Purchasing new batteries: How do you determine what one 'cycle' is?

[tombarton]

Hi all, I'm looking to purchase new SLA batteries for an off-grid system. I know I need to choose something with a high number of cycles, so that it will be well-suited to the task of daily use and recharging.

My local solar shop sells some AGMs with a lifespan of around 350 cycles. At first I thought this was too low to even last a year, but then realised a 'cycle' likely does not just mean one day of drawing and one day of charging, but that in this context one 'cycle' may mean an entire capacity use/charge. In this case, the battery could last several years' usage.

Can someone please shed some light on what constitutes a 'cycle' for this purpose, and how to calculate the potential lifespan of a new AGM battery?


*Note to noobs: I learned the hard way, and was one of those people who bought some used UPS batteries to 'save money'. Turns out they have an extremely low number of cycles, as they're designed to live on float charge and give all their energy in emergency situations, as opposed to being constantly run down and recharged as expected in a solar setup.

[max2k]

Hi all, I'm looking to purchase new SLA batteries for an off-grid system. I know I need to choose something with a high number of cycles, so that it will be well-suited to the task of daily use and recharging.

My local solar shop sells some AGMs with a lifespan of around 350 cycles. At first I thought this was too low to even last a year, but then realised a 'cycle' likely does not just mean one day of drawing and one day of charging, but that in this context one 'cycle' may mean an entire capacity use/charge. In this case, the battery could last several years' usage.

Can someone please shed some light on what constitutes a 'cycle' for this purpose, and how to calculate the potential lifespan of a new AGM battery?


*Note to noobs: I learned the hard way, and was one of those people who bought some used UPS batteries to 'save money'. Turns out they have an extremely low number of cycles, as they're designed to live on float charge and give all their energy in emergency situations, as opposed to being constantly run down and recharged as expected in a solar setup.

If you really going to charge/discharge them daily- take a look at LFP cells, much better deal in the long run IMO: 20x cycle life + 1.5 times more energy for the same Ah and other stuff I mentioned in your other thread. After 1000 cycles at 0.5C current it will still have 90% original capacity: LFP_Cycle_Life.jpg

[Mike90250]

I've understood 1 cycle to be about a 20% change in charge level. Full down to 80% and recharge back to Full would equal 1 cycle. If you only go down 10%, it's not a full cycle but does add to age. Often, there is a Depth of Discharge vs Cycle life, and you can look at ways to maximize your investment,

[NEOH]

How do you determine what one cycle is?

Assume nothing when it comes to, what is a cycle or the number of deep cycles, for a deep cycle battery.
One cycle is a discharge and then a charge, regardless of Depth of Discharge.
Now, how many of those deep cycles you get is dependent upon the average % Depth of Discharge, of each and every cycle, and also the battery's age.

You may get ...
1,500 Cycles at 20% Depth of Discharge - a shallow cycle
750 Cycles at 40% Depth of Discharge - a medium cycle
375 Cycles at 80% Depth of Discharge - a deep cycle

As you can see from above ... what is a Cycle does vary.
And so the number of cycles must always be stated with the "% Depth of Discharge per Cycle" to be meaningful.
Typically, the actual lab testing parameters used per CYCLE is not specified.
The graph for "Number of Cycles vs Depth of Discharge" is FOR ESTIMATION PURPOSE ONLY - meaning you will get less.

[tombarton]

Thanks guys for all the info. Illuminating stuff and really helpful with planning.

I recognise now that a cycle is a cycle, but how many of these constitute a battery's lifespan depends on the average DOD over this lifetime.

[Mike90250]

DoD vs life chart (suggestion only, your mileage will be less)


Battery-DoD-vs-Cycles.gif

[max2k]

DoD vs life chart (suggestion only, your mileage will be less)


Battery-DoD-vs-Cycles.gif

is this for AGM? 50% DOD gets you only 1.3 years of daily use.

[Sunking]

If you really going to charge/discharge them daily- take a look at LFP cells, much better deal in the long run IMO: 20x cycle life + 1.5 times more energy for the same Ah and other stuff I mentioned in your other thread. After 1000 cycles at 0.5C current it will still have 90% original capacity:

Max, do not fall for those lies. That chart you show for a Chi-Com battery went bankrupt and had to change their name. They went bankrupt from warranty claims. They lasted less than 3 years and the company was called Thundersky aka Thunder Turds.

Thundersky split into 3 companies. CALB, Winston, and Sinopoly. Calb is the best of the bunch and is a 2 to 3 year battery at best. Stay away from Winston and Sinopoly.

FWIW assuming you follow standard design protocol, there are 300 cycles in a calendar year.

[max2k]

Max, do not fall for those lies. That chart you show for a Chi-Com battery went bankrupt and had to change their name. They went bankrupt from warranty claims. They lasted less than 3 years and the company was called Thundersky aka Thunder Turds.

Thundersky split into 3 companies. CALB, Winston, and Sinopoly. Calb is the best of the bunch and is a 2 to 3 year battery at best. Stay away from Winston and Sinopoly.

FWIW assuming you follow standard design protocol, there are 300 cycles in a calendar year.

I'm just trying to be open minded about LFP. From my reading around (not real experience) it sounds the area was under heavy development over last 5-7 years so any data related to LFP needs to be seen from that perspective as well. It also needs to be closely examined on the type of use pattern and type of failure.

There were few members from RV crowd passing by here who were claiming 4 years cycle life without losing capacity. On closer consideration it turned out they had greatly oversized banks and were not using LFPs to the full potential due to limitations in their BMS/charging systems so they were cycling them in 95%->75% SOC range over those years and only few times over that period down to 20%-30% or so. BTW, this seems to be working for them and I have no problem with that.

Karrak as you might remember uses his house bank in the same fashion and it also seems to be working for him. My only problem with all this is they somehow forget to mention their use pattern as pre- requisite to what they're actually doing and fog discussion with BMS details, balancing, etc while in reality none of that matters as the use pattern would forgive any screw ups in there or cells shortcomings for that matter.

I haven't seen one coming by who would use smaller bank (matching to the task) and use the cells in 95%-20% SOC range daily taking advantage of claimed LFP properties. That might change their cycle life considerably. OTOH on EV forums where this pattern is prevalent as they are constrained in space later posts don't seem to be against LFP but still I haven't seen hard evidence of long cycle life (>500) under 'EV pattern' of use.

LFPs also have very flat charge/discharge curves making it difficult to guess their SOC based on voltage. Many users don't realize this and produce 'measurements' made with equipment with not enough accuracy or not done accurately enough so I doubt they can even reproduce them reliably. It is hard to have meaningful discussion with all those things in place. My conclusion was the voltage cannot be used as SOC criteria for LFPs and it can only be used to set off alarms for 2 thresholds: 'fully charged' or 'fully discharged'. If one needs SOC gauge type of indicator they're supposed to use shunt in series and honestly count coulombs which passed in and out. Texas Instruments and few other makers produce such chips for EV use.

[Sunking]

Karrak as you might remember

Yep I remember, but I quit drinking and smoking dope decades ago. Will not touch that chit today. Make you stupid and stupid's name is Karrak.

LFPs also have very flat charge/discharge curves making it difficult to guess their SOC based on voltage. Many users don't realize this and produce 'measurements' made with equipment with not enough accuracy or not done accurately enough so I doubt they can even reproduce them reliably. It is hard to have meaningful discussion with all those things in place. My conclusion was the voltage cannot be used as SOC criteria for LFPs and it can only be used to set off alarms for 2 thresholds: 'fully charged' or 'fully discharged'. If one needs SOC gauge type of indicator they're supposed to use shunt in series and honestly count coulombs which passed in and out. Texas Instruments and few other makers produce such chips for EV use.

Well first I apologise if I offended you, was not my intention because I know you are smarter than that. You have a better understanding than most, but still have some catching up to do. You are correct, LFP cells are difficult to judge SOC because as you said the charge/discharge curve is very flat from 10 to 90% SOC. Take notice that only applies to LFP cells, and no commercial EV I know of would use LFP cells. If you stop and think about it becomes very clear why they do not use them. It would be impossible to get any meaningful range out of them. LFP cells have roughly half the Specific Energy (Wh/Kg) and Energy Density (Wh/L). That 900 pound battery Tesla uses would have to weigh 1800 pounds and be twice as large in volume. I know you are smart enough to understand that will not work worth a damn. Some DIY EV guys still use LFP batteries and they have to use Chi-Coms like CALB, Winston, Sinopoly, and GBS because they are cheap of 1/4 the price of quality LFP cells made by A123 or LG. However that trend has come to an end as their are now 10's of thousands of used EV' sitting in dealers and salvage yards returned from lease or wrecked. No one in their right mind would buy a used EV because they know the battery needs replaced. So the DIY EV guys including myself look for salvaged Chevy and Nissan Leaf battery sin salvage yards to harvest the battery. My litle racing golf cart uses a Nissan Leaf battery of 144 volts @ 66 AH. Ever been in a golf cart that does 0 - 70 mph in 4 seconds? My little 25 hp electric motor produces 110 ft-lbs of torque from 0 to 6000 RPM, and from 6000 to 1000 RPM is flat HP of 100 peak HP. With a 6:1 ratio direct drive gives me almost 650 ft-lbs of torque at the drive wheels. Not may commercial gas cars can do that.

Anyway back to LFP SOC. As you said it is difficult to determine SOC if SOC is greater than 10% and less than 90%. However it is no where near flat below 10 or above 90%. Those knees you see at the both ends are extremely easy to detect. They are called Delta Voltage Bumps. Working voltage on a LFP is 3.2 to 3.4 volts which is very compatible with Pb battery equipment so you can use off the shelf equipment without a lot of modifications. Most all you have to change is your mindset and battery management. Example LVD setting would be 12, 24, or 48 volt cut-off eliminates over discharge.

A DIY can mimic what commercial EV do. Never ever fully charge your batteries, rarely ever balance the cells., and never ever let the voltage get near fully discharged. Fully discharged is 8 volts on a 12 volt battery, and your LVD is set to 12 volts a heck of a long way from 8, 16, and 42 volts. To chargee is even easier with LFP. A that is required is a Float Charger. No stages, just a simple regulated Float Charger.

[max2k]

Well first I apologise if I offended you, was not my intention because I know you are smarter than that.

No offense taken, I know where are you coming from and I really don't see anything offensive in what you posted in response.

... So the DIY EV guys including myself look for salvaged Chevy and Nissan Leaf battery sin salvage yards to harvest the battery. My litle racing golf cart uses a Nissan Leaf battery of 144 volts @ 66 AH. Ever been in a golf cart that does 0 - 70 mph in 4 seconds?

that is truly impressive- assuming constant acceleration it would require only 62 yards distance to reach 70mph.

I hope you're not doing this in a parking lot as some young lad who tried to prove his Porsche could reach 100 km/h in a parking lot. Well, it did but he missed small detail that Porsche also needs distance to stop after that .

Anyway back to LFP SOC. As you said it is difficult to determine SOC if SOC is greater than 10% and less than 90%. However it is no where near flat below 10 or above 90%. Those knees you see at the both ends are extremely easy to detect.

Right but I couldn't find any cycle life field data when someone used LFP cells in that mode for several years (>2). Karrak & RV folks never exercised their banks that deep and IMO didn't even see true SOC values of their banks due to flat curves and shallow cycles. They were pretty much hovering around upper knee.

They are called Delta Voltage Bumps. Working voltage on a LFP is 3.2 to 3.4 volts which is very compatible with Pb battery equipment so you can use off the shelf equipment without a lot of modifications. Most all you have to change is your mindset and battery management. Example LVD setting would be 12, 24, or 48 volt cut-off eliminates over discharge.

A DIY can mimic what commercial EV do. Never ever fully charge your batteries, rarely ever balance the cells., and never ever let the voltage get near fully discharged. Fully discharged is 8 volts on a 12 volt battery, and your LVD is set to 12 volts a heck of a long way from 8, 16, and 42 volts. To chargee is even easier with LFP. A that is required is a Float Charger. No stages, just a simple regulated Float Charger.

I see, basically if one foregoes interest in knowing 'how much exactly left' all that is needed are 2 points: top and bottom. If the house system is sized properly knowing exact SOC is mostly for entertainment purposes. EV of course is different story.

[karrak]

There were few members from RV crowd passing by here who were claiming 4 years cycle life without losing capacity. On closer consideration it turned out they had greatly oversized banks and were not using LFPs to the full potential due to limitations in their BMS/charging systems so they were cycling them in 95%->75% SOC range over those years and only few times over that period down to 20%-30% or so. BTW, this seems to be working for them and I have no problem with that.

Karrak as you might remember uses his house bank in the same fashion and it also seems to be working for him. My only problem with all this is they somehow forget to mention their use pattern as pre- requisite to what they're actually doing and fog discussion with BMS details, balancing, etc while in reality none of that matters as the use pattern would forgive any screw ups in there or cells shortcomings for that matter.

I haven't seen one coming by who would use smaller bank (matching to the task) and use the cells in 95%-20% SOC range daily taking advantage of claimed LFP properties. That might change their cycle life considerably. OTOH on EV forums where this pattern is prevalent as they are constrained in space later posts don't seem to be against LFP but still I haven't seen hard evidence of long cycle life (>500) under 'EV pattern' of use.

For off-grid systems, both stationary and in RVs etc. you have to have reserve storage if you do not have a generator or do not want to use it excessively to tide you over cloudy days. For LFP batteries a good rule of thumb is having the battery capacity equal to two to three times daily consumption (Sunking's guideline is three days). This means that on average you will be only cycling less that 50% of the batteries capacity on a daily basis. You also want to keep the battery as full as possible to have as much reserve for cloudy weather. That is what "matching to task" is for off-grid systems.

At low charge and discharge rates as found in off-grid systems the lifespan of LFP batteries is predominantly dependent on the amount of energy cycled through them. Cycle at 20% and the battery will last five times as many cycles as cycling at 100%. See this post and this thread for more information about this.

LFPs also have very flat charge/discharge curves making it difficult to guess their SOC based on voltage. Many users don't realize this and produce 'measurements' made with equipment with not enough accuracy or not done accurately enough so I doubt they can even reproduce them reliably. It is hard to have meaningful discussion with all those things in place. My conclusion was the voltage cannot be used as SOC criteria for LFPs and it can only be used to set off alarms for 2 thresholds: 'fully charged' or 'fully discharged'. If one needs SOC gauge type of indicator they're supposed to use shunt in series and honestly count coulombs which passed in and out. Texas Instruments and few other makers produce such chips for EV use.

I think you will find that most if not all serious users of LFP batteries are well aware that you cannot use voltage to determine SOC.

Due to the fact that LFP batteries have such a good and consistent overall coulomb/current efficiency (around 99.5%) you can very accurately measure the SOC of an LFP battery with a shunt. Again most serious LFP battery users will use shunt based SOC counters so they can plan their energy and generator usage given the prevailing weather conditions.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[max2k]

For off-grid systems, both stationary and in RVs etc. you have to have reserve storage if you do not have a generator or do not want to use it excessively to tide you over cloudy days. For LFP batteries a good rule of thumb is having the battery capacity equal to two to three times daily consumption (Sunking's guideline is three days). This means that on average you will be only cycling less that 50% of the batteries capacity on a daily basis. You also want to keep the battery as full as possible to have as much reserve for cloudy weather. That is what "matching to task" is for off-grid systems.
...

good point, you're correct any battery in off grid system will not be very deep cycled on daily basis due to autonomy requirement.

thank you for the linked thread and the link to the research paper in that thread as it actually contains the data which are close to what I was after- cycle life of LFP batteries depending on various conditions. There's still problem though as the paper used 26650 cylindrical cells and not prismatic ones usually used in off grid systems. So just for the reference purposes their graph of cycle life dependency from DOD indicates that the cells they used could be expected to have 80% capacity after 2000 cycles at 30% DOD and C/2 rates: LFP_DOD_cycle_life.png




This is far cry from what is usually claimed about LFP cycle life of prismatic cells something along the lines of '6 years with 20% DOD and no capacity loss' or Winston showing 5000 cycles at 70% DOD: http://en.winston-battery.com/index....ategory_id=176

This inconsistency and lack of equivalent data for prismatic cells from independent sources doesn't make the case for LFP any better. There's also mismatch between this graph and the test matrix numbers at the beginning of the article by a factor of at least 2- for 60 C and 20% DOD graph stops at 5000 with 65% C remaining while matrix shows 9800 cycles and still going. They defined End of Life for their experiment as: "failing to deliver the required capacity before reaching a cell voltage of 2.0 V" but never stated what is 'required capacity' is as it is clearly not 80% of initial C and such definition would actually prevent them from plotting fading capacity graph in the first place:
LFP_Test_Matrix.png

[karrak]

Right but I couldn't find any cycle life field data when someone used LFP cells in that mode for several years (>2). Karrak & RV folks never exercised their banks that deep and IMO didn't even see true SOC values of their banks due to flat curves and shallow cycles. They were pretty much hovering around upper knee.

Thought I would do a graph of the daily maximum and minimum SOC of my LFP battery over the past year
SOCoverYear.jpg

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[karrak]

thank you for the linked thread and the link to the research paper in that thread as it actually contains the data which are close to what I was after- cycle life of LFP batteries depending on various conditions. There's still problem though as the paper used 26650 cylindrical cells and not prismatic ones usually used in off grid systems. So just for the reference purposes their graph of cycle life dependency from DOD indicates that the cells they used could be expected to have 80% capacity after 2000 cycles at 30% DOD and C/2 rates: LFP_DOD_cycle_life.png

This is far cry from what is usually claimed about LFP cycle life of prismatic cells something along the lines of '6 years with 20% DOD and no capacity loss' or Winston showing 5000 cycles at 70% DOD: http://en.winston-battery.com/index....ategory_id=176

I agree it would be nice to have some independent experimental reports on prismatics made by Winston, GBS, and CALB but I haven't managed to find anything. The results in this report I would think would still be indicative of any results obtained on any prismatics.

I don't think you can compare lifespan tests done at a temperature of 60^oC to lifespan tests done at the "normal temperature" that the Winston tests were done at. The following graph from the same paper shows there only being a ~7% drop in capacity after ~6500Ah being cycled through the 2.2Ah batteries which is ~4200 cycles at 70% at a temperature of 15^oC.
CapacityFadeTemp.jpg

This inconsistency and lack of equivalent data for prismatic cells from independent sources doesn't make the case for LFP any better.

IMO what does make a good case for LFP batteries in off-grid systems is all the real world data from numerous people running prismatic LFP batteries over a period of up to seven years that implies that in standard off-grid system they should last well in excess of ten years assuming there is no unforeseen mechanical, electrical or chemical breakdown inside the batteries.

There's also mismatch between this graph and the test matrix numbers at the beginning of the article by a factor of at least 2- for 60 C and 20% DOD graph stops at 5000 with 65% C remaining while matrix shows 9800 cycles and still going. They defined End of Life for their experiment as: "failing to deliver the required capacity before reaching a cell voltage of 2.0 V" but never stated what is 'required capacity' is as it is clearly not 80% of initial C and such definition would actually prevent them from plotting fading capacity graph in the first place:

My interpretation Is that they are only using the results from the first 5000 cycles of the ~9800 cycles that they performed. I agree that the 'failing to deliver the required capacity' definition is somewhat vague, but I don't think it makes any difference to the results.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller