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Old 03-04-2015, 06:22   #1
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AGM Feedback

AGM batteries often get bad press on CF so I thought I might share my AGM good news story. At least, it is good news to me IMO

I have a single 150 AH, 12V AGM probably 5 or 6 years old. It doesn't get a lot of love, the only charging source is the stock alternator on the 2GM20 with it's internal regulator (output is about 14.5V). It often sits without recharging for months at a time and sometimes 6+ months. Until this week, it has never had any maintenance whatsoever. The only saving grace is the electrical load is quite light. LED nav lights, LED cabin lights, autopilot, sounder, log, GPS, VHF and occasionally SSB & radar and of course starting the 2GM20.

Anyhow five weeks back, I foolishly left the battery switch on as well as the GPS. Needless to say, the battery was dead flat. I removed it and recharged at a constant voltage of 15V and maximum current of 25 amps (the limit of the charger) until there was no decrease in current for 5 hours. Let it sit for 24 hours and then carried out a constant current discharge test (15 amps) to a end point voltage of 10.5 V. Managed to get 7.7 hours which in my mind is 77% of original capacity. I was expecting it to be far less given the history and it being totally discharged for at least 3+ weeks.

I have recharged it with a constant current of 15 amps for 12 hours and it is now back in service.

Clearly it's life has decreased but who knows how many more cycles is left in it, but I'm happy with it so far

5 weeks back is the first time I needed to use the standby battery in anger , which by the way is two SBS30 gel cells in parallel.
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Old 03-04-2015, 07:48   #2
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Re: AGM Feedback

Something seems odd....

A 20 hour discharge test would be 7.5A on a 150Ah battery not 15A as 15A is double the 20 hour rate??

At a 15A load a brand new 150Ah AGM battery would only be 138Ah assuming a Peukert of 1.12 and should run for 9.2 hours at 15A at 77F..

This means the capacity you measured would have been approx 84% which I have not seen nor measured in a scenario where a battery sat discharged for 3 weeks. Usually when the battery sits for more than a few days at I am seeing actual tested Ah capacities (lab grade capacity test equipment and a constant temp water bath held at 77F) in the less than 50% range or otherwise ruined..

Heck in just 30 days of PSOC cycling, for an upcoming article (due out in May), we saw capacity loss approaching that, in just 30 days of PSOC operation...

While anything is possible I will say I have not seen a recovery like that with any lead acid battery, not even GEL. Buy a lottery ticket today!!!!
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Old 03-04-2015, 15:47   #3
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Re: AGM Feedback

FWIW, the rated capacity of this battery is at a 10 hour rate rather than 20 hour rate. I think this is another English/Australian vs USA variation .

Manufacturer's specification for 20 hour rate is 154 AH and 127.5 AH for 5 hour rate ; all with a EPV of 10.5 V.
Battery is an Absrbed Power GT12-150C; made in China.
So I have to live with 77% and not 84%

Not sure how much difference it makes but battery temperature ranged from 18C to 24C which pretty closely matched the ambient temperature range off the day.

I would be happy to buy another one but maybe I got the only good one
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Old 03-04-2015, 16:00   #4
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Re: AGM Feedback

Quote:
Originally Posted by Wotname View Post
FWIW, the rated capacity of this battery is at a 10 hour rate rather than 20 hour rate. I think this is another English/Australian vs USA variation .

Manufacturer's specification for 20 hour rate is 154 AH and 127.5 AH for 5 hour rate ; all with a EPV of 10.5 V.
Battery is an Absrbed Power GT12-150C; made in China.
So I have to live with 77% and not 84%

Not sure how much difference it makes but battery temperature ranged from 18C to 24C which pretty closely matched the ambient temperature range off the day.

I would be happy to buy another one but maybe I got the only good one
Yes at the 10 hour rate that is correct. Still an amazing recovery!!
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Old 05-04-2015, 17:41   #5
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Re: AGM Feedback

I am looking for a capacity tester for my AGMs (mix of 4D, 8D and smaller ones for engine start and windlass). Anything you would recommend?

Thanks

Dirk
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Old 05-04-2015, 19:10   #6
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Re: AGM Feedback

No sorry, cant help with suggested battery tester. I had access to a Concorde BC 8000 but at $4+K, it is a bit of overkill. I expect some of our USA members will be able to suggest something.
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Old 05-04-2015, 19:22   #7
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Re: AGM Feedback

Unfortunately, batteries are one of these pieces of gear that do not take abuse lightly. IMHO running batts flat, whatever technology, leads to damage. I think maybe lithiums are less vulnerable as I do run our laptops and tablets flat now and then and they still charge and deliver as if nothing happened.

But, for acid, a dead one is a dead one. Full stop.

b.
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Old 05-04-2015, 19:48   #8
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Re: AGM Feedback

I have agm batteries 4 x 120amp
Fullriver Chinese now 7yrs old
They still working well, I have no plans
to change them in the foreseeable future.
We are planning on heading off on an extended
cruise multi year.
I believe these will last at least another 2yrs
possibly 4yrs.
So I am very happy with these batteries & would
Happily buy another set.
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Old 06-04-2015, 06:50   #9
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Re: AGM Feedback

Quote:
Originally Posted by barnakiel View Post
Unfortunately, batteries are one of these pieces of gear that do not take abuse lightly. IMHO running batts flat, whatever technology, leads to damage. I think maybe lithiums are less vulnerable as I do run our laptops and tablets flat now and then and they still charge and deliver as if nothing happened.

But, for acid, a dead one is a dead one. Full stop.

b.
I don't think it's possible to run the battery in a laptop or tablet "flat". The device is usually smart enough to shut down before the battery can go completely dead.
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Old 06-04-2015, 09:39   #10
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Re: AGM Feedback

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I don't think it's possible to run the battery in a laptop or tablet "flat". The device is usually smart enough to shut down before the battery can go completely dead.
Now that you said it out loud I know you must be right.

Do you know if this is the same case with lithium based boat and infrastructure batteries?

I know the boat lithium batteries use 'controllers' but I can see infrastucture lithium batteries sold 'bare'.

For our boat, lithiums are the way forward, I am just waiting for their prices to become somewhat easier on our budget.

b.
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Old 06-04-2015, 10:15   #11
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Re: AGM Feedback

It depends on the battery. A plain Li battery with no cutoff relay or other electronics can be "drained flat". This is very bad for their overall life. To prevent this the charging system has to disconnect the battery before it gets overcharged or fully discharged. A battery monitor will either measure the total pack voltage or the individual cell voltages. When the voltage gets to a point deemed to be fully charged or discharged (but not dead flat) the battery monitor activates a relay that turns off charging when full and disconnects all loads from the battery when empty. Thus the battery is saved from destruction at both ends.

There are very expensive drop-in Li battery replacements that do all the monitoring and cutoff functions themselves. But they are frightfully expensive. Thus most boaters are using off the shelf components to roll their own DIY systems. This route is about 25% of the cost of available fully integrated solutions.

But if you just buy Li cells and strap them together without a monitoring system they will be quickly destroyed either from over charging or discharging. If you want to DIY there is a long thread on this subject here on CF. You pay for the savings by having to read thousands of posts and figure out who is right and who is wrong. Hint: Maine Sail is one of the voices of sanity on that thread.
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Old 06-04-2015, 12:40   #12
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Re: AGM Feedback

Quote:
Originally Posted by transmitterdan View Post

It depends on the battery. + (...)
Thanks for this explanation. Yes. I noticed the vast price difference btw the special purpose "marine" packages and the commercial grade available stock.

On our own boat, I keep all things simple and I think I know at any given time of our cruises how much we have used, how much we have left, and what we owe. So I will likely build our own kit out of the commercial stock.

On other boats, well, owners' technical ability and will vary vastly and beyond doubt it is safer to use a heavily monitored/controlled solution to avoid damage, trauma and insurance claims.

I know that Tesla has just started their new lithium bat facility that is said to double our global supply. I think that's good news. We owe so much to the car industry, much as we are often reluctant to admit things.

Thank again!

Cheers,
b.
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