How was it determined that it is safe to discharge a battery to 50% of its capacity but to go beyond would shorten it's useful life? Any myth busters?
Question of the day.
- Thread starter Benny17441
- Start date
- Status
As Stated ........................
................................. in "The Good Book" (Calder's Boatowner's Mechanical and Electrical Manual) the 50% figure is more of an arbitrary figure for comparison of batteries rather than being an exact value.
As shown on page 8, Figure 1-12 of his book, the deeper the discharge, the fewer the number of life cycles are available. This graph may refer to a particular battery which had been tested but I don't believe it's meant to be used as gospel for all deep disch. batteries, just an indication of how they die.
It's also interesting to read (in the second column) what the definition of "battery death" is according to each manufacturer. Lot's of snake oil :Liar:
................................. in "The Good Book" (Calder's Boatowner's Mechanical and Electrical Manual) the 50% figure is more of an arbitrary figure for comparison of batteries rather than being an exact value.
As shown on page 8, Figure 1-12 of his book, the deeper the discharge, the fewer the number of life cycles are available. This graph may refer to a particular battery which had been tested but I don't believe it's meant to be used as gospel for all deep disch. batteries, just an indication of how they die.
It's also interesting to read (in the second column) what the definition of "battery death" is according to each manufacturer. Lot's of snake oil :Liar:
Attachments
mine were at 10.5 when i returned from gulf of mexico trip and lasted 7 yrs total.... were t 105s..... the replacements didnt lasty as long--- bad already at 11 months--wont hold charge, yet havent been below 12.5 to date.....
Ralph I agree that the 50% seems like an arbitrary figure to accomodate both usage and conservation. The reason I ask is because I believe this 50% may be a myth and am hoping for someone to confirm or disprove it. I have applied both experience and observation and cannot say that the percentage of discharge is more responsible than lack of proper maintenance for a battery's demise. I define battery abuse as the following, 1) deep discharges 2) not recharging to 100% 3) poor maintenance 4) high rate of discharges 5) overcharging. In my experience batteries suffer more from excesive rates of discharge or lack of maintenance that from being deeply discharged. I have accidentally discharged a battery all the way down by leaving the refrigerator on and upon discovery promptly recharged it back to 100% with no apparent ill effects. Have we been lulled into a false sence of security or alarmed beyond reasonable cause?
Benny, that deep discharge is exactly what "Breaking in New Batteries" or trying to bring back an old one suggest. http://c34.org/bbs/index.php/topic,6353.0.html
You are also correct that deeper discharges result in fewer cycles.
And given your description of potential battery "abuses" which is something many of us do (and I did to may bank after I broke my leg a few years ago - so I KNOW, for sure), I think perhaps we might choose to look at this question somewhat differently: that 50% is simply one of a good many ways that you can improve the treatment of your batteries.
No one thing will kill them immediately. Any one thing repeatedly will either kill them or certainly reduce their useful life. A whole bunch of them at different times will do the same thing.
No myths involved...
NOT doing those things will certainly increase the life.
The "myth" likely comes directly from the manufacturers..
Lifeline:
"For maximum battery life in cycling applications, do not discharge the battery bank below 50%. Discharging the bank repeatedly to 100% will shorten the battery life."
Trojan:
"For optimum life and performance, we generally recommend a discharge of 20 to 50% of the battery’s rated capacity even though the battery is capable of being cycled to 80%."
Deka/East Penn:
"Using a 50% depth of discharge (versus 80% or 100%) will dramatically extend the life
of any battery. Therefore, when helping to specify a battery for a system, choose a battery with at least twice the capacity required for best performance."
Rolls Battery:
"Do not completely discharge a deep cycle battery if it can be avoided. The deeper the discharge the less life you will obtain from the battery. The ideal method of operation in to charge and discharge the batteries through the middle range of their capacity (50% - 85%)."
Battery Life Cycle Chart / US Battery (LINK)
Those "life cycles" are "lab" life cycles where the batteries were charged and discharged at controlled temps, charged immediately after discharge and basically not tested in the real world. The claim of 500 cycles to 0% will simply not work in the real world..
The industry definition of "deep cycle" is to 80% depth yet most battery manufacturers don't recommend this and recommend 50% instead because your batteries will last longer if cycled less deeply.
Can you go to 80%? Yes, and people do but the batts just don't last as long. Numerous times I have helped people extend battery life by simply adding a battery monitor so they know what is actually going on with their system. Most were unknowingly going to 80% or more deep and after the monitor to only 50% DOD. The life of the batteries, with all else being equal, usually goes from about 2 -2.5 years on average, with these customers, to 5 years+.
We cycle our bank on average about 30% and they still test at near new capacity going on year six.
I agree 100% that other factors play a role but when you combine deep discharges to other improper care you'll only guarantee short life. I feel bad for you guys down South as heat is one of the number one killers of lead acid batteries. Add temps onto lack of full charges, deep cycles, improper maintenance, lack of periodic equalization with wets and you've got a real challenge getting even 200 cycles out of a bank. Let alone the "lab" ratings of 1000+...
Even my buddy Mike who manages a golf course in MA aims for a max discharge on their cart batteries of 50% as they have spread sheeted the cost savings and they win when cycling less deep. He likes to aim for 65% but admits this is hard to do..
I use 80% of new capacity (capacitance or pulsed load) as a basically "dead" bank. I do this not because it is the industry suggestion but because once beyond 80% of new capacity I find the drop off to death is considerably more rapid. With some battery technologies such as AGM it can be down right unpredictable.
I ran into a customer this afternoon at Hamilton Marine, he owns a SWEET boat, who's batteries (Lifeline's) I tested before they took the boat South in mid November. The start battery tested at 74% of new capacity and I suggested that replacing it it and the house bank might be a good option. He decided not to. He told me today that off Cape Hatteras the starting battery failed on them. This is why I consider 80% of new capacity a a bank worth replacing soon..
Thanks Stu, I am in agreement. Maine, if the Manufacturers seem to unanimously recommend the 50% threshold there is probably proof that deeper discharges do shorten excesively the life of a battery. I particularly like Trojans notice which seems to indicate the boundaries but leaving it up to the individual on how to use them. I guess for me economics is secondary to reliability when it comes to batteries. I have replaced a set of working batteries just because they were coming up on three years and we were getting ready to take an extended trip. I figure that for batteries that retail for $75 if I get three years out of them it is good. Wether this same batteries could last another three years if I changed my ways and monitored them closely its really not of much concern. I like wet batteries because if I kill one of them I can stop at the nearest Pass or Inlet and quickly find a replacement. Currently have two Walmarts Group 29s (really group 31s) and always carry a charged up starting battery pack. Don't usually discharge them below 50% and check the water twice a year and do keep them on a float charge and expect to get one more year out of them. Have heard is not good to keep them on a continuous float but will not bother to set a schedule to turn charger on and off. We do sail down here all year long so I feel that three years of usage down here may compare to 5 years of usage up North. Thanks for the info, it is appreciated.
The OP indicate safety, it is safe to discharge batteries completely (0% SOC) The 50% is related to long term battery life
The actual $/AH stored/released puts the "best" SOC range to be 75-100% $/AH is calculated by cycling the batts till they fail. Each battery is cycled to a different SOC. so a deep discharge gives lots of AH but not so many cycles while a shallow discharge gives less AH/cycle but many more cycles.
I read somewhere that the 50% charge figure was based on both the best $/AH, the normal variation in battery discharge, and the desire for battery companies to continue to sell their product. Seems you can't sell your product if it breaks too soon, but you don't want to have it last forever either.
The actual $/AH stored/released puts the "best" SOC range to be 75-100% $/AH is calculated by cycling the batts till they fail. Each battery is cycled to a different SOC. so a deep discharge gives lots of AH but not so many cycles while a shallow discharge gives less AH/cycle but many more cycles.
I read somewhere that the 50% charge figure was based on both the best $/AH, the normal variation in battery discharge, and the desire for battery companies to continue to sell their product. Seems you can't sell your product if it breaks too soon, but you don't want to have it last forever either.
Percent Discharge Number of Cycles Total Amphours Longevity based on Number of Cycles Longevity based on Total Number of Amphours extracted
Sorry I don't know how to copy spreadsheet data to paste here. The following is an interpretation of the data for Deca AGM batteries. First column % discharge, 2nd # of cycles to death based on Deca's chart, 3d Total amp hours for a 400 amp battery bank, 4th longevity based only on # of cycles with 50% discharge used for 100%, 5th longevity based on # amp hours obtained during life cycle. As mentioned earlier, this all based on manufacturer's data and ideal conditions. Nevertheless, total power obtained is not decreased as much by deep discharge as looking at the number of cycles suggests.
Geoff
25% 925 92500 250% 125%
50% 370 74000 100% 100%
80% 200 64000 54% 86%
100% 150 60000 41% 81%
Battery Capacity 400 ah
Sorry I don't know how to copy spreadsheet data to paste here. The following is an interpretation of the data for Deca AGM batteries. First column % discharge, 2nd # of cycles to death based on Deca's chart, 3d Total amp hours for a 400 amp battery bank, 4th longevity based only on # of cycles with 50% discharge used for 100%, 5th longevity based on # amp hours obtained during life cycle. As mentioned earlier, this all based on manufacturer's data and ideal conditions. Nevertheless, total power obtained is not decreased as much by deep discharge as looking at the number of cycles suggests.
Geoff
25% 925 92500 250% 125%
50% 370 74000 100% 100%
80% 200 64000 54% 86%
100% 150 60000 41% 81%
Battery Capacity 400 ah
Bill - You forgot an important factor... you want to sell bigger and/or more batteries on each sale.
The lower the DOD the larger the battery bank required for a given design.
- Status