How Fast Can an AGM Battery Be Charged..?

[Maine Sail]

Hi All,

Finally got around to doing some testing & data logging that I have been meaning to do for a long, long while. The article hits those proverbial questions that are almost always assumed or answered incorrectly such as:

How fast can I charge an AGM battery?

What SOC can I charge to in 1 hour or 2 hours?

At what % SOC do I end bulk and hit absorption voltage?

Does a large charge current really make a difference?

How long does it take to charge from 50% SOC to 100% SOC?

How Fast Can an AGM Battery be Charged (LINK)

Hope this helps answer a few of those nagging questions....


P.S. Still editing out some typos etc.. If you find those buggers let me know!

 

[Joe A]

Very nice write up. In general how would you expect regular lead acid batteries to behave in a similar test?

 

[Maine Sail]

Very nice write up. In general how would you expect regular lead acid batteries to behave in a similar test?

AGM batteries are lead acid.. As for flooded batteries they are best charged at under .25C but preferably less than .2C so current can absorb into the thicker plates and not create a surface charge and trick the charger into a premature absorption. They also generally take longer to recharge from 50% to 100% than AGM. As sulfation sets in, on any lead acid battery, the time to 100% extends..

Here is a 130Ah Trojan SCS-225 that was testing at 124.5Ah, so in quite good health, and it took 6:58 minutes to reach full at a .15C charge rate..

 

[Gunni]

Having switched last season from FLA to AGM, along with a modern battery charger w/temp compensation) I can say that the ability to restore my house bank in a couple of hours is exactly what I needed. Sitting on the hook with the generator running feeding the charger for hours was a practice I was looking to end, and a change to a 85 amp charge rate and the battery that can take it got me there.

 

[Maine Sail]

Having switched last season from FLA to AGM, along with a modern battery charger w/temp compensation) I can say that the ability to restore my house bank in a couple of hours is exactly what I needed. Sitting on the hook with the generator running feeding the charger for hours was a practice I was looking to end, and a change to a 85 amp charge rate and the battery that can take it got me there.

How big is the AGM bank in rated Ah's?

 

[Gunni]

Right around 500 AH Rated, believe they are the group 31 (4) Lifelines you reference.

 

[Maine Sail]

Right around 500 AH Rated, believe they are the group 31 (4) Lifelines you reference.

At a .17C charge rate you should be in bulk from 50% SOC for approx 1.5 hours approaching absorption somewhere around 80% SOC.....

 

[Ken Cross]

Main Sail
Thanks for the write up. This is the main reason I go to this website.
Now you have me questioning my own charging system. I have no idea of the capacity of my alternator (Yanmar 2GM20F) and now I need to go do a bit of checking. I never see my battery voltage up in the 14 volt region unless the engine is running.
I need to run down to the boat to see what king of shore power charger I have. There is no ammeter installed. Now you are going to force me replace my old AC with a new DC clamp on meter. How come it seems like every time I read a post from you it cost's me something?

Ken

 

[Maine Sail]

Main Sail
Thanks for the write up. This is the main reason I go to this website.
Now you have me questioning my own charging system. I have no idea of the capacity of my alternator (Yanmar 2GM20F) and now I need to go do a bit of checking. I never see my battery voltage up in the 14 volt region unless the engine is running.
I need to run down to the boat to see what king of shore power charger I have. There is no ammeter installed. Now you are going to force me replace my old AC with a new DC clamp on meter. How come it seems like every time I read a post from you it cost's me something?

Ken

Ken,

The reason for the poor performance of your factory alternator are three fold.

#1 This is an automotive duty alternator meant for a few minutes at best at full output then supplying perhaps 1-3A to the car battery and maybe 5-10A for the cars DC loads. On a boat with deep cycle batteries you are often asking it for hours at full output something it was never designed or intended to do..

#2 As a protective feature, to make the cheap alternator last though the warranty period, a temp gradient is built into the internal regulator. For every degree rise above about 69F it drops the absorption voltage limit. This in turn reduces current output and murders batteries by chronically under charging them. It also extends your charge times to well beyond 12 hours to reach full, if you ever do. In order to charge batteries in any reasonable time the absorption voltage needs to be maintained for multiple hours beyond the bulk to absorption transition.

#3 Your alternator is "self-sensed" meaning it only knows what the system voltage is at the alternator before any voltage drop occurs in the system wiring between the battery bank and the alternator.. Couple this with the horrible over-heat protection method (reducing charge voltage) and you get really, really crappy charging performance out of this type of factory alternator..

An external regulator, such as a Balmar, does the opposite in terms of alternator protection. Instead of reducing the absorption voltage it reduces the field. Reducing the field essentially reduces the current output but the target voltage, say 14.6V, remains unchanged. With the proper voltage sensing capabilities of an external regulator you get a significantly healthier charge, proper absorption voltages and faster charging.

If you have AGM batteries the charge sources really need to be temp compensated or they can be ruined prematurely. For temperatures above 80F the charging voltage needs to be reduced and on the cold side charge voltages should ideally increase.. What this means is that if the battery says 14.4V absorption and 13.4V float this is the voltage to be used at 77F - 80F adjustments need to be made for warmer and colder temps.....

Spending the money on AGM batteries means you should care for them otherwise they will last no longer than a set of inexpensive Wal*Mart batteries only you'll be out a LOT more money...

AGM's are good investment for those who install and used them properly and a poor investment for those who don't..

 

[Ken Cross]

Thanks Main Sail
I knew you were going to cost me money. I need to have a look at my alternator as I don't know if it is the original or not. (I assume it is original.) I think I need to know if it can be wired to an external regulator (Are the connections available?)
I guess I need to go down and look. Also I did order a DC clamp on ammeter today.
Ken

 

[Gunni]

Balmar has a selection of high output alternators which fit the OEM mounts and can be wired to an external regulator. You can even buy a high output alternator, de-tune it to run on a v-belt, and later upgrade to serpentine belt system that allows you to increase the output. This is an affordable way to get the most out of your auxiliary diesel.

 

[Gunni]

At a .17C charge rate you should be in bulk from 50% SOC for approx 1.5 hours approaching absorption somewhere around 80% SOC.....

That sounds about right. My average daily sailing Amp/Hour budget is closer to 100 Ah/day. Add 20Ah for anchor, cabin light, pumps, radio, and stereo at night. Which should put me at about 1 hour of generator time in the morning. Acceptable. I keep looking at those thin Italian photovoltaics that you zip to your bimini, a marginal cost benefit, but it would be nice to see the amp meter spinning backwards while under sail.

 

[Maine Sail]

That sounds about right. My average daily sailing Amp/Hour budget is closer to 100 Ah/day. Add 20Ah for anchor, cabin light, pumps, radio, and stereo at night. Which should put me at about 1 hour of generator time in the morning. Acceptable. I keep looking at those thin Italian photovoltaics that you zip to your bimini, a marginal cost benefit, but it would be nice to see the amp meter spinning backwards while under sail.

If you're just doing a few days away from the dock at a stretch, 2-4 days, you are pretty good with what you are doing. If you start to push more than 4-7+ days then the PSOC use of the batteries will start to accelerate the aging and drag them down and then solar makes more sense..

 

[Maine Sail]

Just finished adding the extra data to the article:

Added:
1 hour recharge @ .2C & discharge back to 50% Calculating Stored Energy
1 hour recharge @ .4C & discharge back to 50% Calculating Stored Energy
2 hour recharge @ .2C & discharge back to 50% Calculating Stored Energy
2 hour recharge @ .4C & discharge back to 50% Calculating Stored Energy
Charging Graph showing bulk, absorption etc.

 

[jackhartjr]

Maine Sail, would you please call me at 828-310-8111?
Jack

 

[berner73]

P.S. Still editing out some typos etc.. If you find those buggers let me know!

Maine, great article. Thank you, this is exactly why we support your site. Best boat dollars we spend every year.

Real quick, check page 2 about halfway down re: getting back to 100% SOC. Did you mean "under" or "over":

"The importance of getting back to 100% SOC, as often as possible, can not be under emphasized."

 

[Maine Sail]

Maine, great article. Thank you, this is exactly why we support your site. Best boat dollars we spend every year.

Real quick, check page 2 about halfway down re: getting back to 100% SOC. Did you mean "under" or "over":

"The importance of getting back to 100% SOC, as often as possible, can not be under emphasized."

Fixed that error.... Thanks for that!!

 

[Brian D]

For a battery that does not display the 20 hour discharge rate, is there a formula to calculate that value, other that physically discharging the battery?

 

[Charles Erwin]

Maine

At page 2, discussing bulk voltage: ". . . but once voltage is held stead or becomes "voltage limited." . . . "
At the last page, at Note concerning Lifeline: ". . . .2C charge rate as teh bare minimum . . "

By the way one hopes folks take the time to understand this stuff thus to reduce battery homicides - probably should be battacides.

I am wrestling with why you say bulk voltage is a marketing term. Every regulator and shore charger that is programable allows one to choose a specific voltage during the bulk phase and that voltage is typically higher than the absorption voltage. Could it be lower? What was the voltage during bulk at .4 and .2 in your exercise?

Charles

 

[bgary]

Spending the money on AGM batteries means you should care for them otherwise they will last no longer than a set of inexpensive Wal*Mart batteries

Maine Sail -

Can you put together an article on "Battery Care for Dummies"?

I'm back on the water after a few years away, and find that the advances in battery technology have outpaced my 1980s understanding... and really don't want to ruin the nice AGMs my new-to-me boat came with.

B