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Old 29-07-2014, 16:06   #16
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Re: Small LiFePO4 (<100 AH) + Echo Charge for Windlass

Originally Posted by evm1024 View Post
The primary risk of fire etc (in LiFePO4) is associated with over charging. Over Drawing the battery down too many times more likely will be a wallet event. Some will argue the safety but that is moot in my mind (plenty of LA explosions with the resulting acid mess) if you stay in the safe operating envelope of the device.
I was thinking about this a bit, and started to wonder: if you have a slow sort of charger setup (example: 100W solar panel), and you try to overcharge a LiFePO4 with that, does it just act like a 100W resistor, and so long as the battery can dissipate that much input heat load without reaching the venting temperature/pressure combination, it just sits relatively quiescent albeit warmer? The charts like this ( seem to imply that LiFePO4 starts turning excessive charging energy to heat instantaneously vs. storing it as energy or stripping lithium metal out. The EV guys with their kilowatt rated chargers have issues still as it's a lot harder to dump that much heat out without raising the cell temperatures but that's not the use case for small marine house banks.

Does this sound completely out of left field?

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Old 30-07-2014, 13:46   #17
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Re: Small LiFePO4 (<100 AH) + Echo Charge for Windlass

I'm no expert so let me quote t1 terry from the house bank thread post 631.

A lithium ferrous cell is fully charged at 3.4v, anything higher than that generates heat, the heating is simple maths stuff, voltage above 3.4v x amps = watts. At the constant current of 3C into a 100Ah cell would be 300 amps, at 4v it's 0.6v over fully charged if all the current had been accepted into the cell, 300 x 0.6 = 180w, how long would that take to boil the electrolyte in the cell and cause gassing? By stopping the high current at this point the charge can distribute through all the cell packs and cells in the string if paralleled. At a 5amp or 10 amp charge there is much less heating, 10 x 0.6 = 6w, sustained for a very short term, not much heating so little damage, held there for hrs, the electrolyte will still boil and the cell will vent. Increase the voltage and the problem gets worse, at around 4.5v lithium plating accelerates, then big irreversible damage starts to occur.


In addition to Terry's comments some of the papers I've read are talking about the growth of Iron "spears" in overcharge conditions that eventually short out the cells. They are small in crossection so have a high resistance and thus cause heating and increased self discharge.

Another note: the study you site appears to be a "progress" report that states that the study period was only 10% complete. With a completion of 2014. Is there a final paper?


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