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.
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.
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.
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..