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LFP users, who can measure cell's capacity?

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Alex_Brooy
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LFP users, who can measure cell's capacity?

Post by Alex_Brooy »

Hello guys,

I have passed about 25.000 km (Max:70-80km per cycle) on Thunder-Sky TS-LFP90AHA cells. I installed them in May,2009. Today I measured the capacity of my 90Ah. Now I have only 83% of 90Ah ( about 75 Ah). They had been discharged till 1.7-2.0 V, about three times from the moment I started to use them. So, I wonder, what has the effect on such a loss of capacity. Is it Thunder-sky datasheet disinformation (for ex. 1500 cycles instead of 3000 for first LFP versions) OR my mistakes with three deep discharges?

If somebody can repeat my experiment, please measure one of your LFP cells, to know how much capacity it has.
I used multiple 10W resistors cascade to get 16A from a cell at 3.6V. Also I recorded the voltage by digital oscilloscope. But you can use digital voltage meter.

T1 Terry
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LFP users, who can measure cell's capacity?

Post by T1 Terry »

If you read the TS data sheet carefully the 3,000 cycles is at a 0.5C charge and discharge rate to 80% DoD. If you have discharged or charged at a higher rate than that then the 3,000 cycle specification does not apply. The other thing to think about, does the specs actually say that you will have 80% capacity after 3,000 cycles, not that after 3,000 cycles you will still have 100% capacity?

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Alex_Brooy
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LFP users, who can measure cell's capacity?

Post by Alex_Brooy »

I have max. peak discharge currents not more than 150A. Cruising currents are 30-50A.
I actually passed 25.000km, that means 300 full cycles. But actually I don't fully discharge the pack. Hope that only my mistake with a few very deep cycles has caused such a loss.
Any way, I would appreciate if anybody can measure his own LFP cells.

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LFP users, who can measure cell's capacity?

Post by T1 Terry »

Hi Alex,
Did you condition the cells with a 4v per cell charge as specified by Thundersky before you used them? I watch a LFP over view video by Jay Whitacre a professor at CaCarnegie Mellon University and in it he suggests that the initial high conditioning charge could be designed to put a light coating on the anode to prevent further heavy build ups from occurring. It's this coating that robs the cell of capacity and high discharge capacity as the cell ages. How much of that is theory and how much is known fact I can't really determine but the way this guy talks he really does sound like he knows what he's talking about.
If he is correct then the conditioning charge appears to be critical to long life in regards to capacity and electric vehicle discharge rates. The problem is that these cells are so new to the market that a lot of the knowledge has been here say rather than proven laboratory testing so watching the video was certainly enlightening.

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Alex_Brooy
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LFP users, who can measure cell's capacity?

Post by Alex_Brooy »



Last edited by Alex_Brooy on Wed, 03 Aug 2011, 01:44, edited 1 time in total.

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Alex_Brooy
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LFP users, who can measure cell's capacity?

Post by Alex_Brooy »

Yes, of course. I charged to 4.1v. But I didn't check the capacity after buy.
Terry, can you provide the link to professor's video?

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LFP users, who can measure cell's capacity?

Post by T1 Terry »

Sorry Alex, I should have put it in the last post http://www.ri.cmu.edu/video_view.html?v ... enu_id=387

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LFP users, who can measure cell's capacity?

Post by roddilkes »

I have noticed the same thing on two cars now with Thundersky batteries. My EV Mighty boy was about two years old with LFP batteries and the capacity was down from 90Ah to about 75Ah. Admittedly that pack had a hard life and it used very early model LFPs.

My current car I noticed the other day the BMS was complaining at about 80Ah. It uses mostly 2.5 year old LFP90AHA cells. I am not sure if this is just one or two weak cells yet.

No doubt the batteries are improving as time progresses. Winston Chung has left the company he started, Thundersky, and has morphed into Winston Battery. It looks like several law suits are following him though.

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