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Old 08-01-2019, 17:31   #1
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Can AGM batteries be drawn down further than FLA without serious impact on longevity?

I have read that as a rule of thumb, the more a battery's AH capacity is drawn down regularly, the shorter its life span. Don Casey provides a nice line graph showing this relationship and I have historically chosen to never draw down the house bank to less than 60% of total AH capacity. Its not clear however what battery chemistry Don is referring to.

I have recently read that it is possible to draw AGM batteries down to 50% without serious impact on their longevity. I wonder what others think and do?
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Old 08-01-2019, 18:37   #2
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

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Originally Posted by Marathon1150 View Post
I have read that as a rule of thumb, the more a battery's AH capacity is drawn down regularly, the shorter its life span. Don Casey provides a nice line graph showing this relationship and I have historically chosen to never draw down the house bank to less than 60% of total AH capacity. Its not clear however what battery chemistry Don is referring to.

I have recently read that it is possible to draw AGM batteries down to 50% without serious impact on their longevity. I wonder what others think and do?
I believe the key for AGM longevity, or any LA battery, is to fully recharge them to 100% after discharge to any level. I got 10 years out of mine and they were still in pretty good shape following that regime.
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Old 08-01-2019, 19:37   #3
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

Search for DOD charge cycle charts "Cycle life vs DOD". Look for the brand battery or a name brand of the type you are interested in. Typically stated as not to believe you'll see anywhere close but you can use as a comparison within the same brand.
Generally speaking, higher DOD = shorter life (all other things being equal).

Lifeline says "What depth of discharge should be used when sizing a battery?
To get the best cycle life, the average depth of discharge should be as low as possible. Concorde recommends the average depth of discharge to be no greater than 50% of the battery’s 20 hour rating."

Page 32 of 40 as an example:
http://lifelinebatteries.com/wp-cont...cal-Manual.pdf
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Old 08-01-2019, 20:11   #4
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

Just about every lead acid battery will have a shorter lifespan, the greater the discharge. That's not to say that it may not be worth it. Any lead acid battery benefits from being fully recharged, and that is where the issues often are, as recharging the last 10-20% can take a very long time.



Most people try not to discharge past 50%, and recharge to about 85% SOC, but how the battery handles that partial state of charge routine varies greatly.


There is a lead acid AGM, named the Firefly, and which uses Carbon Foam construction, that can take a very deep discharge, but while often worth it, this still shortens the number of cycles it will endure. Importantly, however, it handles the partial state of charge side of the equation very well. But, it's about the only one that does.
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Old 09-01-2019, 09:20   #5
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

I run three Odyssey AGMs (two in parallel as a house bank) and the other for starting. At the end of last season, I managed to draw down the house bank to about four volts when I left the battery switch on. My retailer said they were toast (naturally!) but I managed to resurrect them to about 12.5V with my charger and they seem to be holding. I'lll monitor and charge again monthly through the winter. So, it seems they're been given new life but time will tell.
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Old 09-01-2019, 14:16   #6
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

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I run three Odyssey AGMs (two in parallel as a house bank) and the other for starting. At the end of last season, I managed to draw down the house bank to about four volts when I left the battery switch on. My retailer said they were toast (naturally!) but I managed to resurrect them to about 12.5V with my charger and they seem to be holding. I'lll monitor and charge again monthly through the winter. So, it seems they're been given new life but time will tell.

How old were they before that event?

I put in one bank of 3x 12V Odyssey PC-2150s in early 2006, replaced in mid-2017. Another bank of 3X went in mid-2009, still seem fine.

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Old 09-01-2019, 14:24   #7
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

These are 3-year-old Odyssey PC6880s. I don't require huge cranking capacity because I have an Atomic 4 and the boat (C&C 27) is all LEDs now. Charging in summer is by two bimini-mounted 100-watt semi-flexible panels.
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Old 09-01-2019, 16:55   #8
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

Many thanks for all the helpful advice and comments.

We installed 2 Lifeline 4D AGM's in about 2008 and they were still working pretty well in January 2017. I was fastidious adhering to the 60% rule during that time.

We replaced those two batteries with two more of the same batteries, and I have been sticking with 60%. However, we are planning to cross from Mexico to the Marquesas in March of this year and even with 400W of solar panels and a wind generator, I expect that there will be days when sun and wind are not adequate to fully top up the batteries. Apparently this is not unusual, particularly as one of the above posts points out, its the last 10-20% that is really hard.

We will keep our fingers crossed that we stay in the 60-80% range but it is somewhat comforting to know that drawing them down to 50% from time to time will not lead to a seriously accelerated decline in battery health.

Many thanks again.
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Old 09-01-2019, 17:13   #9
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Can AGM batteries be drawn down further than FLA without serious impact on longevity?

The most important thing you can do with a Lifeline battery to make it live a long time is to ensure it’s fully, 100% recharged as often as you can.
That pretty much requires either running your engine early in the morning for some time before Solar starts outputting, or a generator, although of course if the wind is working well, maybe you won’t need to run anything.

I assume you were often plugged into shore power with your previous set?
If not, then don’t change whatever you do, cause it’s obviously working

Lifeline has probably one of the best manuals manuals written for a battery, it will answer your questions
http://lifelinebatteries.com/wp-cont...cal-Manual.pdf
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Old 09-01-2019, 17:13   #10
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

You can compare for yourself.

Trojan Deep Cycle AGM
https://www.trojanbattery.com/pdf/AG...tLineSheet.pdf

Trojan Deep Cycle Signature Line Flooded Batteries.
https://www.trojanbattery.com/pdf/Si...tLineSheet.pdf
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Old 09-01-2019, 17:34   #11
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

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Originally Posted by Marathon1150 View Post
Many thanks for all the helpful advice and comments.

We installed 2 Lifeline 4D AGM's in about 2008 and they were still working pretty well in January 2017. I was fastidious adhering to the 60% rule during that time.

We replaced those two batteries with two more of the same batteries, and I have been sticking with 60%. However, we are planning to cross from Mexico to the Marquesas in March of this year and even with 400W of solar panels and a wind generator, I expect that there will be days when sun and wind are not adequate to fully top up the batteries. Apparently this is not unusual, particularly as one of the above posts points out, its the last 10-20% that is really hard.

We will keep our fingers crossed that we stay in the 60-80% range but it is somewhat comforting to know that drawing them down to 50% from time to time will not lead to a seriously accelerated decline in battery health.

Many thanks again.
you should be aware that out here there is no shore power - your solar/wind and engine will have to ensure your batteries are charged. I don't know what type of equipment you have on board, but 400 watts is on the light side living 24/7 on the boat.

Lots of boats out here (currently we are on Nuku Hiva) have wind generators - everyone says they are worthless.
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Old 09-01-2019, 17:49   #12
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

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you should be aware that out here there is no shore power - your solar/wind and engine will have to ensure your batteries are charged. I don't know what type of equipment you have on board, but 400 watts is on the light side living 24/7 on the boat.

Lots of boats out here (currently we are on Nuku Hiva) have wind generators - everyone says they are worthless.
Thanks. Yes we are quite aware that once we leave Mexico we will not be plugging the boat in again, for a long time. There are no marinas

The three most energy consumptive devices on board are the watermaker (Spectra Ventura - 7 amps), the Frigoboat compressor for the fridge (16 amps to start and then 6.5 once running) and an Engel freezer (4amps).

Once the freezer is empty of frozen food it will become the fridge and the fridge will become unrefrigerated storage.

We have been using this set-up in Mexico (Sea of Cortez) for the past 5 years. From April until October, the batteries are back at 100% before noon even when running the watermaker every couple of days. November through March is more challenging, unless there is a reasonable amount of night time wind. Or more sunny than cloudy days. When we are really stuck we run the engine, but I am never keen to do that and would prefer to do without refrigeration of any kind rather than run the engine. Many millions of people live without refrigeration, and as a kayaker, camper, climber, I don't see that as a major problem. The watermaker is more important and will receive priority.
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Old 09-01-2019, 19:17   #13
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

Lead acid is the chemistry so factors such as the relationship between DOD and cycle life are very similar.
AGM is simply the construction. All sealed cells have the advantage of being spill proof. AGM can also give a smaller footprint and is slightly lighter as they can reduce plate spacing and have less water, particularly spiral wound cells. The disadvantages are short life, high cost and longer charge times to get to 100% (but yes higher charge acceptance rates from 50-75% DOD)

Assuming correct cycling expect around 1\2 the life cycles from a sealed cell compared to FLA's because they have less water and it can't be replaced. They are also more fragile, one serious overcharge and they are toast. Do the same to a FLA and you loose some capacity but the cell will often continue if you replace the water immediately.

I always plan batter capacity based on 60-70% DOD for a normal days use. this should give enough in reserve to give 3-4 days 'emergency reserve' on minimal power if the charging system fails. It is also about the most efficient point for cost over cell lifetime. Good quality FLA's work out at around 25% of the cost of AGM's over the batteries life.
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Old 11-01-2019, 15:09   #14
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

I don't think any spiral AGMs are deep cycling anymore. Optima was good while still part of Enersys, but way downhill since bought by JCI
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Old 13-01-2019, 18:17   #15
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Re: Can AGM batteries be drawn down further than FLA without serious impact on longev

I don't know that this comparison helps at all but I thought it was interesting.
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