AGM charge acceptance

[sailorboy1]

I've been looking at battery charge acceptance and one of the things you read is that AGMs have a faster charge acceptance. Great, can someone provide a link to prove this statement? Something with test data that shows for the same battery capacity the difference in acceptance from 80% SOC to 100% SOC between FLA and AGM batteries?

[Suijin]

Here's some testing:

How Fast Can an AGM Battery Be Charged..? Photo Gallery by Compass Marine How To at pbase.com

[a64pilot]

I'd look at Lifeline for that, however at higher SOC rates like 80% and above, I think the difference shrinks, its when you get down around the 50% SOC that you can really throw amps at an AGM bank.
With batteries around the high 70 temp. my 660 AH bank will accept 185 amps for 20 , maybe 30 min, then it hits absorption voltage and the amps begin to decline.

On edit, I think using your example, there is little practical difference, certainly likely not worth spending a whole lotta bucks on.
Page 19 of the Lifeline technical manual talks about calculating charge time and gives a formula to determine it, however it does not do the same for a flooded battery, and I would have to assume that may be a variable as well, different manufactures may calculate "full" differently?

As an AGM owner, if my opinion means anything, the requirement to be 100% charged often, way offsets any advantage of a higher acceptance rate may have, if I had the option, I'd rather my bank be more partial SOC cycling tolerant and I'd accept a lower charge rate for that.

[sailinbob1]

Sorry,I don't have a link for you,but I've installed 2 Intimidator AGMs on my sailboat, and they stay charged,seldom dropping below 12.8v, even on cloudy days. I've got an 11 ft. × 14in, 100watt solar panel, and when I board the boat,the batteries are usually near 14v. That said,I don't draw on them much,because I'm doing a refit. I do,however, use my cabin lights,Anchor lights,and navigation (no need to go into that. Ugh....)lights,which are all incandescents (LEDs are part of the refit). I also run my Hummingbird 365i fish finder. Bottom line, I've not had a problem. None. The advantages I enjoy is,there maintenance free,and can be installed in any configuration. ie: top up,on their side,vertically, etc. Mine are 1000 cold cranking amps, and 130 amph. I like em. Good luck.

[sailorboy1]

My little 440AH house bank is rarely less than 85% and even at that SOC they are limiting charge acceptance to less than 1/2 of what my charger can do if I run my generator. Once the batteries get into the low 90% SOC less than .1C is going in. If this AGM acceptance advantage is only at 50% SOC that's just a BS marketing spin story.

[a64pilot]

It is not a BS marketing spin, Lifeline in their chart where they market their advantages over a FLA battery do not even mention charge acceptance as I guess they don't think it significant.
They do mention it in their marketing chart where they talk of their advantages over a Gel battery, because apparently you dare not charge a Gel too fast.

Just a guess, But I'd guess I go from close to 50% to 70% or so pretty quick, maybe an hour? But it takes two hours from 90% to 100%? So that last 10% that is so critical to an AGM's life, goes in 1/4 as fast?
Those are guesses so don't attribute anything to them, only to illustrate you dribble in the last little bit.

You cannot get more value in a battery than a GC wet cell battery, however due to many other concerns, not all of us can or want to run GC's. Some people the extra money for a "maintenance free" battery is worth it, some have sizing concerns.

Now if the price comes down on the Firefly and it lives up to its hype, then that may be the AGM to have.

Only technology that I am aware of that you can really slam amps into and they don't taper well off as you approach a full charge is Life-Po, and I don't think you want to fully charge them anyway

[a64pilot]

Charge current slowly begins to decline, I start out current limited at 185 amps, it then slowly begins to walk down, my 660 AH bank is not "full" until charge current has walked down to 3.3 amps, that doesn't happen all the sudden, its a gradual thing.
Sounds a lot like your FLA bank doesn't it

[john61ct]

Yes, this only saves time during the relatively short Bulk constant current stage.

Look for a maximum charge rate specification from the maker.

.2C means for a 300AH bank, no point buying a charger over 60A.

Same size bank with a .6C rating chemistry you can pump in up to 180A.

So say Bulk is from 50% to 83% (100AH), that increased CAR saves you about an hour off a total say 12 hours total required.

Actual numbers will of course vary, this just shows how to do the calc.

[john61ct]

This shows the huge LFP advantage, much higher CAR, some you can fully charge < one hour!

Plus no need to fill up in the first place, in fact keeping at or even trying to get to spec'd 100% will *damage* your bank, they *prefer* to rest at 50% for longevity.

[sailinbob1]

My advise to you would be this. If you're suspicious, stick with what you know. Unless you're willing to make some changes to your system, reducing your draw,and aren't willing to do some experimentation, due to budget restraints,or whatever, all advertising is a BS marketing scam. Lol... The only way you'll know for sure,is to give em a try. You can do all the research, and math, you want,but,just like in theoretical physics,you don't know, for sure, until the experimental physicists, or astrophysicists,prove your theory. Ask Uncle Albert. Again, good luck.

[StuM]

To me, the advantage of AGM's higher charge rate is it halves the engine /generator time for any initial bulk charge first thing in the morning leaving a longer time available on solar to finish off the absorption during the day. It doesn't appreciably shorten the time required for the last 10-15% top up.

(But that's not the main advantage for me - it's primarily the lack of maintenance that appeals - the way my bank is situated, it would be a real PITA to have to top them up regularly).

[john61ct]

Originally Posted by a64pilot

Just a guess, But I'd guess I go from close to 50% to 70% or so pretty quick, maybe an hour? But it takes two hours from 90% to 100%?

Wut?

Try 6-10 hours, maybe more!

And yes huge difference in lifetime between even 98% and 100%.

Also between once a week vs every cycle.

If you can't do that IMO save your money, 6v GC2 FLAs at $180 for 245AH, Duracell actually Deka/East Penn.

Get your system to the point you can get *those* to last four years, *then* invest in more expensive chemistries.

[ranger58sb]

Originally Posted by a64pilot

With batteries around the high 70 temp. my 660 AH bank will accept 185 amps for 20 , maybe 30 min, then it hits absorption voltage and the amps begin to decline.

Originally Posted by a64pilot

Charge current slowly begins to decline, I start out current limited at 185 amps, it then slowly begins to walk down, my 660

Originally Posted by john61ct

Yes, this only saves time during the relatively short Bulk constant current stage.

Just a note that absorption voltage decreases slowly (as A64 notes), not immediately...

So high charge acceptance from, say 50% SOC, could well mean a boatload of amps for a significant period.

Our maker suggests a minimum of .4C, and in our configuration that absorption period (after reaching that, post bulk) could last up to 4 hours...

-Chris

[ranger58sb]

Originally Posted by StuM

To me, the advantage of AGM's higher charge rate is it halves the engine /generator time for any initial bulk charge first thing in the morning leaving a longer time available on solar to finish off the absorption during the day. It doesn't appreciably shorten the time required for the last 10-15% top up.

(But that's not the main advantage for me - it's primarily the lack of maintenance that appeals - the way my bank is situated, it would be a real PITA to have to top them up regularly).

I fall firmly in both these camps. Maintenance free, first. (Reduced off-gassing, next.) Faster charge rate, OK with me... but not really the primary reason why I made the switch in the first place.

AGMs (instead) of gels meant I could use my original charger on all three banks throughout the gradual transition (of individual banks) from FLAs. Wasn't optimum, but it's worked out fine.

Increasing one of our (currently 300Ah) banks would require non-traditional mounting, due to space constraints. I could do that with AGMs; not sure whether I will, or not. As I write this, that could become a theoretical advantage -- for me -- to AGMs. Doubt I need to fool with it...

-Chris

[a64pilot]

Originally Posted by john61ct

Wut?

Try 6-10 hours, maybe more!

No, if you chose to believe Mainesail, and my experience mirrors what he says, assuming a big enough charger 5-7 hours from 50% SOC to 100% SOC.

Lifeline states in their manual that if you have a big enough charger 2 hours, but I don't think that is possible in anything but Lab conditions on I assume a new, properly conditioned battery.

Link that Suijin posted earlier
How Fast Can an AGM Battery Be Charged..? Photo Gallery by Compass Marine How To at pbase.com

Worth a read


On edit, I throw the 7 hours out there as I believe as they age charge time takes longer, so for a bank that has been in service for some time it may take longer.

For people that don't want to read the article, biggest take away from it is that in this case, doubling charger capacity shortened charge time by 12 min.
However its not that simple as I believe that a high charge rate will make an AGM Lifeline bank last longer.

1- 50% SOC to 100% SOC at .2C = 5:42 - Exited Bulk Charge at 1:16
2- 50% SOC to 100% SOC at .4C = 5:30 - Exited Bulk Charge at 19 Minutes
3- 50% SOC Charged at .2C For Exactly 1 Hour = 71% SOC - Never Exited Bulk
4- 50% SOC Charged at .4C For Exactly 1 Hour = 85% SOC - Exited Bulk Charge at 20 Minutes
5- 50% SOC Charged at .2C For Exactly 2 Hours = 87% SOC - Exited Bulk Charge at 1:16
6- 50% SOC Charged at .4C For Exactly 2 Hours = 96% SOC - Exited Bulk Charge at 20 Minutes