Duff/weak cell?

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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Sat Oct 01, 2011 11:29 am

I see what you mean, just because the pack temperature hasn't got very hot, doesn't mean it's not going to at some point :roll: Now I've got loads of space in the PIC18F2680 I'll put some code in to cut the charge if the temperature gets too high (what would be considered "too high"?). As for a cell going to 0V I presume this would shut down the slave PIC in which case I'd get a serial error, which will cut the charge. The 5A bulk charge is "one time only", once one cell has got to the predetermined limit it will only ever charge at the lower current thereafter.

The problem isn't entirely one of balance, that cell still charges quickly (and is the one that bounces up to the top and restarts the charge when it drops down again), and it still drops to 3.1V or so under acceleration. So when I get the time (which is in relatively short supply at the moment :( ) I will be taking the pack out again to do some more investigation.
Ian.

A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....

GregsGarage
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Re: Duff/weak cell?

Postby GregsGarage » Mon Oct 03, 2011 9:39 am

Check your cell data sheet for operating temperatures, typically charging is allowed between 0 - 45 C. Don't forget to add a low temp cutout as well. Lithium must not be charged below 0 degrees C.
Greg Fordyce

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http://www.evalbum.com/4191

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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Mon Oct 03, 2011 11:07 am

Must admit I hadn't thought about not charging below a certain temp either - something else to check :) It appears that Headways are 0-45° - guess I need to allow some "headroom" for the high temperature as my temp sensor is in the centre of the pack, so may not pick up a cell heating up too much around the outside.
Ian.

A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....

mattcarr
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Re: Duff/weak cell?

Postby mattcarr » Mon Oct 03, 2011 2:26 pm

Greg - is that true that Litium batteries do not like to be charged when below 0°?

Could I still discharge them and hope they warm up enough with use so that I can put them on charge afterwards?

GregsGarage
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Re: Duff/weak cell?

Postby GregsGarage » Tue Oct 04, 2011 8:19 pm

mattcarr wrote:Greg - is that true that Litium batteries do not like to be charged when below 0°?


Yes, apparently bad things happen to the cells when charged below freezing although there doesn't seem to be much info out there as to exactly what bad things will happen. :?

Could I still discharge them and hope they warm up enough with use so that I can put them on charge afterwards?


That's my plan for this winter, charge the cells right after driving. The discharge should warm them up a bit.
Greg Fordyce

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http://www.evalbum.com/4191

Beemer
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Re: Duff/weak cell?

Postby Beemer » Tue Oct 04, 2011 8:23 pm

GregsGarage wrote:Check your cell data sheet for operating temperatures, typically charging is allowed between 0 - 45 C. Don't forget to add a low temp cutout as well. Lithium must not be charged below 0 degrees C.


Fascinating statement.
What, in any form of Li cell shows a change of state relating to the temperature of freezing water at sea level?

The electrolyte in laymans terms is coke, alcohol and prozac in solution. No water.
Whatever spec sheet I look at, the minimum temp is around -20C. Accepting the charge quantity is reduced by around 20% and the ion transfer rate is more leisurely; depending on chemistry.

The guy in this video knows.
http://www.ri.cmu.edu/video_view.html?v ... enu_id=387

His advice, if the cells are cold, then nail the throttle hard for a few seconds to bring them up to life, lol

GregsGarage
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Re: Duff/weak cell?

Postby GregsGarage » Tue Oct 04, 2011 10:44 pm

Beemer wrote:Whatever spec sheet I look at, the minimum temp is around -20C.


Is that for charging or discharging? I have yet to see a spec sheet that allows charging below 0C. Please post link to your spec sheet.
Greg Fordyce

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http://www.evalbum.com/4191

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hohisilver
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Re: Duff/weak cell?

Postby hohisilver » Tue Oct 04, 2011 11:30 pm

Interesting. I'm beginning to wonder if I'll need to take the pack out again after all. My charge completed to 87.5V tonight without that cell "running away" like it used to, and it doesn't seem to be dipping as badly as it used to either.

Cells becoming "conditioned" or just a result of better balance?
Ian.

A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....


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