Raised Internal Resistance on Hi-power Lifepo4 Cells

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Rory166
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Raised Internal Resistance on Hi-power Lifepo4 Cells

Postby Rory166 » Wed Sep 03, 2014 9:30 pm

I have several 200 Ah cells which have been fitted in 2 vehicles and some not used. The cells have very different voltage under load or IR if you like. The worst having 7 times the resistance of the best. Even the 4 unused cells. One thing that I have noticed is that the pack from inside the 2009 car has fared much better than the pack from the engine compartment. I surmise that low temperature may be a factor. It would be interesting to know if other brands and chemistries suffer similar random degradation.

It would be logical to assume that cells which have high IR would suffer excessive temperature rise under sustained load although I have not measured such. It may be that Internal resistance has entirely different character from actual resistance.

Rory
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Grumpy-b
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Re: Raised Internal Resistance on Hi-power Lifepo4 Cells

Postby Grumpy-b » Thu Sep 04, 2014 12:06 pm

With lifepo4 cells the term of internal resistance is not really the same as other chemical change cells. Its more a case of lack of flow of current through the cell, rather than pure resistance. If the cells are excessively depleated, or operated at low temps, the internal structure will get plated (copper I believe) and so seals the holes into the crystaline structure into which the ions migrate during charge. Result is that there are fewer ions to migrate back during usage. Result is less flow.
I fell its better to consider how the AH has been impacted, so charging the cells with a known amperage for a known number of hours (from about 2.7v ) is a good insight as to the capability of the cells. Its also a guide as to what you can take out. If you have a 200ah cell in good condition that can take 3c discharge then thats aflow of 600a. If the same cell has degraded to 150ah then at 3c thats only going to deliver 450a.
The cells do vary a lot especially as they get older. That is why I suggested you needed to bottom balance the cells you had and work out by charging and monitoring what state they were in. In my view there is litle point is putting a good cell in parallel with a poor cell, as you will get the worst of both cells, and the good cell will be dragged down by the poor cell, especially if it has any internal shorts and self discharges.
Self discharge is not another ter for internal resistance, the cells can build a small spike of conductive material between the plates, and plates can be physically damaged so causing this to be of more significance, reduce the number of plates in play and the cells will have less voltage . Bottom balancing is also a good way of seeing which cells are self discharging. As part of the process you leave them for at least a week after reaching the target voltage, the lower the current you have discharged them with at the final stage generally the less they will bounce back. But in that week and internaly discharging cells will go down. They are then consigned to the bin. Its just not worth using such cells. Unless the whole pack does exactly the same thing. The you will just have to constantly recharge them altogether.

Grumpy-b

granada203028
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Re: Raised Internal Resistance on Hi-power Lifepo4 Cells

Postby granada203028 » Fri Sep 05, 2014 9:29 pm

Anyone had experience of the 20Ah Lifepo4 poly cells on eBay, Aliexpress etc? On the face of it good prices if the quality OK. Supposed to be A123 e.g:

http://www.ebay.co.uk/itm/A123-LiFePO4- ... 5405c7de2a

http://www.aliexpress.com/item/High-qua ... 67448.html

End of last year I bought 8 off eBay like the 1st above. One cell developed an internal leakage spot and slowly ballooned up. The supplier (not the one linked above) replaced it free but I had to pay postage. Photo shows charge pump balancer. I charge at 28.0V pack, 3.5V per cell. Balancer pulls the cells in within a couple of mV.

Image
Image

Dropped from 3.3V to 2.8V at 200A with some obvious warmth towards the terminal area. So 10C looks realistic but not 20C.

I more recently bought one:

http://www.ebay.com/itm/Brand-New-High- ... 43ceb6b9f9

Behaved similarly but much more friendly full length tabs. Though in USA was posted via some eBay USA to UK postal program. So came quite quick and duty paid up front.

Image

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Rory166
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Re: Raised Internal Resistance on Hi-power Lifepo4 Cells

Postby Rory166 » Sat Sep 06, 2014 12:01 am

Grenada

I have a 400v 60Ah battery made of these. Some posters on sites have tried to put me off using them with tales of conflagration. They appear similar in construction to Nissan's Leaf cells but different chemistry presumably.

Your post would have warranted it's own thread perhaps?

Rory
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Grumpy-b
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Re: Raised Internal Resistance on Hi-power Lifepo4 Cells

Postby Grumpy-b » Sat Sep 06, 2014 11:15 am

Do a search on a123 20ah
They are quite capable of discharging 200a.
But most of these cells (which arnt made in the us despite the markings) that are being sold come from the chinese factory and are graded stock. Probably OK.
They really need restraining in the packaging, and A123 made them into separate packages, / cell blocks that are then assembled into packs. With BMS etc. All the tabs in the packs are welded together, not clamped or bolted so some of the ones you see have very short tabs where they have been cut out of the packs.
DO some searching and you will find lots of stuff on them.


Grumpy-b

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Rory166
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Re: Raised Internal Resistance on Hi-power Lifepo4 Cells

Postby Rory166 » Sat Sep 06, 2014 3:44 pm

Grumpy-b wrote:With lifepo4 cells the term of internal resistance is not really the same as other chemical change cells. Its more a case of lack of flow of current through the cell, rather than pure resistance.
Grumpy-b


Thank you for confirming that. The Internal Resistance is an element in the electrical equivalent circuit which predicts the electrical performance along with a theoretical perfect cell voltage. Both figures will vary with the state of charge and temperature. The electrical equivalent circuit is no predictor of thermal performance.

With small NiCads the small spike of conductive material used to completely short out the cell. It used to be common to use a large charging current of perhaps 20C or more to destroy the spike and restore an operative cell which could then be charged normally. I have no idea if such a technique could work with Lifepo but in any case a 20C current on a 200Ah cell would be something to behold. I recently generated 1000A AC using a couple of Torroidal transformers but 4000A DC would be a lot harder especially at 3.5 V as that would be at least 15 KW.

It seems odd that some cells that have been treated identically suffer from High IR while others do not. If I had to bet I would suspect that quality control in the manufacturing process may be less than might be desirable, handling of the plates before cell assembly for example.

EV is such a new industry that no one knows how cells will behave in the long term. Accelerated life testing is only an idea of what the actual use experience may be. Recent experiments in the US shown on EVTV showed cold temperature to be twice as deadly to cell capacity as high temperature. So keep your EV in an airconditioned heated garage for optimum battery life.
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Grumpy-b
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Re: Raised Internal Resistance on Hi-power Lifepo4 Cells

Postby Grumpy-b » Sat Sep 06, 2014 3:55 pm

Build quality and thickness of the carbon / lithium coating on the plates has a lot to do with it. Think of the plates as having a black surface as if they have gone through a laser printer with a picture of total blackness, thats all they have on the surface.
DOnt try and do 20c with the high power they wont do it, but the A123 will.
Good video on you tube of one being shorted with welding cables. Quite impressive.
The short doesnt grow like the Nicads as it doesnt have the same chemical processes, its more like a lump in the carbon face. But the biggest issue is with plating of the crystal structure of the carbon/ lithium stopping the ions from entering the crystal structure. Over discharge and poor build are seemingly the causes. As is improper initial charge.

Grumpy-b


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