Storing your LFP cells

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4Springs
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Storing your LFP cells

Post by 4Springs » Fri, 24 Aug 2018, 15:49

I've been prompted to give some advice on how to look after your LFP (lithium ferro-phosphate, or LiFePO4) cells in storage. Posting it here means that others can see it, and people more expert than me can pick it apart!
Particularly aimed at those with the Brighsun (ex-AVASS) cells, since some of us might not get around to our projects for a while...

Store LFP cells any way up except for terminals-down. Most people will store them upright. Cold is fine, and possibly extends the life of these cells in storage. Avoid heat, but don't be too pedantic - 60 degrees is seen as an upper limit. Make sure you store them somewhere where metal objects can't be dropped across the terminals!

To keep an eye on their health, run a voltmeter over your LFP cells every now and then. If you don't have one, buy a multimeter from Jaycar or similar. Set it to DC volts and measure each cell.
You can't tell how much charge is in a lithium cell just by measuring the voltage, unless it is full or empty.
Empty: less than 3.0V
Full: more than 3.4V
Your cells are quite likely to be reading 3.25V. This means "somewhere between empty and full".

If a cell goes less than about 2V it will die. A dead cell cannot be resurrected. A dead cell is not dangerous in itself, but trying to charge a dead cell can result in heat and smoke - possibly flames.
Your cells will self-discharge while sitting in storage, thus the recommendation to check their voltage every now and then. Write down the voltage of the lowest cell, and see if it moves at all over time. If it starts to move lower then it might need charging.
The cells were probably delivered about half charged, and this is a good way to be for storage. They will probably last several years before they discharge enough to be a problem.

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Re: Storing your LFP cells

Post by T1 Terry » Sat, 25 Aug 2018, 11:18

Store at 3.2v for LFP cells, the new Winston cells actually come charged to 3.3v but that might be more to do with the build/design changes including both terminal posts now being aluminium and I suspect all the plates are now aluminium as well.
The 3.2v will remain for yrs as long a you don't have any loads connected and the cells are all in good condition. If you plan to leave the battery pack unused for 12 mths or more, disconnect all parallel cell links, just in case one cell is crook it won't kill the other cells.
This advice relates to house battery use and not EV use. As far as house battery use, a lithium cell will recover even if discharged down below 1v, but it can't ever be dragged below 0V without serious damage resulting from such abuse. We have recovered cells down as low as 0.4v and steadily brought them back to life, all be it with a bit of lost capacity and increased internal resistance, but they still work better than any lead acid battery cell so it ain't all that bad :lol:
After more than 7 yrs doing house battery installs and research I am still often amazed by just how much abuse these cells can handle, well at least the old Thundersky and newer Winston cells go anyway. The latest Winston offering are yet another step up, well as house batteries anyway

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Re: Storing your LFP cells

Post by weber » Sun, 26 Aug 2018, 10:37

I agree with most of that advice. However:

1. Do be pedantic about heat. Store them in a cool place. Every 10 degrees increase in temperature causes an approximate doubling in the rate of capacity loss and internal resistance increase. This is approximately true of all batteries, and in fact all chemical reactions, both beneficial and destructive. It's called Arrhenius' law.

2. Recharging lithium batteries after they have spent any significant time below 2.0 volts is a bit like Russian roulette. What happens is that below 2.0 V the copper current collectors begin to dissolve in the electrolyte. On recharging, the copper plates out, but not where you want it to. It tends to grow whiskers or "dendrites" that pierce the plastic separators and cause a short circuit between plates, which can lead to fire. If you must do it, recharge at a very low current, e.g. 0.05C or less, and do it in a location where it won't set fire to anything else if it burns.

3. It seems unlikely that both terminals would be aluminium, and I'm certain that both sets of plates cannot be aluminium. Some terminals are nickel-plated copper or brass, which can look like aluminium.
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Re: Storing your LFP cells

Post by T1 Terry » Mon, 27 Aug 2018, 09:57

" It seems unlikely that both terminals would be aluminium, and I'm certain that both sets of plates cannot be aluminium. Some terminals are nickel-plated copper or brass, which can look like aluminium."

We buff the terminal posts with a scotch brite type pad to remove any oxidising and flatted the surface by removing any burrs etc created when they helicoil the bolt holes, something that occurred at the same time as they went to both aluminium terminals.
I haven't cut one of these cells open because I haven't had one die yet to verify one way or the other, but is there a technical reason why both plates can not be aluminium?

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Re: Storing your LFP cells

Post by jonescg » Mon, 27 Aug 2018, 10:15

The anode is always copper - aluminium is too reactive a metal to be used as the current collector on the negative terminal.
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Re: Storing your LFP cells

Post by weber » Mon, 27 Aug 2018, 10:42

Please post, or link to, a photo of these cells with both-aluminium terminals.

The anode (negative) current collectors cannot be aluminium as their voltage range relative to the electrolyte would cause them to corrode in normal operation, whereas the copper only corrodes if you discharge the cell below 2.0 V.

I read a suggestion that LTO cells (2.4 V nominal) may be able to use aluminium for both sets of current collectors. But LFP and other lithium-ion cells with carbon anodes can't.
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Re: Storing your LFP cells

Post by T1 Terry » Tue, 28 Aug 2018, 14:30

These are new out of the box Winston LYP cells. Note the revised nom. voltage and 660wh capacity yet still only a 200Ah cell. They are actually 3.3v out of the box and you can see the helicoil insert and the thread where it was cut in showing it is in fact aluminium and not copper
Winston 200Ah negative terminal compressed.jpg
Winston 200Ah negative terminal compressed.jpg (57.18 KiB) Viewed 411 times
Winston 200Ah positive terminal compressed.jpg
Winston 200Ah positive terminal compressed.jpg (60.97 KiB) Viewed 411 times
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Re: Storing your LFP cells

Post by jonescg » Tue, 28 Aug 2018, 14:44

They must have an ultrasonic welded junction between copper and aluminium on the inside of the cell. The anode is definitely copper, but the terminals may well both be aluminium.
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Re: Storing your LFP cells

Post by T1 Terry » Tue, 28 Aug 2018, 14:51

In the earlier cell type the plates were attached to the terminal block with a bolt through the middle. I sure hope they haven't continued with that method because the reaction between the copper and aluminium will cause early plate failure due to oxidization causing localised heat and breaking/melting the plate away at the connection. This is the most common cause of capacity loss and increased internal resistance in these cells, I've dissected quite a few dead ones and this is always the first thing you see, plates no longer attached to the terminal block.

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Re: Storing your LFP cells

Post by weber » Tue, 28 Aug 2018, 16:16

Thanks for the photos Terry. It's a little hard to compare the appearance of the terminals because the light and camera angles are different, but the negative terminal looks to have the same colour and mottling as its nut, while the positive terminal (which has no reason not to be aluminium) looks a lighter grey than its nut. So I'm still thinking the negative terminal might be nickel plated copper. You'd have to drill or file it to be sure. Or see if a drop of concentrated caustic soda solution produces bubbles on both terminals. [Edit: To avoid damaging the mating surface, you could drill at the bottom of the bolt hole and examine the swarf, provided you didn't get too carried away.]
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Re: Storing your LFP cells

Post by T1 Terry » Wed, 29 Aug 2018, 07:16

If you look closely at the tread in the top of each terminal block you can see the gouging of the aluminium when the thread was cut and then the helicoil wound in afterwards. It could not have been nickel plated after that because it would have wrecked the helicoil thread. I have accidentally drilled a part of the top of a terminal on a 100Ah cell in another battery pack when using a hole saw to create clearance for the terminal in the rolled over end plate and it was still aluminium in the base of the drill hole so to me that rules out nickel coating.
I have a contact with direct links in Chine who has been in the factory before so I've asked if it is possible to get photos of cell showing the connection between the plates and the terminals. Things don't happen quickly in China unless there is big $$ to be made so it might take a while before I get the photos .... if I get them at all that is :roll:

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