I read some very interesting arguments against what ever problem a cell can have...
Yes we can put pressure sensors heating sensors bms ect to protect a string and what ever system that we can think of...
But arent we forgetting the most important thing?
Cell level fuse?
The fuse will blow when the cell is asking to much aka a self discharger, short circuit or a heater.
This will stop other cells to put more energy into that cell so a heater is out of the question.
The bms will register a faulty string and shut that string down, if the bms does not do this other fuses will blow.
Oke a lot of extra work those fuses, but less work than rebuilding your home.
Or am i missing the point?
Cell level fusing, string level fusing and pack fusing.
Over v cut off, under v protection and one temp sensor to cut off charging below 0C and cutt off charge/discharge above 45C.
With best regards Igor
I believe so, yes.
When a cell is shorted, either internally or externally, it is not the ‘other cells’ that are providing the current that heats the shorted cell up - it is the cell itself.
In fact, no current will flow from other cells as the shorted cell discharges - they will all maintain full voltage and overall battery will decline just because the voltage if the shorted cell will drop as it discharges.
There is no way to stop a shorted cell from discharging using fuses. The best you can hope for is an alarm causing you to check the battery before the situation has become catastrophic.
If a wire has come loose and caused the cell to short externally, you may be able to break the short and stop cell discharge.
If terminal damage or whatever has caused a cell to short internally, there is nothing you can do to stop it. If it has not gotten too hot yet, you may be able to shut the battery down, remove connections to that cell and remove that cell from the battery (watch it discharge in the middle of the yard
, but standing by with a fire extinguisher ;and possibly an infrared thermometer may be the best you can do.
The inter-cell fuses your are proposing might held avoid problems if a loose wire causes a short between cells, but that’s really not the area of concern.
Several members are suggesting that any cell failure / degradation that can cause internal shorting and increased self-discharge will be gradual and cannot result in sudden high-current discharge sufficient generate enough heat to cause a fire.
That’s comforting and goes a long way towards addressing my concern, since with proper design, it should be straightforward to design a battery such that shorting caused externally can be pretty much impossible.
Even though I suspect these aftermarket cheapo cells are sub-prime (not suited for use in EVs), I’m not terribly concerned about them being truly defective (ala defective LG Lithium cells in some Chevy Bolts with folded tabs).
So if substandard cells that begin to fail through accelerated growth of dendrites and increased self-discharge is a gradual process that can be noticed and reacted to by just keeping an eye on battery performance (monthly?), I’m starting to come around to the prevailing view that DIY risk may be low and acceptable (with monitoring).
I’m still extremely uncomfortable with the idea of putting a DIY LiFePO4 battery in someone else’s house were no one will be keeping an eye on it (‘set it and forget it’).