A123 Battery Handling Guidelines

[etriker]

http://i180.photobucket.com/albums/x218/biodiesel_2007/a12311s3p40miles.jpg

This is a charge graph on a Hyperion set at 3.6v charge. Some cells go to 3.7v for a short time.

Not that well matched cells from 2006.

 

[wb9k]

http://i180.photobucket.com/albums/x218/biodiesel_2007/a12311s3p40miles.jpg

This is a charge graph on a Hyperion set at 3.6v charge. Some cells go to 3.7v for a short time.

That's a good charge strategy for LFP. That pack is nearly perfectly balanced all the way--very low time at low current.

dh

 

[dnmun]

another neat paper but very dry: (about the SEI and solvent diffusion & degradation at the interface)

http://hal.archives-ouvertes.fr/docs/00/78/82/80/PDF/post_print_JES_2013_Prada_Huet.pdf

 

[dnmun]

this is about the TI bq25070 fast charge IC:

http://electronics.stackexchange.com/questions/71056/does-tis-lifepo4-cv-free-fast-charge-method-reduce-cell-lifetime

and this last one i have no time to read right now: (it is an accepted manuscript on a 10C charge rat lifepo4 cell using lithium titanate and carbon coated lifepo4)

http://www3.theeestory.com/files/1291216405JBBQGb.pdf

 

[dnmun]

this guy did his thesis on expanded foam porous nickel for the lifepo4 electrode because it offered higher collector currents and you can see how much better the lifepo4 performed at 5C and 7C over regular aluminum electrode.:

http://www.electrochemsci.org/papers/vol7/7098753.pdf

 

[Spacey]

Appreciate your quick reply. The pack goes out of balance when left off the charger due to the BMS running off the first few cells which when left for a week will cause a slight inbalance.

If I lowered my charger voltage to say 42.96v which would hold each cell at 3.58v whilst left on the charger...I was wondering if this would force the BMS into balance mode and create heat....stupid question as I just figured it out.....any high cell would get bought below 3.65v and eventually self discharge down to 3.58v so I guess this would work.

Will do some tests on my 12S 1P pack this week.

The problem with an unbalanced pack is that the cells that are NOT out of balance tend to get held at 3.8 to 3.9v by the BMS until the rest of the pack catches up and each cell is held at 3.65v.

With a 20Ah pack the minimal 40ma balancing power of your average crappy BMS is going to take a LONG time holding the high cells at over 3.8v.

But I do have packs that have been left on the charger for over 2 years now (Headway packs) and still hold full AH discharge as they did when new. The BMS holds each 12S 1P pack at 43.8V which means each cell is at 3.65v permanently. Why do I do this? Because leaving them off the charger for days when not used causes the packs to go out of balance which would mean when charging the pack again some of the cells hit above 3.8v whilst the pack is balanced.

Lesser of the two evils.

Maybe I should set the charger to 42.96v which would be 3.58v a cell. Would this compromise the BMS at all?

If you're floating for a long time like that, I would back off the HVC to 3.6 or just below. What should happen during top balancing is that the MAX cells get held to 3.6-3.65 while the other cells catch up, and yes, this can take hours under normal circumstances. A severely imbalanced large pack can take weeks to recover if depending on the balancers to do all the work. However, if your pack noticeably gets out of balance after a good top-off charge in just a couple of days, you have VERY uneven SOH (state of health) among your cells, or some cells with very high self-discharge. You shouldn't be able to get that far out of whack that fast.

dh