Long range lithium conversion

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Joe T
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Postby Joe T » Mon Nov 09, 2009 1:40 pm

I may be up for some if your letting some go.

Cheers

Joe

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timpootle
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Postby timpootle » Wed Nov 11, 2009 7:52 pm

Night Train wrote:I haven't got a motor, controller or charger yet but....
:lol:


Don't make the same mistake I did - buy your cells last. This chemistry has a reported calendar life of 5-8 years, whether you use them or not.

You are welcome to try mine in place if you want to design your crates, but don't buy any yet.

HiPower say their cells can be used in any orientation except upside down, as it says obliquely in the thread you linked. I also believe the bit about the warranty being dubious. I just hope I don't need to find out.

PM sent to Joe T
Tim Crumpton

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Night Train
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Postby Night Train » Wed Nov 11, 2009 11:24 pm

Cheers Tim,

I may come and have a look at them to get an idea and feel of what they are actually like in the plastic. I've made a mock up so I can see if it is the same in reality as they are off the internet.
Image

Is it just the shelf life that is short if not in use? Would regular monitoring and charging make a difference?

I may be interested in sorting out a 24-36v pack for a little project I am working on but that could be sealed LA at a lower capacity.

Bentzon
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Postby Bentzon » Thu Nov 12, 2009 4:44 pm

I think quality lifepo4 will have a longer calender life than 8 years if stored correctly. Not sure what the correct way to store is tho some say 50% discharged in the freezer or at least cold temps.

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MaryRCrumpton
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Postby MaryRCrumpton » Thu Nov 12, 2009 6:23 pm

Bentzon wrote:I think quality lifepo4 will have a longer calender life than 8 years if stored correctly. Not sure what the correct way to store is tho some say 50% discharged in the freezer or at least cold temps.


I am now trying to imagine how many freezers Tim would need for all of his batteries! :lol:

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timpootle
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Re: Long range lithium conversion

Postby timpootle » Fri Jan 29, 2010 2:08 pm

Progress! Well, a bit, anyway.

I have a shelf of bent steel, to build a fiberglass front battery box into, for 30 cells under the bonnet.

I have collected my BMS from Russ in Leicester. Playing with it on the bench now, and will be fitting all the wires into the car very soon. Off work ill today, and not well enough to play outside or with sharp tools.

Will post pictures when I have the brainpower to work a camera.
Tim Crumpton

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Jeremy
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Re: Long range lithium conversion

Postby Jeremy » Fri Jan 29, 2010 6:50 pm

I'd be careful about storing unused LiFePO4 cells. I've had about 8 LiFePO4 cells fail as a consequence of my own stupidity, so this tale may be of interest to prevent someone else making the same mistake.

I assumed that the cells, as delivered, would be sufficiently well charged to be OK for a while without further attention. I was wrong. When I checked a few weeks ago I found that a lot of the cells that I had had in (reasonably cool) storage for about 8 months were perilously close to death (around 2.8 to 2.9 volts) and 8 were already dead. I've been going through the remainder of the 64 cells, cycling them one by one, and have found another 4 cells that may be suspect (these were cells that were sat at around 2.6 - 2.7V). Luckily, most of my cells seem to be recovering, although many seem to have a permanently higher Ri than they should have. From my understanding of LiFePO4 chemistry this is to be expected from any cell allowed to go out of limits - these cells are now permanently damaged.

The moral of this story is to frequently check the terminal voltage of stored cells, keep a record of each cell and partially charge any that look like they are dropping below 3.2V. I've discussed this with others, who have reported much the same experience, with a range of cells from different manufacturers.

Jeremy

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ChrisB
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Re: Long range lithium conversion

Postby ChrisB » Fri Jan 29, 2010 7:49 pm

Thanks for that tip Jeremy.

This is pesumabley through internal discharge ??

Roughly how long would you think a cell can stand before you should check it or do anything with it, and does it make a difference depending on capacity ??

ChrisB
I reject reality and substitute my own !!!!!!

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Jeremy
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Re: Long range lithium conversion

Postby Jeremy » Fri Jan 29, 2010 10:06 pm

To be honest, I just don't know, Chris. I know that the Ping pouch type cells I have on my bike hardly lose any charge at all with storage, as I didn't use the bike for about 5 months, yet it charged back to full in about 30 minutes. Based on this experience, I didn't worry about self-discharge for the other cells.

The big cells weren't charged by me, I just left them in their packaging until a few weeks ago. My big mistake was to assume that they would be charged when delivered, but I was obviously wrong.

The curious thing is that the amount of charge each cell is taking to reach full is varying enormously. I've had some cells take around 1.1C, yet others have reached full charge after about 0.4C. This seems to show that there is a great deal of variability in the state of charge of cells when they leave the factory, or, perhaps, a big difference in self-discharge rate between cells. Those that I've cycled so far seem to all be within about 5% of stated capacity, at a discharge rate of about 1C. I've got a wide variation in Ri though, with those cells that were allowed to get down to the lowest voltage having an Ri around three times greater than that of the cells that were closer to 3.2V.

For the electric boat that these cells are going in this doesn't matter too much, as the pack discharge rate won't exceed about 0.12C. For an EV that uses more peak power, like a car, though, it'd be a serious problem, as the cell losses would be fairly high at high current demand.

Jeremy

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timpootle
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Re: Long range lithium conversion

Postby timpootle » Sat Jan 30, 2010 3:27 am

Yes, thanks for the advice, Jeremy.

My cells have survived remarkably well. They were delivered to me in November 2008 with 3.33v and I have checked them at 6 month intervals, with no loss whatsoever.

I hope to get them in the car before needing to charge them on the bench.
Tim Crumpton


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