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Old 06-25-2022, 12:03 PM   #1
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My AGM epiphanies

Apologies in advance if this is common knowledge but I just figured it out.

I've got a house bank of 1000 Ah of Rolls AGM batteries which means 500 Ah usable. I don't let the batteries go below 50% but it is a pain when the batteries approach this figure at the end of a day. What to do? Run the generator a couple hours (most expensive electricity ever) or let the batteries go below 50% (and shorten their lives) and just charge them from the main on the next days run?

I stumbled on a post suggesting that pulling the batteries below 50% wasn't that big a deal - https://tab-rv.vanillacommunity.com/...ge-myth-busted I then dug into the Rolls data sheets.

My Rolls AGMs list an expected life of ~1200 cycles at 50% depth of discharge. That translates to 1000 * .5 * 1200 = 600K Ah. At a routine 80% DOD the expected cycles drop from ~1200 to ~675 cycles - almost a 50% decrease. However, the usable Ah in this case are 1000 * .8 * 675 = 540K Ah. That is just 10% less than the 50% DOD number. Looking at total Ah available rather than cycles suggests that going below 50% DOD occasionally - or maybe regularly - isn't that big a deal.

Another conclusion I've reached from this is I spend too much time worrying about the batteries. I'm just going to use them as I want (within limits) and replace them when needed.
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Old 06-25-2022, 01:09 PM   #2
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I think you are right. I also saw similar data for FLA batteries: 1,000 cycles at 50% and 500 cycles at 80% DOD. I use my batteries a couple of dozen times each year. At 80% DOD I will have a twenty year life from them. Now I am not expecting twenty years, other things will get them, but it does put the old 50% rule of thumb to bed.

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Old 06-25-2022, 01:52 PM   #3
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Apologies in advance if this is common knowledge but I just figured it out.



I've got a house bank of 1000 Ah of Rolls AGM batteries which means 500 Ah usable. I don't let the batteries go below 50% but it is a pain when the batteries approach this figure at the end of a day. What to do? Run the generator a couple hours (most expensive electricity ever) or let the batteries go below 50% (and shorten their lives) and just charge them from the main on the next days run?



I stumbled on a post suggesting that pulling the batteries below 50% wasn't that big a deal - https://tab-rv.vanillacommunity.com/...ge-myth-busted I then dug into the Rolls data sheets.



My Rolls AGMs list an expected life of ~1200 cycles at 50% depth of discharge. That translates to 1000 * .5 * 1200 = 600K Ah. At a routine 80% DOD the expected cycles drop from ~1200 to ~675 cycles - almost a 50% decrease. However, the usable Ah in this case are 1000 * .8 * 675 = 540K Ah. That is just 10% less than the 50% DOD number. Looking at total Ah available rather than cycles suggests that going below 50% DOD occasionally - or maybe regularly - isn't that big a deal.



Another conclusion I've reached from this is I spend too much time worrying about the batteries. I'm just going to use them as I want (within limits) and replace them when needed.


Exactly! The 50% rule that everyone lives by is greatly misunderstood. There is NOTHING magic about 50%. Battery life is based on the total number of Ah run through the batteries. NOT the number of cycles, and NOT the number of years that you have owned them. You can consume them with lots of shallow cycles, or fewer deep cycles. Itís a linear relationship, not exponential.
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Old 06-25-2022, 01:54 PM   #4
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For extra credit, you can figure out whether it is worthwhile to fully recharge, since charging (at least for FLA batteries) slows radically as the batteries approach full charge.
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Old 06-25-2022, 02:17 PM   #5
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Im with twistedtree re 50% "limit" being a myth.
Even Trojan recommends 80% DOD as a practical limit that still provides a cushion to prevent excessive DOD causing batty damage.
Yes you get fewer (larger AH) cycles vs more (smaller AH) cycles but total AH delivered very close to same. If you do the math at various DOD and # cycles for life you get very close to the same total AHs over batty life - like within 10%.
50% DOD is not a bad guide for desiging a system but it is NOT a hard limit beyond which causes damage. So the point of whether need to recharge immediately when reaching 50% Id say no... OK to go beyond if you only need an additional 10%-20% DOD overnight before running & recharging the next day.
Recharging back to 100% frequently (like weekly) is important!
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Old 06-25-2022, 03:24 PM   #6
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It's the voltage sag when running big stuff that was a killer for us
As we got near 50%, if two or more 240v fridge freezers kicked on at the same time, voltage would drop well below 24v

I guess no worries if you have a lightly loaded system
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Old 06-25-2022, 03:33 PM   #7
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For extra credit, you can figure out whether it is worthwhile to fully recharge, since charging (at least for FLA batteries) slows radically as the batteries approach full charge.
Quite true. Rolls does say:

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To maintain good health, VRLA AGM batteries should be brought to a full charge on each cycle or, at minimum, once every 6-7 days.
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Old 06-25-2022, 05:31 PM   #8
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So help me out.

A full float charge seems easy enough plugged in at a marina. But harder with a generator away from the dock. True or false?
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Old 06-25-2022, 05:56 PM   #9
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So help me out.

A full float charge seems easy enough plugged in at a marina. But harder with a generator away from the dock. True or false?
True unless running (alts), long gen runs ( for A/C?), solar or visiting a dock once / week.
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Old 06-25-2022, 09:15 PM   #10
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Battery life is based on the total number of Ah run through the batteries. NOT the number of cycles, and NOT the number of years that you have owned them. You can consume them with lots of shallow cycles, or fewer deep cycles. Itís a linear relationship, not exponential.
I'm gonna argue part of that. Lead acid batteries of any kind have a clock ticking on them from the day they were made. Used under ideal circumstances, they still die, though good AGMs will live 10 years or more. They die an early death if: too many AH are run through, or they are murdered by their owners. The latter is by far more popular. And, the relationship is exponential not linear, though the exponent is only a little over one.
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So help me out.

A full float charge seems easy enough plugged in at a marina. But harder with a generator away from the dock. True or false?
Yes. You have to really love the drone of the genset to charge them fully that way. I have collected a lot of charge cycle data lately, and you need to figure a 6 hours run if discharged to say 50%, and a 3-4 hour run even if only 80%. Good argument for solar, which may not be that much current but is relentless.
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Old 06-25-2022, 10:48 PM   #11
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Good argument for solar, which may not be that much current but is relentless.
Excellent point, and others are their silence, need no engine running, and top up batts in a nice gentle way if a proper controller is in the circuit. Only issue is the space they need, and where they need it.

Which is why if I was buying another boat - and I'm not - but if I was - it'd be a pilot house design, with no flybridge. Because I want all that space for panels - and who really needs a flybridge if you have a pilot house anyway..?
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Old 06-26-2022, 08:00 AM   #12
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So help me out.

A full float charge seems easy enough plugged in at a marina. But harder with a generator away from the dock. True or false?

That's where solar comes in. If you don't have enough solar to get you fully charged, you run the generator for a bit in the morning to throw some power in quickly, then the solar has all day to get things topped off.

Mind you, you'll only ever get to like 99.9% charged with solar. My Fullriver AGMs indicate that a truly full charge is to charge until acceptance gets down to 1.2 - 2% of 20hr rate at 14.7v, then drop to 13.65 for float and after 8 hours of float, they're fully charged. It really takes a bit more than 8 hours for the acceptance rate to taper down to almost nothing in float, and typically we're only in float for 4 - 6 hours on solar. I do have the solar hold absorb to 1% of 20hr rate to help make up for it though.
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Old 06-26-2022, 08:41 AM   #13
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How do you define a "cycle" on a battery??
Some say 50%, others say 80%.
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Old 06-26-2022, 08:49 AM   #14
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rslifkin:

Well that's where I'm trying to understand the intersection of theory and reality.

So during the day, assuming sun, there is still a lot of living going on in the boat that uses power. Seems to me the solar output would have to exceed the use in order to obtain any float charge. In the southeast where AC is a need that's unlikely without a massive solar array.

Put differently, solar is not new but is relatively new. So absent solar, some battery degradation is just a fact of life to be accepted? Theory vs reality in battery life. It seems like one needs to add a LOT or don't bother if it won't provide day use power plus enough to float charge.
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Old 06-26-2022, 09:05 AM   #15
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The part about "murdering" batteries is a good one. Most die prematurely because they are not regularly brought back to full charge, sulfation becomes irreversible, and accumulates over time.


Once you understand that the underlying battery life (when properly managed) is based on cumulative Ah of charge/discharge, it suggests an interesting and counterintuitive approach to battery bank sizing. Cut the size of your bank is half, double your depth of discharge, and replace it twice as often. It seems like abuse, but the cost over time is the same (half the battery bank cost, but replaced twice as often). And because you own your batteries for half as long, you have half the opportunity to murder them. Since most battery murders are slow poisonings rather than quick kills, by halving your ownership/operating time you greatly decrease your chances of killing off the batteries before they have delivered their lifetime number of Ah.


So buy fewer batteries, run them hard, and replace them more frequently.
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Old 06-26-2022, 09:40 AM   #16
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How do you define a "cycle" on a battery??
Some say 50%, others say 80%.

It's a discharge from 100% SOC to some lower SOC, then recharge to full. Manufacturers then specify how many cycles you can get at various depths of discharge.


With ALL types of batteries that I'm aware of, including all types of lead batteries, and all types of Lithium Ion batteries, their expected service life is based on the total Ah run through the battery. That's not the Ah capacity of the battery, but the total number of Ah that you run through the battery before it drops to 80% of its rated capacity. That's why you get more cycles for smaller DODs, and fewer cycles for larger DOD.



It's really just like your gas in your car. How many times can you drive your car before you need to refill? Well, it depends on how far you drive each time. You can get lots of short trips, or fewer long trips. What's really behind it all is the finite number of gallons of gas that your tank holds, and how long it takes to burn through it.
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Old 06-26-2022, 09:45 AM   #17
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So buy fewer batteries, run them hard, and replace them more frequently.
Interesting perspective. My 3 AGMs have been murdered along their relatively short life and while I am comfortable replacing them, I am less excited about replacing my 3000 watt Xantrex inverter/charger. Technology has passed it by, but that really increases the budget.
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Old 06-26-2022, 10:29 AM   #18
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Mind you, you'll only ever get to like 99.9% charged with solar. My Fullriver AGMs indicate that a truly full charge is to charge until acceptance gets down to 1.2 - 2% of 20hr rate at 14.7v, then drop to 13.65 for float and after 8 hours of float, they're fully charged.
There is some debate about what constitutes fully charged. Lifeline defines it as dropping to 0.5% C at absorb. Solar does that and has for a decade on my sailboat.

You need enough solar capacity to both charge the batteries, and service ongoing house loads (in my case primarily the refrigerator, but there are a surprising amount of small vampire loads which add up as well).
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Old 06-26-2022, 10:37 AM   #19
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Interesting perspective. My 3 AGMs have been murdered along their relatively short life and while I am comfortable replacing them, I am less excited about replacing my 3000 watt Xantrex inverter/charger. Technology has passed it by, but that really increases the budget.
Sooo, for example we have one 4D battery rated 200 amps and 1000 cycles.
That battery contains a total of 200,000 amps, ignoring simultaneous charging, before it goes belly up forever. Now we come to, drawing down to what percentage, before charging back up, without damaging the battery.
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Old 06-26-2022, 11:08 AM   #20
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How do you define a "cycle" on a battery??
Some say 50%, others say 80%.
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It's a discharge from 100% SOC to some lower SOC, then recharge to full. Manufacturers then specify how many cycles you can get at various depths of discharge.

Yep. Lifeline publishes an Expected Life Cycle curve -- cycles versus DoD -- in the Technical Manual for their batteries, available on their site Page 40 in Rev G, 11/26/2019.

On the graph it looks like a little over 1000 cycles at 50% predicted. Eyeballs somewhere near 525-550 at 80% DoD.

Per Rod Collins: that's "lab conditions" though... not necessarily representative of reality.

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