and then adding solar panel for multi-day use would give me the most flexibility as it's really sunny here in Southern California and it can help with re-charging without running the engine.

[Scubaseas]

Calculate your usage first. Then size your bank off that figure and your charging capacity off the bank size. The less power used the smaller the bank. How far DOD comes into play after you calculate usage and how often and long you are wiling to charge.

Fridge and Auto Pilot are the two biggest power hogs if you have cooking fuel/gas.