Do AGMs have more usable capacity than regular Flooded?

[StuM]

Originally Posted by George_SD

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

maybe because your question doesn't make sense?

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).

Capacity has nothing to do with rare of discharge. They are completely different concepts.

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,

FF are not standard AGM batteries

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?

"Voltage level" depends on load and state of charge. How long the voltage can stay at a certain required level also depends on the battery capacity ( with is very basically the number of amp hours it stores)

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?

Again, depends on bank size and load. Recommended SOC is more a factor of longevity than of output voltage. You can discharge a FLA to much more below 50% SOC with a workable voltage (assuming the load is commensurate with the storage capacity) , but you will be decreasing it's effective life.

... 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.

Probably not. That's not the reason most people switch to AGM.

[sv.antea]

In 2010 I have invested in much advertised TPPLs before an extended Med cruise.
Shame on me, I ignored the salesman with Odysey at a boat show who mentioned that I should have about 40A charging capacity per 100 Ah battery to periodically charge with. On a boat that was to depended mostly on 270W nominal solar (and nominal 100A alternator), it did not sound right. Why would anyone recommend them as a house bank? I would need a 140A charging source. The batteries failed in two months from partial discharge disease, although I practically motored across the Atlantic and down the Med.
After I reacquainted myself with the 40A per 100 Ah requirement I took advantage of the separate on - off battery switches and periodically charged only one (or two when motoring) batteries at a time. Two of my batteries are 7 years old now and holding up. I even made an automatic battery discharger to check the capacity.

[George_SD]

My basic guestimate is about 60-100ah for a day depending on what and how is used. Again, I don't know the actual draw and going off of info i see on the web for the devices that I have. So 400ah should give me enough for a day without recharging. Actual usage may be much less or even more. I wont know until out a battery monitor of some sort.

I just installed a Raymarine wheel pilot and new (smaller) MFD with sonar. I put many instruments and devices on separate breakers so i can really disconnect the power to them and not have a parasitic draw.

[wrwakefield]

George, I think you might be overthinking this given the use case you stated for the near future of your boat.

When I had a smaller boat with limited space for batteries [and all boats are limited...] I chose to not have a separate starting battery. I never had any issues starting the engine.

It sounds like you have room for 4 batteries. I would install 4 6-volt quality FLA golf cart batteries to have a 450AH [nominal] battery bank. That would give you a relative 225AH of capacity drawing them down to 50% SoC.

If conservative, you could always isolate 6-volt battery pairs [12-volt series wired] with a battery switch if you wanted to preserve/fast charge/ equalize half the bank at a time, but I wouldn't bother.

If access for watering them is difficult, install a single point watering system [I have a house bank of 8 6-Volt FLAs on my current boat, and can water all 8 house batteries without accessing them in under a minute... Link to my battery replacement project... Nothing new or pretty, just effective...]

Just another way of looking at your battery requirements. I wouldn't hesitate to go this route...

Also, I highly recommend you take some time to 'cruise at the dock' [turn off your AC battery charger(s)] and live on the boat for a couple of days to gain better perspective what your 12V DC consumption is... Worse case at the dock is you have to turn on the AC charger(s) sooner than you hoped...

In case this provokes some ideas.

Cheers! Bill

[doublewide]

You will like your AGM's because they accept charge faster than regular lead acid batteries. Here's the danger: Please be aware that when you switch to AGM you will need to reduce your charging voltage or you will overcharge the AGM, boil off the electrolyte and kill them. This means that if you ever use engine alternators for charging, the alternators need to be programmable. Regular automotive type alternators charge at too high a voltage for AGM's. You might need to change out your OEM alternators - usually a basic automotive alternator - for an adjustable unit like Balmar makes. I trust that your inverter/charger and solar controllers are already programmable. It's the alternator that will kill your AGM's if you aren't careful.

[sailorboy1]

All batteries of same capacity have same useable power. Its just a matter of how far the user is willing to take out.

[thinwater]

I think we may be missing one of the OP's points.


Some electronics wink out before deep discharge is reached. They have a minimum voltage. He would need to find that min. voltage for his electronics package. This is also far more of a problem in cold climates, something many of you may not know (and which probably does not apply to the OP). A battery at 20 F puts out 0.5V less than one at 80F, and the difference can mean that electronics will wink out at 70% SOC. Been there.

[Scubaseas]

Originally Posted by doublewide

You will like your AGM's because they accept charge faster than regular lead acid batteries. Here's the danger: Please be aware that when you switch to AGM you will need to reduce your charging voltage or you will overcharge the AGM, boil off the electrolyte and kill them. This means that if you ever use engine alternators for charging, the alternators need to be programmable. Regular automotive type alternators charge at too high a voltage for AGM's. You might need to change out your OEM alternators - usually a basic automotive alternator - for an adjustable unit like Balmar makes. I trust that your inverter/charger and solar controllers are already programmable. It's the alternator that will kill your AGM's if you aren't careful.

You can easily convert an internally regulated alternator to accept external regulation. You maybe thinking of Gel Cells which do need lower charging voltages. AGMs are Ok with auto style alternators.

[newhaul]

Originally Posted by thinwater

I think we may be missing one of the OP's points.


Some electronics wink out before deep discharge is reached. They have a minimum voltage. He would need to find that min. voltage for his electronics package. This is also far more of a problem in cold climates, something many of you may not know (and which probably does not apply to the OP). A battery at 20 F puts out 0.5V less than one at 80F, and the difference can mean that electronics will wink out at 70% SOC. Been there.

Exactly and the only way that I know of to eliminate the issue is either constant charging or convert to lfp as it has a basicly flat discharge curve well above 12.5vdc which is still well above most electronics cut off voltage .

[Pete7]

He is weekend and holiday sailing, starting in SoCal, how often does it get that cold down there?

Now we know how the boat is used we have a start point. I would follow Bill's advice in post 19.

However, some form of battery monitoring is going to be essential before any other changes are made.

Longer term I am not sure that the fast charge or discharge advantages are going to be needed or achievable on a Bene 323 in standard trim weekend sailing.

Pete

[kryg]

Every boat has its power battery and charging challenges in my case - I investigated lithium battery and the end up because of the matching BMS required and risks associated with a bad BSM compatibility with charging assets. I had changed flood top up battery's for 4 AGM 215 Amp charged by 2 solar panels each 360 watts total 720 watts and a 400 watt wind generator for windy overcast and night charging for the house bank of 860 AH 12 volts.

Plus one 12 volts sealed starter battery.

The added difference to overcome the possible battery degradation, in my 12 volt system to prevent/slow the down battery degradation I also installed a broad desulfator from a UK company Battery Extra recovermybatteries.com who makes these good quality products in the UK, and not from China. I have purchased two of these one for the starter battery and a bigger capacity desulfator for the large 860 Amp house banks. I hope to get a much longer life from these AGM, as a result. So far it's been a year the system is works better than expected,,, Only time will tell if I made the right choice with installing the desulfator as a preventive.

Note: The attached photos are of the Battery Extra installation at the time I just installed them. You will note, the two different sizes of the desulfator's. One for the 860 Amp house bank the other for the starter battery.

From my experience I can recommend installing AGM batteries with a good quality desulfator (I tried the Chinese manufactured ones they do not work as promised). PS I have no connection or benefit from or to the UK company Battery Extra.

Good luck nutting out what is best for your boat and needs.

Originally Posted by 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...

[StuM]

Originally Posted by sailorboy1

All batteries of same capacity have same useable power. Its just a matter of how far the user is willing to take out.

Unless your load requires a certain minimum voltage!

Take a look at Load/SOC/Voltage curves for different battery technologies.

[Adelie]

Originally Posted by 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...

At 80% down the resting voltage of a lead acid battery will be about 11.98v.
https://www.mmbalmainauto.com.au/PDF..._batteries.pdf

Under a moderate load (=<C/10) the voltage will be 11.5v or higher (pg 3)
https://www.scubaengineer.com/docume...ing_graphs.pdf

So if your bank is 235Ahr and you are drawing 10A (0.043C) then voltage will be about 11.90v.
Running your fridge and normal house loads shouldn't be a problem, trying to start the engine would almost certainly be successful but the voltage drop might trip some of the electronics depending on what their individual minimums are.
You should find out what amps your starter draws if you are starting off the house bank. A clamp multimeter should be able to tell you.

AGMs do not have any more capacity in them than FLA or Gel. Carbon Foam batteries are somewhat better able to tolerate deeper discharge so there is an extra 25-30% "usable".

The carbon technologies are starting to make their way into into FLA batteries and cycle life appears to be improving.
https://www.trojanbattery.com/pdf/So...tLineSheet.pdf

An issue with AGMs including CFs is that they want to be charged at high rates periodically (once a week?) when they are drawn down far enough (40%down, 60% remaining). By high rates I mean 0.3-0.4C (71-94A for a 235Ahr bank). Apparently FF can be regularly charged at 0.2C (47A) as long as it gets the occasional 0.3-0.4C when drawn to 60%SoC or lower. See SB1's first post in https://www.cruisersforum.com/forums...ml#post3304417

If you have a stock alternator it might be able to push 70A but not for long once it heats up. In order to treat AGMs right you should have a high capacity alternator (120A), regulator and probably serpentine belt so that you can weekly run a high charge rate. Or you need to be on shore power with a high amp charger every week or 2 if you are regularly pulling below 60%SoC. If you convert to a 4 battery house bank it will probably be too big even for that alternator, then you will need to split the bank into 2, charge 1 side for an hour or so, then turn the engine off, switch to the other side, and do it again.

Why do you need 2 batteries in the starter bank? Really big engine? Or could one of them be converted to part of the house bank? Could you convert both to be the same type battery as the house bank?
Have you considered using the house bank to start the engine? Yes, you run the risk of not being able to start if you run it down far enough. With a 470Ahr bank it would be a lot less likely that would happen, unless you left the boat unattended for a significant period. There are mitigation strategies for the house bank not having enough juice.

Could your battery spaces tolerate batteries with a slightly bigger footprint, or maybe taller? If there is plenty of overhead, the Trojan SPRE 12 225 could almost double the capacity of your house bank to 408Ahr (20hr rate) for about $800 and you would have 204Ahr usable to 50% vs 186Ahr usable to 20%SoC FireFlys that would cost $1086 for the pair.

[Adelie]

Originally Posted by 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.

Actually the power density for lithium is similar to LA batteries. It's the specific energy (energy per weight) that is much better.

[sailinglegend]

[QUOTE=StuM;3309880]maybe because your question doesn't make sense?

This I'm afraid is all too common on many threads like this!!!! An astute observation Stu.

Elecktrikery is a complex subject so it's not surprising that boat owners can't get their heads around it. The problem - in my opinion - is that there are lots of books that unfortunately describe Ohm's Law in the first chapter but fail to address the OP's original confusing post. The answer to his questions needs a completely new book - not short posts - to explain the concepts in a simple and understandable format. Sadly forums like this only confuse the situation even more because too many posters add misleading, and worse incorrect advice.

There are many references on this forum to Mainsail's excellent website, who sadly no longer regularly posts here. Sometimes his articles are too detailed and complicated for the average boat owner - sorry Rod - so those articles are occasionally misquoted here! Nigel Calder's books are also far too advanced. The answer should really be your local marine electrical engineer or marine store, but sadly that is too often not the case??????

The battery and equipment manufacturers are equally to blame, especially when they make oversimplified statements in their literature like:

....batteries are fully charged when the charger drops to float mode.

Two manufacturers I can recommend who provide good advice are Victron Energy and Lifeline Batteries whose 36-page manual has in-depth discussions on AGMs, not just 36 pages listing their extensive range of batteries. Victron's site offers excellent downloadable documents on charging, especially 'Energy Unlimited'.