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

[ebaugh]

Originally Posted by rfadler

First I have to confess I have a huge interest in the outcome of your testing. I've spent a considerable amount of time thinking about going the LifePo4 route when I upgrade my house bank. That said I have spent a career performing software and electronics (hardware) testing and would recommend that you;
1) run the test again but go 100-0-100% SOC,
2) select four different cells and run the test again,
3) select four additional cells and run the test again

Using just four cells and one test cycle does not constitute a valid test. There are so many variables that you need multiple tests to confirm the result. That said, your time is not free and any and all information you've provided is very much appreciated.

I really appreciate the information you've shared on this forum and on your web site.

I fully agree with you. But time and effort is an issue, so there are some limits to what I can do.

Yesterday, I validated the test setup. I verified the time base for the CellPro software is using the PC clock for a time base, so that part is accurate. I added a 50A, 50mV shunt to the test rig to verify the current flow. I recharged the cells and it took 79 Ah to go from 0-100% on the weakest cell of 4 in my test pack at 30A. Then I individually recharged all the cells using CC-CV terminating at 3.6V and 2A. That's full.

I am planning to run the discharge test again today. This time to 2.5V on the low cell. In my view, that is 100-0%. Then I will immediately continue the test individually with the 3 remaining cells. This will give me 3 more numbers, one for each cell. Ideally, I would pull more cells, but by the time I get these back installed, I've reached my limit of interest at 30-40 hours invested. 4 of 48 is almost 10%, I have no reason to suspect substantially different results from the other cells.

I heard back from the USA importer of the cells. He indicated their capacity ratings are based on 4.0V to 2.5V, but suggested that range was not recommended. He thought the 2.85-3.6V range might represent about a 5Ah difference. Finally, he suggested that GBS has recently added about 10Ah of capacity to their 100Ah cells to insure the capacity is present at recommended ranges. Oh...and the new cells are on the shelf ready to ship.....

I've been communicating with another EV user of these cells, he has 60 of them installed in converted Mazda with about 200 cycles now. He has not yet run capacity tests post install, but he did prior to install. His new cells ranged from 98-105 Ah when new. He will eventually run some more testing, but rebalancing a 60 cell series connected bank that is bottom balanced is not a trivial task.

What we really need is testing on some other cells installed for a year or more.

A couple of things to remember. My usage at anchor is higher than most. The cells are constantly either under discharge at an average of 20-25A, or under charge at 150-160A. 24x7. These cells are LiFeMnPO4, a slight variation of straight LiFePO4. I don't know if that's good or bad until we start hearing from CALB, Winston, Sinopoly or Thundersky users. I had not noticed in actual operation any deviation from new. But I did size the bank so that at 70% capacity it would still be fully functional for us.

Final numbers in a day or so....

[goboatingnow]

interesting ebaugh, we all await your tests, a 20% drop in capacity after only a reasonable number of cycles would be somewhat worrying , especially if the capacity decreases continues with additional cycles into the future

dave

[ebaugh]

Originally Posted by goboatingnow

interesting ebaugh, we all await your tests, a 20% drop in capacity after only a reasonable number of cycles would be somewhat worrying , especially if the capacity decreases continues with additional cycles into the future

dave

Dave,

The final results are in:

Average of 4 cells: 85.1 Ah, Range for 4 test cells: 83.3 - 86.4 Ah

So that's 15% maximum for the first year, perhaps as low as 11% factoring in the importers suggestion to drop 5% for cycling between 2.5 to 3.6 instead of 2.5 to 4.0V.

The graphs are still on the computer in the shop, I will post them tomorrow.

Bob

[goboatingnow]

Originally Posted by ebaugh

Dave,

The final results are in:

Average of 4 cells: 85.1 Ah, Range for 4 test cells: 83.3 - 86.4 Ah

So that's 15% maximum for the first year, perhaps as low as 11% factoring in the importers suggestion to drop 5% for cycling between 2.5 to 3.6 instead of 2.5 to 4.0V.

The graphs are still on the computer in the shop, I will post them tomorrow.

Bob

Hmm , I wonder how this will progress , is this a once off deterioration, Wonder what the 4 year graph will look like. Provides some food for thought, given the costs involved.

Dave

[ebaugh]

Originally Posted by goboatingnow

Hmm , I wonder how this will progress , is this a once off deterioration, Wonder what the 4 year graph will look like. Provides some food for thought, given the costs involved.

Dave

Dave,

Attached is the published life cycle graph from the lab....looks pretty linear to about 1500 cycles. This is for a 40 Ah cell. Nothing published on the 100, but they can be reasonably expected to test the same.

Bob

[hellosailor]

ebaugh-
I don't know about "a lot more than two". That could just be plates and separators. But Winston's PDF online shows their 'cells' to each consist of two units in one case. Maybe that means a 100A battery is really two sets of 50A "cells" in parallel, and then four pairs of that in series. I don't draw the pictures, or make the translations. I can only go by the contradictory information they post, and ask my Magic8Ball to clarify it from time to time.

So, from 40AH down to 32AH after 2000 cycles? That's still way better than wet cells for a 100% DOD, isn't it? (Rhetorical question.) And after all, they don't suggest using a 100% duty cycle, do they?

[goboatingnow]

Originally Posted by ebaugh

Dave,

Attached is the published life cycle graph from the lab....looks pretty linear to about 1500 cycles. This is for a 40 Ah cell. Nothing published on the 100, but they can be reasonably expected to test the same.

Bob

Your results would indicate the graph should be much steeper. Worrying !

Dave

[ebaugh]

Originally Posted by hellosailor

ebaugh-
I don't know about "a lot more than two". That could just be plates and separators. But Winston's PDF online shows their 'cells' to each consist of two units in one case. Maybe that means a 100A battery is really two sets of 50A "cells" in parallel, and then four pairs of that in series. I don't draw the pictures, or make the translations. I can only go by the contradictory information they post, and ask my Magic8Ball to clarify it from time to time.

So, from 40AH down to 32AH after 2000 cycles? That's still way better than wet cells for a 100% DOD, isn't it? (Rhetorical question.) And after all, they don't suggest using a 100% duty cycle, do they?

I'm not truly 100% certain. I don't build them.... But my understanding is if you go back to the picture I posted, you will see a clamp. That clamp holds multiple, what I will call pouches for lack of a better term. I think each one is a cell. The picture does show "extras" not on the clamp. I'm not sure why they are there, I lifted that particular picture off the Internet, it's not mine.....

[ebaugh]

Originally Posted by goboatingnow

Your results would indicate the graph should be much steeper. Worrying !

Dave

Me too...it may be the difference between the lab and actual application. But one user experience, especially one without a good baseline, is only precautionary.

[dlentz]

Originally Posted by ebaugh

I'm not truly 100% certain. I don't build them.... But my understanding is if you go back to the picture I posted, you will see a clamp. That clamp holds multiple, what I will call pouches for lack of a better term. I think each one is a cell. The picture does show "extras" not on the clamp. I'm not sure why they are there, I lifted that particular picture off the Internet, it's not mine.....

The "pouches" are anode and cathode "plates" separate by a plastic sheet. Not totally unlike a LA battery(except the materials). I would suppose the number of plates determine voltage and the size determines Ah. I have an open 700 amp cell. Check out EVTV.com. Jack Riccard has some of the new cells from CALB. LiCoMg or something, not the colbalt of boeing fame, but a couple of new chemistry s. Little higher energy density, little higher $. Good for EV, not needed for a house bank.

ebaugh, did you do an initial capacity test to determine the original capacity?

[ebaugh]

I did not do an initial test. I've corresponded with an EV user of the same cells (60 of them) who quoted a tested 98-105 Ah range.

Some one sent me a link to this:

http://www.seidenbattery.com/product_vehicles.asp

On that site is the attached graph that shows a graph for calendar life based on temperature I've never seen before. It may be relative to my situation since the batteries are in the engine room.

[Singleprop]

Assuming 35 degrees, then the standby service life (and no cycling of the bank) is still about 9 year at 100% charge.

[Singleprop]

The graph above is also interesting: cycle life versus discharge, as I read it: after 1000 x 100% discharges the capacity is still about 95%......

[ebaugh]

That chart appears to be based on a logarithmic scale. See the attached example from Wikipedia with the intermediate tick marks they left out. Looks more like 6 years at 35C, but very sensitive to temperature.

[Singleprop]

Thanks for that, my eye-balling or ball-parking was a bit off.

I assume that the data is based on the battery bank being kept "topped up" - could be a once a month top up or as required, its not clear how they keep the 100% charge for many years...

If my memory serves me correct - keeping a LiFePo4 at 100% charge is not optimal - thats why they are delivered at approximately 50% charge. As per some estimates: 40-90% charge seems to be better for longevity - and cycling the bank also seems to increase the service life.

So the 6 years @ 100% charge is actually not applicable to our use and is way off compared to our continuous cycling of the house bank.