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Old 06-25-2016, 04:47 PM   #1
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Default Another view of the 50% AGM Discharge Limit

Not much battery discussion lately so how about something to create some discussion about the 50% AGM discharge limit. The info here is not anything new or original it but may have not gotten much exposure...

The 50% AGM battery discharge limit has been etched in stone as the point past which you will encounter dire consequences if you don't recharge your batteries. However, when you look at the specifications for high quality AGM batteries, the 50% limit is found to not really apply to these batteries. So, the caveat here is that lower quality AGM batteries may suffer more from discharge past 50% than the more expensive higher quality AGM batteries from Lifeline, Trojan, Fullriver, and other manufacturers.

The easy way to think of the this is to consider the battery bank to have a lifetime power limit, in other words, there is a limit to the total number of amp hours that can be used over the life of the battery before it needs to be replaced (the end of life is when the capacity is reduced to 80% of the original capacity). The power usage takes place in discharge/charge cycles with the number of these cycles available over the lifetime dependent on the level of discharge in each cycle. The table at the bottom shows the data for a Lifeline AGM 220 amp hour battery bank which is typical of the configuration in many vans (two 6 volt golf cart sized batteries wired in series). While the amp hour numbers would be different for other battery configurations the number of cycles and the percentage changes from 50% would apply to any bank of Lifeline AGM batteries.

As you can see in the table, for the range of discharges from 10% - 80% there is only a +/- 12% max difference in lifetime available amp hours vs the 50% number. I would suggest that this is not a drastic penalty for discharging past 50% up to the 80% discharge point. Going past 80% does show a more significant penalty in amp hours lost and runs the risk of getting so far into the discharge that special procedures are needed to recharge the battery.

For Class B vans with limited battery space these results provide a significant opportunity to get more battery capacity without adding more batteries. Using the 50% limit required adding more batteries to get more capacity with little space to do this and typically doubling the cost of the batteries.

I would not expect the 50% limit to die out, it is so ingrained that it will continue to be preached as the gospel. Looking at the data does suggest that it is time to put it in perspective however...
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Old 06-25-2016, 04:57 PM   #2
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Lots of information on that topic in this discussion that starts with this post in a long thread.

http://www.classbforum.com/forums/f8...html#post43528
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Old 06-25-2016, 04:59 PM   #3
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Originally Posted by booster View Post
Lots of information on that topic in this discussion that starts with this post in a long thread.

http://www.classbforum.com/forums/f8...html#post43528
Good info in that thread...

I thought it might be interesting to have a specific discussion of the 50% limit...
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Old 06-25-2016, 07:28 PM   #4
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Totally agree with your comments. Your comment about the limited space in a class B hits home in my case. I have a 105 Ah battery and if I want to use 80 Ah before recharging, and I follow the 50% "rule", then I absolutely need a second battery. In my Roadtrek, this would be a major undertaking to do it right. Now if I can in fact get 80 Ah out of the existing battery without a major hit in longevity, then I'm good to go as is.
I'll have to replace the battery more often, but at half the cost ... one instead of two.
Now this brings a question to mind. Does this really only apply to AGM's? Would a deep cycle flooded lead-acid battery react about the same?
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Old 06-25-2016, 07:29 PM   #5
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I don't think I participated in previous discussion mostly because I'm a fan of the 50% guideline. However, it is very encouraging to see that the penalty is less severe than previously thought if measured in terms of the amp hours delivered.

You get your money's worth is what the data is indicating

Lead acid batteries still need to be fully recharged at least periodically (I prefer ASAP) but that is not too difficult to do.

Thanks to everyone who contributed to this and other discussions. It has been a bit of an eye opener.
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Old 06-25-2016, 07:46 PM   #6
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Now this brings a question to mind. Does this really only apply to AGM's? Would a deep cycle flooded lead-acid battery react about the same?
Good question, if you can find specs for Trojans or some other brand with the right info it is easy to determine that...

This data is from Page 38 in this Lifeline Technical Manual...

http://lifelineb.wpengine.com/wp-con.../12/manual.pdf
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Old 06-25-2016, 07:48 PM   #7
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So, what would be interesting would be to see this analysis done in terms of cents/amp-hour consumed over the lifetime of the batteries, rather than # of cycles, etc. For extra credit, take into account purchase price--comparing "premium" vs "budget" batteries. I have always wondered about the latter.
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Old 06-25-2016, 08:42 PM   #8
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Originally Posted by avanti View Post
So, what would be interesting would be to see this analysis done in terms of cents/amp-hour consumed over the lifetime of the batteries, rather than # of cycles, etc. For extra credit, take into account purchase price--comparing "premium" vs "budget" batteries. I have always wondered about the latter.
Easy to get from that data, use $600+ dollars for two Lifelines, create another column with values equal to 60000 cents / lifetime AH column. Details left to the reader...

The last column with the percentages should be close for cost/amp hour...
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Old 06-25-2016, 09:04 PM   #9
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Here you go, cents per AH for Lifeline Battery Bank using $600.00 as the cost for two 6v AGM golf cart sized batteries...

And no one has asked the obvious question about the data being unusual at the lower discharge values. I rechecked the data in the Lifeline spec to make sure I have not forgotten how to read logarithmic graphs and this is what is in the spec so who knows why the data jumps around at lower discharge rates...
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Old 06-25-2016, 10:15 PM   #10
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Quote:
Originally Posted by WJones View Post
Totally agree with your comments. Your comment about the limited space in a class B hits home in my case. I have a 105 Ah battery and if I want to use 80 Ah before recharging, and I follow the 50% "rule", then I absolutely need a second battery. In my Roadtrek, this would be a major undertaking to do it right. Now if I can in fact get 80 Ah out of the existing battery without a major hit in longevity, then I'm good to go as is.
I'll have to replace the battery more often, but at half the cost ... one instead of two.
Now this brings a question to mind. Does this really only apply to AGM's? Would a deep cycle flooded lead-acid battery react about the same?
William
2000C200P
What started the discussion on the other thread was lithium, which showed they did do much worse at deeper discharges, then the first actual graph found of total amp hours over the life of the battery was for a lead acid wet cell, and it had a very similar profile as the AGM, but the AGM graph had to be plotted from the Lifeline battery specs.

Bottom line is the wet and AGM seem to not really back up the 50% rule, but surprisingly lithium does, even thought they still have a lot cycles. Their advantage in life over lead acid shrinks a whole bunch once you get to 80-90% discharges and using total ah over the life of the battery.
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