LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[goboatingnow]

Originally Posted by tsmwebb

Does the Li battery just look like a smaller and smaller Li batter over time or are you seeing other changes to the battery behavior over time?

In general outside of tests that cause either serious damage to cell all issues ( undercharge , overcharge , dynamic pulse performance , and lifespan ) seem to equate to loss of capacity.

Right now the indications are that approx 8-10 % of capacity is lost per annum irrespective of normal usage patterns.

What has not been reliably determined is whether that slope is linear. ( nor can I see it in the scientific literature )

My views on lithiums have changed somewhat. If you have a key requirement that is energy density , these are the best types that exist at the moment ( in normal commercial availability ) and charging regimes while different to LA are actually simpler.

However if you are looking at low usage or are not sensitive to energy density then the arguments in relation to other battery chemistries and in particular good quality LA are much less clear cut then perhaps originally thought.

Ie a 10, 000 cycle life does not necessarily result in a long " lifespan " battery. So people doing costing that equate cycle life to linear lifespan may be fooling themselves.

If you want all the other useful Li properties then that's a different matter.

In boats where perhaps energy density is not so clear cut. There needs to be serious examination on the pros and cons with extended battery life justifications not being one of then in my humble scientific opinion.

Dave

[Maine Sail]

Originally Posted by goboatingnow

Right now the indications are that approx 8-10 % of capacity is lost per annum irrespective of normal usage patterns.



Dave

Dave,

My four 400Ah Winston cells (Thundersky) were manufactured on September 6, 2009. I bought them knowing they were used and possibly used for testing purposes. I got a great deal and specifically bought them for my own education and experimentation.. I have no idea what was done with them before I got my hands on them nor how many cycles they were put through..

What I do know is that in 550+ cycles, since I got them, the capacity has not changed much and I can still exceed the as new 400Ah rating.

The last cycle test, using some new and more accurate equipment, at cycle 550, yielded 419 Ah's until the lowest cell hit 2.8V. This is charged to 13.8V and less than 10A of accepted current at 13.8V. I don't consider that too bad for 2009 batteries and 550 cycles, most to 80% DOD...

In over 550 cycles I've noticed no real meaningful capacity difference, perhaps a 10Ah change. Every 50 cycles they are fully capacity tested so that I can keep track of performance. Each of the first ten were done identically including temp, load and equipment. From cycle 550 on they are being tested with a lab grade DC constant load that can count Ah's and time.

Based on your observations above I should be seeing approx 200Ah's based on age alone?? Why I am still measuring in excess of new rated capacity with cells that are 5.5 years old?

I fully expected them to have fallen off the cliff by now, but they have not... Clearly I must be doing something wrong...

Capacity test #11 @ cycle 550 end of charge:


Completed test 419.2Ah's @ 30A constant load:

[goboatingnow]

Originally Posted by Maine Sail

Dave,

My four 400Ah Thundersky (Winston) cells were manufactured on September 6, 2009. I bought them knowing they were used and possibly used for testing purposes. I got a great deal and specifically bought them for my own education and experimentation.. I have no idea what was done with them before I got my hands on them nor how many cycles they were put through..

What I do know is that in 550+ cycles, since I got them, the capacity has not changed much at all and I can still exceed the as new 400Ah rating.

The last cycle test, using some new and more accurate equipment, at cycle 550, yielded 419 Ah's until the lowest cell hit 2.8V. This is charged to 13.8V and less than 10A of accepted current at 13.8V. I don't consider that too bad for 2009 batteries and 550 cycles, most to 80% DOD...

In over 550 cycles I've noticed no real meaningful capacity difference. Every 50 cycles they are fully capacity tested so that I can keep track of performance. Each of the first ten were done identically including temp, load and equipment. From cycle 550 on they are being tested with a lab grade DC constant load that can count Ah's and time.

Based on your observations above I should be seeing approx 200Ah's based on age alone?? Why I am still measuring in excess of new rated capacity with cells that are 5.5 years old?

Clearly I must be doing something wrong...

Capacity test #11 @ cycle 550 end of charge:


Completed test 419.2Ah's @ 30A constant load:

I don't doubt your comments for a moment. I am merely basing my results on both my own tests and also the published literature.

Its worth noting that one poster here ( ebaugh ? ) mentioned a loss of capacity in his cells after some time.

My own test setup is a fully computerised programmable power supply, electronic load test setup with full logging with test varying conditions including dynamic pulse response., effects of partial under or overcharge and storage . The tests were to determine the lifespan of batteries used not on the typical high cycle high energy density applications typical of Li usage, but in applications more approaching ups style usage of low or standbys followed by short ( relative ) bursts of heavy activity.

We tested 40 types of Li battery across several chemistry variants.

I find it hard to accept that over nearly 5 years you are seeing no reduction in capacity , maybe you have a bionic battery. But I can offer no explanation

Dave

[Maine Sail]

Originally Posted by goboatingnow

I find it hard to accept that over nearly 5 years you are seeing no reduction in capacity , maybe you have a bionic battery. But I can offer no explanation

Dave

My data only goes back about three years with these cells. I don't know where they would have tested when brand new. I did a capacity test when I first got them to create a baseline for my own use. Also keep in mind that my capacity testing has been at .25C for the first ten which is likely why they exceed the rating. At 1C I got just about 400Ah's but really don't have the ability to do 400A load tests repeatably and that load is not relevant to our use.. From here forth I can track it more accurately..

Quite frankly I am as puzzled as you because all the white papers suggest I should be seeing some loss of capacity and all I have seen is perhaps -10Ah's... I still keep expecting them to drop off when I get to the next test but they keep supplying the capacity..

I am about 2 months away from capacity test 12 at 600 cycles..

[goboatingnow]

Quite frankly Maine sail id be suspicious of the capacity results that remained unchanged over three years. Even if you have got an outlier as far as life span and given the equipment you have I still find it hard to accept.

Everything I see which effects Li outside of catastrophic effects results in capacity reduction , that Includes mere ageing.

As House MD would say " interesting "

Dave

[Maine Sail]

Originally Posted by goboatingnow

Quite frankly Maine sail id be suspicious of the capacity results that remained unchanged over three years. Even if you have got an outlier as far as life span and given the equipment you have I still find it hard to accept.

Everything I see which effects Li outside of catastrophic effects results in capacity reduction , that Includes mere ageing.

As House MD would say " interesting "

Dave

Dave,

You are more than welcome to come and observe one of these capacity tests, start to finish, and check the date codes on the cells. Next time you are in the US North East you are more than welcome to ring me up...

Besides I would love an opportunity to pick your brain in person..

[deckofficer]

Originally Posted by goboatingnow

Quite frankly Maine sail id be suspicious of the capacity results that remained unchanged over three years. Even if you have got an outlier as far as life span and given the equipment you have I still find it hard to accept.

Everything I see which effects Li outside of catastrophic effects results in capacity reduction , that Includes mere ageing.

As House MD would say " interesting "

Dave

I have over 400 cycles to 80% DOD and have the same results as Maine Sail. My cells are also approaching 6 years of age.

[downunder]

Originally Posted by goboatingnow

Quite frankly Maine sail id be suspicious of the capacity results that remained unchanged over three years. Even if you have got an outlier as far as life span and given the equipment you have I still find it hard to accept.

Everything I see which effects Li outside of catastrophic effects results in capacity reduction , that Includes mere ageing.

As House MD would say " interesting "

Dave

Dave,

Is the difference more with the small cells types you have been testing rather than with the large prismatic type such as the Winston's MainSail is using???

[goboatingnow]

Originally Posted by Maine Sail

Dave,



You are more than welcome to come and observe one of these capacity tests, start to finish, and check the date codes on the cells. Next time you are in the US North East you are more than welcome to ring me up...



Besides I would love an opportunity to pick your brain in person..

I might take you up on that., actually.

Dave


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[goboatingnow]

Originally Posted by downunder

Dave,



Is the difference more with the small cells types you have been testing rather than with the large prismatic type such as the Winston's MainSail is using???

Well , all the reports say the chemistry is independant of physical organisation. The large prismatics are in effect a form of parallel pouch cells. My next step assuming I'm asked is to expand the test to include pouch cells, but personally I have argued that I expect no difference.

I realise MaineSails reputation and his methodology, so I'm reluctant to speculate. Everything I see , including , as he has admitted himself, points to loss of capacity over time across all forms of Li chemistry. There is no doubt that LiFeP04 variants seem to exhibit both longer cycle life and also longer life span, but the chemistries are all similar.

For reference my test setup uses a Rigol DP832 programmable power supply , a Manyud 9712 300W programmable load , data logging is performed by a two channel .025%, 6 digit programmable data logger , all connected via Ethernet to a mac , using SPCI commands and MathLab.. The test includes batteries with stated capacities from 800 mAH to 2000mAH..

Various tests include discharge to near zero SOC, overcharging, 80% capacity discharges, response to pulse loads of 2A and 4A, discharges and recharges at .25C , 0.5C , 1C , and 5C.

Because many cells have ridiculous claims of cell capacity, a baseline series of charge discharge cycles was undertaken to establish the cells actually capacity, that was then used rather then any manufacturers stated data.

Temperature was also monitored in the battery holder by physical contact with a calibrated sensor

The advantage of the smaller capacity cells. Is of course aggressive cycling does not take too much time, even so the process took nearly 6 months ( it's wasn't continuous though )

An extensive review of the scientific literature was also undertaken.

My own view is that Li manufacturers know that cell longevity, is nothing like being claimed, but for oems that need energy density Li is the only game in town.

Again I find M S tests perplexing, as he also finds himself. I wonder are the cells of greater AH then stated.

Maine Sail, have you actually tried a near 100 % discharge test to establish actual capacity or did you merely stop once you exceeded the manufacturers capacity. .?

I also found that stated capacity changes with discharge rates not dissimilarity to LA , ie batteries exhibited different capacities at different discharge rates especially at the outer limits ( 0.25, 5C ) this often was exhibited by batteries from " cheaper " sources.

There is clearly an incredible amount of variation in the real specs of many batteries, as against the printed specs, suggesting that like most things you get what you pay for.

Dave




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[goboatingnow]

Originally Posted by deckofficer

I have over 400 cycles to 80% DOD and have the same results as Maine Sail. My cells are also approaching 6 years of age.

So why did ebaugh report loss of capacity , strange ?

Without knowing your test methods, baseline , and discharge regime, I d find it hard to comment scientifically. How did you establish your baseline capacity.

I wonder where these early large prismatics intentionally underspeced as regards capacity ( or maybe not even tested properly ) it's so hard to follow the machinations of the Chinese large format companies, much of they" research" doesn't seem to be published etc. I suspect a degree of snake oil personally

Dave


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[Maine Sail]

Originally Posted by goboatingnow

Again I find M S tests perplexing, as he also finds himself. I wonder are the cells of greater AH then stated.

Maine Sail, have you actually tried a near 100 % discharge test to establish actual capacity or did you merely stop once you exceeded the manufacturers capacity. .?

For my bank I have used lowest cell at 2.8V as my cut off for 100% discharge.. In my first capacity test I did cut it short slightly at 425Ah's at .25C. I did test at 1C, at around 100 cycles, and got just shy of 400Ah's and then went back to using .25C. A 400A load for an hour is a bit over the top to do and was tough to control or hold steady using inverters & heaters..

As for capacity I am doing some testing for one of the publications as we speak and even amongst the lead acid batteries (PSOC tests for AGM in this case) there is a lot of variability in claimed vs. actual capacity. Some brand new batteries exceed the rating and some won't even cycle up to their rating. Very frustrating.. As you well understand this takes months and months when doing this sort of testing. I have two batteries on the bench now undergoing 20 hour capacity tests one after the PSOC regime and one I am trying to cycle up to capacity before PSOC testing begins. The batteries are temp controlled to 77F +/- using a water bath. This testing has been well over 3 months, and has at least another 2-3 months to go. I am doing this with LA batteries and using the 20 hour rate for capacity baselines and changes so it takes a very, very long time..

One thing I don't see mentioned enough with LFP is the effect of storage temp on calendar life. Bob E's batts for example are in his engine room.. One paper I have (M. Kassem / Journal of Power Sciences) suggested that minimal changes were noticed at 30C but at 45C and 60C these effects were considerably more pronounced. That paper suggested that storage temp was the larger contributor even over what SOC you left the bank at when in storage. (summarizing from memory) I store my bank at about 60% when not being cycled and at about 50F. During capacity testing the battery is at 75-77F. FWIW my bank rarely if ever exceeds 70F and is mostly between 50F (10C) and 70F (21C). I wonder if cooler temps help to limit aging even more than at 30C..?

The long and short for me is that for fractional C use I still have a 5.5 year old bank (calendar age of cells) that as of cycle 550 (my cycles) can deliver 419.2 Ah's at a 30A load. On board our load is 5-8A average so it could be a couple Ah's higher at 8A than at 30A. At this rate I would expect this bank to easily make it to at least ten years, but I could be coming up to a cliff, can't say until it happens...

So far I am pretty happy. Would I buy Winston cells again? Heck no, the company is HORRID to deal with, but I would continue to use LFP (probably CALB) even if calendar aging suggested a 7 year life. The charge acceptance alone, and no noticeable change to it, as the battery ages, is worth it to me.

Why? Because I have yet to see any lead acid battery (other than perhaps a few GEL banks) survive the way these LFP's are. Hell I have customers with very expensive lead acid batteries (nearly as much as my cells per Ah) that have lost 30% or more in less than 2 years and I test and see this regularly. There are some new and very interesting AGM's coming to market that are more sulfation resistant but real world data is scant on these new AGM technologies.

We must remember that for all I have written about, my case is n=1 and n=1 does not make for good science.... Also LFP is not a good fit, at this point, for the majority boaters. Pretty good for the DIY geeks but beyond the scope of most casual users..

Originally Posted by goboatingnow

So why did ebaugh report loss of capacity , strange ?

All I know from corresponding with Bob, and hopefully he will chime in, is that his batteries are installed in the engine space and he never was able to get a baseline capacity figure.. I don't know if he has done any other capacity tests to confirm if this fade is continuing?

[goboatingnow]

Why? Because I have yet to see any lead acid battery (other than perhaps a few GEL banks) survive the way these LFP's are. Hell I have customers with very expensive lead acid batteries (nearly as much as my cells per Ah) that have lost 30% or more in less than 2 years and I test and see this regularly. There are some new and very interesting AGM's coming to market that are more sulfation resistant but real world data is scant on these new AGM technologies.

I agree entirely, it is a rugged technology, I have my concerns about life span but I simply can't comment on larger case formats.

I do agree in fractional C cases the batteries are very bomb proof. One outcome of our tests was that for best longevity , capacity should be increased to lower discharge C rates. This negates sone of the advantages of the storage density. Hence a bigger battery working less does seem to last longer , the issue is how much longer. The other thing is that restricting discharge limits ( and upper charge limits) seems to also make for longer life. All of these mean for certain users efficiencies in energy density are not as great as first expected in certain usage applications where long( er) life is required

It's a pity we haven't seem more large format tests from other industries. The EV boys seem all over the place


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[Maine Sail]

Originally Posted by goboatingnow

I agree entirely, it is a rugged technology, I have my concerns about life span but I simply can't comment on larger case formats.

From a purely science stand point I agree that prismatics should age similarly to cylindrical cells but they also dissipate heat slightly differently and this could possibly play a roll..?

Originally Posted by goboatingnow

I do agree in fractional C cases the batteries are very bomb proof. One outcome of our tests was that for best longevity , capacity should be increased to lower discharge C rates. This negates sone of the advantages of the storage density. Hence a bigger battery working less does seem to last longer , the issue is how much longer. The other thing is that restricting discharge limits ( and upper charge limits) seems to also make for longer life. All of these mean for certain users efficiencies in energy density are not as great as first expected in certain usage applications where long( er) life is required

I think for Frac C use energy density is not really the huge asset many make it out to be or argue for. Shorter engine run times/faster charging, steady voltage for equipment, low weight, the ability to recharge to 100% SOC via a large alternator vs. 70-80% SOC +/- for lead acid and no sulfation worries if left at less than 100% SOC are the bigger advantages. These advantages to me far outweigh the energy density benefit of LFP..

Originally Posted by goboatingnow

It's a pity we haven't seem more large format tests from other industries. The EV boys seem all over the place


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I have spent a lot of time looking to the off grid arena and even that crowd has little to no data on LFP for fractional C use....

Frac C is simply different use than EV use and I suspect, and have for a while, that in Frac C staying out of the bottom 20% and not re-charging at high voltages (3.55VPC max but preferably 3.5VPC or less) leads to a battery that responds better over its life and also one that tends to stay in balance a bit longer...

The ability to use a full 80% of the bank can not be underestimated when compared with lead acid where you really only get 30-35% usable capacity, when more than one day away from a shore charger...

[nimblemotors]

Which cells did you test Dave?
I don't think I've ever seen any Lifeco4 18650 cells.

The old lithium ion cells had very poor calendar life, laptop cells would be dead in 2 years. They put 'additives' in them now to make them last longer.
Which ones are trade secrets so each manufacturer can be different.