How to determine a battery's REAL capacity

Jim 5757 · May 20, 2017

Before I begin, I have to apologize... I just don't understand this discussion. Do you want to know how many years or how many cycles?

dlochner · May 20, 2017

Don S/V ILLusion said:
The generalization about never going below 50% capacity is likely an over-simplification given the lack of real information on how to get the most out of a battery bank.

We do know somethings about battery life. Batteries last longer when cool, they have a charging profile they like, the deeper the discharge and the more frequent the discharge the shorter the battery life.

I'm of the camp that you start with decent quality batteries and try not to abuse them too badly. Use a good temperature controlled charger and get them fully charged as often as possible. A Rolls battery might last a year or two longer than a East Penn, but at what cost? Both in terms of money and effort.

Maine Sail · May 20, 2017

Don S/V ILLusion said:
Anybody know of a reference written by a battery manufacturer or other informed source which verifies the hypothesis that exceeding 50% capacity shortens battery life?

Battery Manufacturers:

Trojan:
1. Shallow discharges will result in a longer battery life.
2. 50% (or less) discharges are recommended.
3. 80% discharge is the maximum safe discharge.

Lifeline:
Q: What depth of discharge should be used when sizing a battery?

A: 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.

East Penn / Deka:
Q: Does depth of discharge affect cycle life?

A: Yes! The shallower the average discharge, the longer the life. This is why it’s important to size a battery system to deliver at least twice the average power required, to assure shallow discharges.

Don S/V ILLusion said:
And if so, how much useful life is sacrificed by going to 30%, 20%, etc...?

In the lab this is pretty easy to quantify. For example Lifeline rates their batteries at 1000 cycles at 50% DOD and just 550 at 80% DOD. However this is not real world cycling it is conducted in ideal lab conditions. Trojan rates their T105 etc. at 1200 lab cycles to 50% and approx 600 cycles to 80% DOD. With Trojan & Lifeline the difference between 50% DOD and 80% DOD cuts lab cycle life in half or just about half.. Both companies have charts showing this.

Don S/V ILLusion said:
Like others, I've likely exceeded the 'magic' 50% figure probably numerous times on battery banks which have far exceed d my expectation for longevity.

There is no "magic" in 50% but it is the suggested max regular discharge /sizing / cycling recommendation by many reputable battery manufacturers selling deep cycle batteries. Any deep cycle battery can be discharged to 80%, some will do better than others, but you will get less life at 80% DOD than average discharges 50% DOD or 20% DOD etc.. The shallower the cycles the more life.

Discharging to a lower SOC when cruising means more time spent at low SOC in PSOC use, a longer time between full recharges when out of the lab, & damaging PSOC use. Cruising use is PSOC cycling if you're not getting back to 100% SOC after each discharge. As boaters who cruise we PSOC cycle more than full cycle and PSOC is bad for batteries.

In the lab the minute the battery discharges to the desired discharge floor it is recharged to 100% SOC including accounting for the proper Coulombic efficiency hit required to attain 100% SOC. PSOC cycling means 1 or more cycles between full recharges. Deep 80% discharges, stacked on top of PSOC use, is even more damaging. For an occasional off shore run, off shore race etc. 80% is fine provided you recharge to full as soon as you can, after this deep discharge. Doing 80% repeatedly and stacking it on top of PSOC, not good. With deeper discharges, longer absorption durations will be necessary and most chargers don't allow you to control this so even this will cause problems many don't have the ability to over come.

In the lab Lifeline for example suggest their battery will deliver 1000 cycles to 50% DOD and approx 550 cycles to 80% DOD. This is under ideal conditions where the batteries are cycled at controlled temps and recharged to 100% SOC immediately after an 80% or 50% discharge. This is 1 cycle. You have either a deep cycle or shallower cycle. Discharging to the design floor then recharging to 100% SOC is a lab cycle. Even if we discharge to our systems design floor the time it takes us to get down there there, with our low discharge rates, takes longer than in the lab so we are in a state of discharge longer before we even begin to recharge. Even if after every discharge to 50% or 80% we recharged to 100% were still not going to see lab like numbers because our time spent discharging is longer than it is in the lab. The longer the time spent away from 100% SOC negatively impacts cycle life when compared to a "lab cycle". Lifeline defines end of life as 80% of as new rated capacity, so a 100Ah battery is considered "end of life" when it can no longer deliver 80% of its rated capacity.

As sulfation builds up on the plates, during PSOC use, your charging system will start to hit absorption voltage earlier and earlier which also leads to under charging and worsening of the effects of PSOC use and deep cycling..

What impact does real world type use play on a battery that works in a PSOC environment, when cycled to 70% - 75% +/- DOD?

In the Practical Sailor AGM PSOC testing the batteries were discharged to 11.7V, based on sailor input as to when they decide to charge. 11.7V is far too low for regular cycling use but many sailors report this is when they choose to. These sailors also report poor battery life. This group of sailors tend to get poor service life compared to sailors who stop discharging at 12.1V to 12.2V... The PSOC test batteries were discharged at the 20 hour rate. They were then recharged for 1 hour at .46C or 46% of the rated 20 hour capacity. Most boat owners, unless they have AGM, don't have this much charging capability so their batteries would do even worse. The batteries were cycle to capacity then went through 30 PSOC cycles to 11.7V followed by a 1 hour recharge at .46C.

This Lifeline battery, which is rated to do 550 cycles to 80% DOD in the lab, lost 11% of its capacity when discharged to 71.4% DOD in just 30 cycles. A battery delivering only 80% of it's new capacity is considered end of life by industry standards. Even after being equalized, at the end of 30 PSOC cycles to try and recover capacity, it had still lost approx 11.6% of its capacity in just 30 PSOC cycles to approx 71% DOD.... For the Lifeline battery 11.7V actually represented 71.5% DOD, at the beginning of the test, which is still a ways from 80% DOD. As the battery continues to "walk down" in capacity during PSOC cycling the DOD from "as new" gets deeper. PSOC capacity walk-down was noted as early as cycle 2. One tested battery failed during this test, it had lost 1% in capacity for each PSOC cycle to 11.7V. This battery was dead in just 30 PSOC cycles so another battery was tested, it too failed within 30 PSOC cycles to 11/7V.

This test originally began with a 1 hour recharge at .2C (210% of Ah capacity) but the batteries were declining so fast it was scrapped. Seeing as they were AGM, and they can take the current, it was decided to jump from a .2C to a .46C charge rate. Despite this .46C charge rate two batteries were still dead in just 30 PSOC cycles to 11.7V. The only battery that I regularly recommend 80% DOD cycling on is the Carbon Foam Firefly Oasis. All others I set up for a max of 50% DOD.

Don S/V ILLusion said:
Like others, I've likely exceeded the 'magic' 50% figure probably numerous times on battery banks which have far exceed d my expectation for longevity.

50% DOD is not a magic number, cycle less than 50% and you'll get even better life. 50% is simply a recommendation for sizing and reasonable service life by many battery manufacturers. The occasional 80% DOD deep cycle is really not all that bad if fully recharged after the cycle or as soon as possible. Mixing an 80% DOD PSOC cycle into multiple back to back 50-85% DOD PSOC cycles is worse than doing an occasional 80% cycle followed by a full recharge. Discharging to 80% DOD regularly, stacked on top of PSOC, will shorten life beyond 50% DOD & PSOC or 30% DOD & PSOC or 20% DOD & PSOC..

Don S/V ILLusion · May 20, 2017

Maine Sail said:
Battery Manufacturers:

Trojan:
1. Shallow discharges will result in a longer battery life.
2. 50% (or less) discharges are recommended.
3. 80% discharge is the maximum safe discharge.

Lifeline:
Q: What depth of discharge should be used when sizing a battery?

A: 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.

East Penn / Deka:
Q: Does depth of discharge affect cycle life?

A: Yes! The shallower the average discharge, the longer the life. This is why it’s important to size a battery system to deliver at least twice the average power required, to assure shallow discharges.

In the lab this is pretty easy to quantify. For example Lifeline rates their batteries at 1000 cycles at 50% DOD and just 550 at 80% DOD. However this is not real world cycling it is conducted in ideal lab conditions. Trojan rates their T105 etc. at 1200 lab cycles to 50% and approx 600 cycles to 80% DOD. With Trojan & Lifeline the difference between 50% DOD and 80% DOD cuts lab cycle life in half or just about half.. Both companies have charts showing this.

There is no "magic" in 50% but it is the suggested max regular discharge /sizing / cycling recommendation by many reputable battery manufacturers selling deep cycle batteries. Any deep cycle battery can be discharged to 80%, some will do better than others, but you will get less life at 80% DOD than average discharges 50% DOD or 20% DOD etc.. The shallower the cycles the more life.

Discharging to a lower SOC when cruising means more time spent at low SOC in PSOC use, a longer time between full recharges when out of the lab, & damaging PSOC use. Cruising use is PSOC cycling if you're not getting back to 100% SOC after each discharge. As boaters who cruise we PSOC cycle more than full cycle and PSOC is bad for batteries.

In the lab the minute the battery discharges to the desired discharge floor it is recharged to 100% SOC including accounting for the proper Coulombic efficiency hit required to attain 100% SOC. PSOC cycling means 1 or more cycles between full recharges. Deep 80% discharges, stacked on top of PSOC use, is even more damaging. For an occasional off shore run, off shore race etc. 80% is fine provided you recharge to full as soon as you can, after this deep discharge. Doing 80% repeatedly and stacking it on top of PSOC, not good. With deeper discharges, longer absorption durations will be necessary and most chargers don't allow you to control this so even this will cause problems many don't have the ability to over come.

In the lab Lifeline for example suggest their battery will deliver 1000 cycles to 50% DOD and approx 550 cycles to 80% DOD. This is under ideal conditions where the batteries are cycled at controlled temps and recharged to 100% SOC immediately after an 80% or 50% discharge. This is 1 cycle. You have either a deep cycle or shallower cycle. Discharging to the design floor then recharging to 100% SOC is a lab cycle. Even if we discharge to our systems design floor the time it takes us to get down there there, with our low discharge rates, takes longer than in the lab so we are in a state of discharge longer before we even begin to recharge. Even if after every discharge to 50% or 80% we recharged to 100% were still not going to see lab like numbers because our time spent discharging is longer than it is in the lab. The longer the time spent away from 100% SOC negatively impacts cycle life when compared to a "lab cycle". Lifeline defines end of life as 80% of as new rated capacity, so a 100Ah battery is considered "end of life" when it can no longer deliver 80% of its rated capacity.

As sulfation builds up on the plates, during PSOC use, your charging system will start to hit absorption voltage earlier and earlier which also leads to under charging and worsening of the effects of PSOC use and deep cycling..

What impact does real world type use play on a battery that works in a PSOC environment, when cycled to 70% - 75% +/- DOD?

In the Practical Sailor AGM PSOC testing the batteries were discharged to 11.7V, based on sailor input as to when they decide to charge. 11.7V is far too low for regular cycling use but many sailors report this is when they choose to. These sailors also report poor battery life. This group of sailors tend to get poor service life compared to sailors who stop discharging at 12.1V to 12.2V... The PSOC test batteries were discharged at the 20 hour rate. They were then recharged for 1 hour at .46C or 46% of the rated 20 hour capacity. Most boat owners, unless they have AGM, don't have this much charging capability so their batteries would do even worse. The batteries were cycle to capacity then went through 30 PSOC cycles to 11.7V followed by a 1 hour recharge at .46C.

This Lifeline battery, which is rated to do 550 cycles to 80% DOD in the lab, lost 11% of its capacity when discharged to 71.4% DOD in just 30 cycles. A battery delivering only 80% of it's new capacity is considered end of life by industry standards. Even after being equalized, at the end of 30 PSOC cycles to try and recover capacity, it had still lost approx 11.6% of its capacity in just 30 PSOC cycles to approx 71% DOD.... For the Lifeline battery 11.7V actually represented 71.5% DOD, at the beginning of the test, which is still a ways from 80% DOD. As the battery continues to "walk down" in capacity during PSOC cycling the DOD from "as new" gets deeper. PSOC capacity walk-down was noted as early as cycle 2. One tested battery failed during this test, it had lost 1% in capacity for each PSOC cycle to 11.7V. This battery was dead in just 30 PSOC cycles so another battery was tested, it too failed within 30 PSOC cycles to 11/7V.

This test originally began with a 1 hour recharge at .2C (210% of Ah capacity) but the batteries were declining so fast it was scrapped. Seeing as they were AGM, and they can take the current, it was decided to jump from a .2C to a .46C charge rate. Despite this .46C charge rate two batteries were still dead in just 30 PSOC cycles to 11.7V. The only battery that I regularly recommend 80% DOD cycling on is the Carbon Foam Firefly Oasis. All others I set up for a max of 50% DOD.

50% DOD is not a magic number, cycle less than 50% and you'll get even better life. 50% is simply a recommendation for sizing and reasonable service life by many battery manufacturers. The occasional 80% DOD deep cycle is really not all that bad if fully recharged after the cycle or as soon as possible. Mixing an 80% DOD PSOC cycle into multiple back to back 50-85% DOD PSOC cycles is worse than doing an occasional 80% cycle followed by a full recharge. Discharging to 80% DOD regularly, stacked on top of PSOC, will shorten life beyond 50% DOD & PSOC or 30% DOD & PSOC or 20% DOD & PSOC..

I appreciate all the responses and will end my questioning with the following:

I've read all that before, however, all of it quantifies battery life in terms of cycles and depth of discharge with no indication except in generalities how it relates to total useful duration. For example, you state "Lifeline for example suggest their battery will deliver 1000 cycles to 50% DOD and approx 550 cycles to 80% DOD"

That is a well known statement but it omits the total current supplied over an undefined period of time..

Stated differently, is there any data indicating how many amp-hrs can be obtained from equivalent batteries as a function of depth of discharge over 100, 500' 700, etc. cycles at varying depth of discharge?

Clearly, in terms of the calendar, lower discharges produce longer battery life. We know that!

Maine Sail · May 20, 2017

Gene Neill said:
Back to capacity for a moment ... Take this Wallyworld battery for instance. Unless I missed something, its capacity is only rated at "65 minutes".

"65 minutes"?? What the deuce can that even mean, without specifying an ampere load ? I'm only halfway through my first cup of coffee, am I overlooking something? https://www.walmart.com/ip/EverStart-24DC-Marine-RV-Deep-Cycle-Battery/180350522

And thank you all so much for all these in-depth comments! The conversation may have drifted a bit, but that's okay, I'm learning! Thank you!

#1 For a battery of that type the discharge rate for "reserve capacity" is 25A. For a golf cart battery is is 75A. You should only buy deep-cycle batteries rated in ampere hours, at the 20 hour discharge rate, not batteries rated in "Reserve Capacity" or "Reserve Minutes" at 25A or 75A.

20 Hour Discharge Rates:

100Ah ÷ 20 hours = 5A Load to 10.5V @ 77F to 80F
150Ah ÷ 20 hours = 7.5A Load to 10.5V@ 77F to 80F
200Ah ÷ 20 hours = 10A Load to 10.5V @ 77F to 80F

#2 The battery linked to is not rated to any industry standard for Ah capacity. For example the Maxx-29DC (group 31 battery) is rated at 114Ah at a 1A load. A 114 Ah rating would be at a 5.7A discharge rate not at 1A.. This rating makes no sense and means it is either a very, very low capacity battery or Wal*Mart has completely mislabeled the product.

I tried multiple times via email and phone to get an accurate Ah capacity rating for the JCI made Maxx-29 battery because I was testing one for a flooded battery PSOC test. Neither JCI (Johnson Controls) nor Wal*Mart could tell me the 20 hour Ah rating of the battery. After repeated break in cycles, the battery could barely muster 72Ah's of capacity in a class (G-31) where Ah capacity ratings range from a low of about 105Ah to a high of 130Ah. Only Crown, US Battery and Trojan actually cycled to rated or close (within 5Ah) of rated capacity.. No other automotive cased "deep-cycle" battery (G-24, 27, 29, 20, 31) could cycle up to its rating (BCI standards) so the testing was scrapped. I burned three months of testing only to realize most of the automotive-cased batteries I had tested could not meet their ratings which meant I really could not even begin to test them.

All I can say is that if you want to buy a G-24, 27, 29, 30 or 31 "deep-cycle" battery stick with Trojan, Crown or US Battery as they were the only batteries I could get to deliver their rated Ah capacity, or close to it.

There are industry standards yet many manufacturers of automotive cased batteries don't publish the actual specs you need to make an informed decision. One G-27 "deep cycle" battery I tested, using the same protocol as the AGM test, only with a lower 1 hour re-charge rate, went from 82.4Ah (rated at 90Ah but could not achieve this) to 37.1 Ah of capacity in 30 PSOC cycles. This is a 55% loss of max achieved capacity is 30 cycles to 11.7V....

FWIW Interstate rates their golf cart and industrial batteries properly, in Ah capacity at the 20 hour rate. They do not rate their "automotive cased" batteries in Ah capacity. I believe this is because they either being intentionally misleading or they are really only "deep-cycle" based on what the sticker says not in how they actually perform in a deep-cycle application..

Keep it simple only buy "deep-cycle" batteries that have a 20 hour Ah capacity rating. Even then they may never deliver the "rated capacity" even when brand new.. Get up into golf, floor scrubber or industrial batteries and they tend to deliver rated capacity far more consistently.

Maine Sail · May 20, 2017

Don S/V ILLusion said:
I appreciate all the responses and will end my questioning with the following:

I've read all that before, however, all of it quantifies battery life in terms of cycles and depth of discharge with no indication except in generalities how it relates to total useful duration. For example, you state "Lifeline for example suggest their battery will deliver 1000 cycles to 50% DOD and approx 550 cycles to 80% DOD"

That is a well known statement but it omits the total current supplied over an undefined period of time..

Stated differently, is there any data indicating how many amp-hrs can be obtained from equivalent batteries as a function of depth of discharge over 100, 500' 700, etc. cycles at varying depth of discharge?

Clearly, in terms of the calendar, lower discharges produce longer battery life. We know that!

Energy throughput will be higher over the life of the battery the shallower you cycle. This is especially true when we move away from the lab and into PSOC use. Even in good lab data, energy throughput is less the deeper you cycle, unless that data is "extrapolated" or "projected", and some graphs are. This can be misleading in trying to determine energy through put in the real world.

There is a reason so many battery manufacturers recommend a max average discharge of 50%. All one has to do is look back to the early days of AGM when they recommended 80% DOD, which made their total cost look better. They pulled back hard on that one because batteries were barely lasting a year, in the real world.

jssailem · May 20, 2017

Gene.
It is a challenge to understand the limited marketing / scientific data produced about batteries and shared with the public. This is one of the reasons I reviewed MaineSail's data. I feel that some of the back and forth discussion in this thread is an attempt to convert the scientific information supplied by MaineSail into a sailors sense of what I can expect in the Marina/Cruise environ.

The problem is it is all different for on the water action. Starting from the science side of the issue MaineSail provides the manufacturing facts and real experience of a Marine Electrical Engineer. Then you get his comments of field experience.

From my reading of his info I have discovered, that your observation of the confusion 'Walmart capacity rating' is just that confusion 'some cubical marketing person in Bentonville, AR cut and pasting what sounds good' reporting from the manufacturer. Note that Walmart makes nothing. They buy, relabel, and sell everything. I felt their plan, having sold things to Walmart, is to buy so low as to make the manufacturer feel the pinch.

The Interstate data is a bit better but still marginal.

Best performance/cost (according to MaineSail) comes from the batteries he lists to include the Dyno brand manufactured in Seattle WA. Because batteries cost so much to ship, I suggest you are better off looking for the best local provider. I think that would be from the East Penn/Deka manufacturer. They are sold under a variety of Labels. One of MaineSail's forum messages outlined a number of relabel sellers selling the East Penn/Deka manufactured battery. I believe Sam's club was listed.

To further the challenge you can not always find a relabeler like Sam's club/Walmart to have all of the battery model (like the specialized Dyno battery I mentioned) in my previious post.

It is a buyer beware space to navigate.

You passage thru knowing that while your experience may be similar to others, it is your passage and your use of batteries may give you longer or shorter life span then others.

Fair winds... No battery needed

JamesG161 · May 21, 2017

Don S/V ILLusion said:
they all lasted the typical 5-8 years

You will have a declining "working capacity". My "boat delivery" Captain with 30 years give me this advice.

Buy inexpensive wet cell batteries and you will change them every 3-4 years.

_____
I have learned more about batteries on this forum thanks to @Maine Sail , than years of training!

____
My new dual bank wet cells 12V cost ≈$400
Annual cost...
$400/3 years = $133.33/year
$400/5 years = $80/year

If I did the best rated, per MaineSail, golf cart batteries for approximately same capacity
≈$800.
Annual cost...
$800/5 years = $160/yr
$800/8 years =$100/yr
_________
I rather go sailing....
Jim...

PS: I am at 3.5 years now and noticing a change on working capacity.
BTW there are SO MANY random variable introduced by sailing sea states, temperatures, charging practices, overdrawing, poor maintenance, etc that you can't predict.