Milanbeh said:
Thanks. On what basis would you reject the cells?
If you are referring to what cells you should reject on other than obvious reasons (physical damage, rust, etc) than my first criteria would be IR.
These are the criteria I use. Others may have different opinions.
Anycell withmore than 100m? IR is cause for total rejection.( Always use a proper 4 wire kelvin 1kHz Ac m? meter)
Any cell with less than 80% of its rated capacity is cause for rejection in a power wall build. (they may be used in other applications)
At 1 amp all cells should heat up?
Absolutely most cells will heat up a littleat that charging rate. How much should it, that is the question.Any decent low drain cell will have a "Standard" charge rate" and for most of them its around 0.5C. Example ICR18650-26D Standard Charge rate is 1300mA and standard discharge rate is 520mA
Maximum Charging is 1C 2600mA and Maximum Discharge is 2C 5200mA (all this info can be found in the cell database).
I agree with Sean to a certain point Charge at 1A and Discharge at 1A is a quick way to weed out certain cells.
I do look at the powerwall cell build just a little differently. I believe the standard discharge rate should be your criteria in the cell selection and in sizing your powerwall. Does that mean you will never go above the standard discharge rating of the cell? Of course not the cells maximum discharge rate is that buffer for that induction motor startup current (as long as your inverter is up to the task).
Ie. How would you reject these without a thermal imaging system. 1 off the 4 cells are hotter than the other?
Without a FLIR type system this would be hard to asses.
That is a problem but you can get a simple point and shoot laser temp gun. They are pretty cheap i've seen them on Amazon for ~$15.00
The hand method works pretty good at least in determining a really hot cell
Chart which shows the temp of an average cell going through the charge and discharge cycle. Notice the charge temp doesn't rise much above ~30C except at the end. These where LGDBB11865 cells. Standard charge rate of 1275mA and Standard Discharge of 510mA.
During discharge the max temp was ~37C. During charging the temp was lower than during discharging even at a lower discharge amperage.
That is the typical cycle of a reasonably healthy cell.
Or am I on the wrong path and rather look at capacity and holding voltage ONLY.
Looking at capacity is a good indicator of a good cell when comparing it to rated capacity.
Don't make your 2000mAh capacity your final criteria. Take into account what the initial capacity of the battery was. As in a LGABD11865 which is a 3000mAh cell if your cut offis 2000mAh that basically means you have 66% of the cells capacity left. That particular cell would need to have at least 2250mAh to make it in my world.If you have a LGDS318650 which is a 2200mAh cell with 2000mAh you still have 90% of the cells capacity. So my Criteria is at least 80% of rated capacity. In my personal instance my cell mAh rating criteria is 2200mAh. So if a 2200mAh cell tests out at 100% its abonus.
I have been checking voltage after 1 to 2 weeks in storage and keeping capacities over 2000mah after charging.
I am checking cells at a minimum of30 days after initial test. It took 30 days to build a backlog of tested cells so now I try to do 100voltage and IR rechecks every day. (Doesn't happen all the time) :dodgy:
Wolf
