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Old 05-03-2014, 09:55   #1
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How Long will Battery Charge Last?

Leaving the boat for a year and will have all the AGM batteries (7), at 12.8 or .9 when we do. How long will that charge last? The batteries will be turned off. There is a small bilge pump connected direct to one battery bank but we have no leaks so I doubt it will ever come on. We can have someone come aboard and turn on the charger but would like to have that done as little as possible.
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Old 05-03-2014, 09:57   #2
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Re: How long will battery charge last?

AGMs have a low self discharge rate, but also need to be kept fully topped off to avoid sudden death syndrome. Your best bet is to get a small solar panel and a controller.
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Old 05-03-2014, 11:05   #3
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Re: How long will battery charge last?

Northern Arizona Wind and Sun says:

AGM's have a very low self-discharge - from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries

Deep Cycle Battery FAQ, or Absorbed Glass Mat Batteries

My AGM outboard start battery sits in a sealed box, connected to nothing, for six months and registers about 12.5 V when I check it. It can start the 9.9 Nissan after seven months of no charge.
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Old 06-03-2014, 07:54   #4
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Re: How long will battery charge last?

Voltage is a poor indication of battery condition, i.e., actual capacity. Ability to start an engine is almost irrelevant. For example, a car battery can start a car one day, and not the next. This is because it takes very little CAPACITY to start an engine.....typically, less than 1AH to start even a small-med size diesel.

As Stu said, while the self-discharge rate of AGMs is low compared to flooded batteries, they must still be fully charged frequently in order to avoid plate sulfation which reduces battery capacity.

Your best bet is to get a small solar panel and controller to keep them topped up and not even think about letting them sit for long periods without charging.

Bill
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Old 06-03-2014, 17:29   #5
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Re: How long will battery charge last?

Your best bet is to get a small solar panel and controller to keep them topped up and not even think about letting them sit for long periods without charging.

Bill[/QUOTE]

Well then having someone come aboard and turn on the charger once and while will be the plan. From what I understand they will charge $5 (about 40 Quetzal ) to do it. Not so bad compared to the cost of batteries.
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Old 07-03-2014, 10:50   #6
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Re: How long will battery charge last?

Quote:
Originally Posted by btrayfors View Post
....As Stu said, while the self-discharge rate of AGMs is low compared to flooded batteries, they must still be fully charged frequently in order to avoid plate sulfation which reduces battery capacity....
Bill/Stu...

This is the same with ALL lead acid batteries, but many say on these forums that AGMs must be fully charged more often than FLAs need to be. But where is this evidence for this as AGMs are also Lead Acid batteries?

Lifeline are one of the few companies who keep pushing the point about fully recharging to lengthen life. There is a lengthy article on the morganscloud site (members only now) with this quote from Justin Godber @ Lifeline:

ALL batteries need to be fully recharged to avoid sulfation build up on the plates.... I can send links for Trojan Battery, Deka Battery, Odyssey Batteries, etc…They all state the same thing. Batteries must be fully recharged to avoid damage and premature failure.
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Old 07-03-2014, 10:57   #7
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Re: How long will battery charge last?

Quote:
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Bill/Stu... But where is this evidence for this as AGMs are also Lead Acid batteries?
Try this:

AGM Batteries - Making The Choice - SailboatOwners.com

It's looong and has a few links inside that are worth reading, too.

It's a good critical analysis of the choices a skipper has to make when choosing these.
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Old 07-03-2014, 11:03   #8
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Re: How Long will Battery Charge Last?

Yes, AGMs, gels, and FLAs are all lead-acid batteries, though they have a very different internal construction and, sometimes, a different chemical composition.

I've never seen any metrics comparing the rate of plate sulfation amongst the three battery types.

I think statements re: need to frequently fully charge AGMs derive from the belief that their relatively low self-discharge rate somehow equals their ability to sit for a long time without damage. Ditto for gelled batteries.

My belief is that all three types need to be maintained at full-charge levels as much as possible. Further, I can tell you that flooded batteries, FLAs, will sulfate even when "fully charged" and floated at, e.g., 13.2-13.4VDC. I float mine at 13.8VDC AND goose them with 14.6-14.8VDC every other day for about 30 minutes, and have documentary proof that this treatment (of Trojan T-105s) results in restored capacity and longer life.

Bottom line: all lead-acid batteries should be kept fully charged as much as possible. It just ain't good to let these batteries sit without charging as their ultimate capacity will be diminished.

Bill
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Old 07-03-2014, 11:35   #9
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Re: How Long will Battery Charge Last?

Quote:
Originally Posted by btrayfors View Post
...I've never seen any metrics comparing the rate of plate sulfation amongst the three battery types.
I only asked the question because many posts on here dismiss AGMs because they say they need to be fully charged more often than FLAs - without providing any references.

Quote:
Originally Posted by Stu Jackson View Post
Good article, but Maine Sail is only quoting the same Lifeline source as I was - with some extra information.
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Old 07-03-2014, 12:06   #10
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Re: How Long will Battery Charge Last?

"...some extra information????"

This is one of the few anaylsis I've ever seen and I read a LOT of boating forums.

His discussion is about as long and as real-world as you're gonna find.

Don't know what more we can do to explain it to you.
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Old 07-03-2014, 12:46   #11
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Re: How Long will Battery Charge Last?

I have not delved into the mysteries of AGM - but what I do know for a fact based on ten years of experience:

I had a little PWC AGM battery that I use for starting my Nissan outboard. It lasted thru 10-years of abuse. The ONLY charge it every received was when it was connected to the Nissan outboard. We often use the 2 HP outboard rather than the Nissan so it was very common for the AGM battery to sit in it's sealed box, up in the anchor locker, for five or six months at a time.

The PWC AGM battery did not seem to suffer due to the lack of charging.
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Old 07-03-2014, 15:22   #12
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Re: How Long will Battery Charge Last?

Quote:
Originally Posted by Stu Jackson View Post
"...some extra information????".....

His discussion is about as long and as real-world as you're gonna find.....
You misunderstood my post. The "extra information" I was refering to was that info quoted in the article by Justin from Lifeline, who I also quoted in my post.

All I was asking for was facts from any source who can clearly say that AGMs are worse than FLAs when they are not fully recharged. Maine Sail's observations may suggest that, but one of the reasons they don't seem to last as long as wets could be because they like to be charged >20% of C, and I'm sure most don't get that high rate of charge.

Maine Sails article is excellent - but now very old!!!! One advantage he hasn't properly covered is the faster charging times.

Yes, as he says, "charge acceptance rate" can make a HUGE difference in charge times, but with a standard alternator or shorepower charger AGMs could still charge faster because of their "charge efficiency". Charge efficiency, or Coulombic efficiency, or Charge Acceptance is complex because it is a non-linear function of battery state-of-charge. At a low SoC the value may be 95%, at a very high SoC it may be much less than 50%, so an overall efficiency value must be used. Wet cells may be as low as 70% (many online references to this figure - including Nigel Calder) which means about 140Ah must be put in to actually raise the capacity of a battery by 100Ah. (70% of 140Ah is 100Ah).

Lifeline AGMs may be up to 98% efficient so only 102 Ah needs to be put in. That’s nearly 40% less Ah needed so nearly 40% faster charging as a worst case scenario, and it doesn’t take a larger alternator to do it. So even solar panels will charge AGMs faster.

I have been a fulltime liveaboard for 7 years and my Lifeline AGMs are now 10 years old and they haven't always been well cared for.
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Old 08-03-2014, 21:24   #13
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Re: How Long will Battery Charge Last?

Tough crowd...

Want something "newer"? Write it yourself.

Good for you for having AGMs last a long time.

Motorcycles are not sailboats.

Enjoy.
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Old 08-03-2014, 23:23   #14
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Re: How Long will Battery Charge Last?

"Motorcycles are not sailboats."

I guess you are telling me that the chemistry and physics in a PWC (WC stands for Water Craft?) AGM battery is different than in a "real" AGM found on a sailboat?

Theory and Real Life?
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Old 09-03-2014, 01:31   #15
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Re: How Long will Battery Charge Last?

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Tough crowd...Want something "newer"? Write it yourself...
Very strange and protective post of your mate Maine Sail - who I deeply respect for his many postings on here.

I did "write something newer" to fill in what I believe are small gaps in his very good article, and I would be very interested in anyone's helpful comments or additions to my brief posting on AGM battery efficiency.
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