Quote:
Originally Posted by MetroMPG
I had another specific suggestion that it may be advantageous to remove the sticks for testing specifically to keep tabs on which cell/s is/are getting particularly hot during charging. Might be a useful data point to have.
|
Kind of ...
It can be useful yes ... but not by itself ... To be useful you would have to combine the temperature per cell data with other data for it to be useful.
Going down to the individual cell level can be beneficial ... but it takes significantly longer ... when 20 sticks can easily take a week or so to test ... going down to the individual cell level could easily end up taking 6x longer ... potentially useful ... but a significant increase in invested time... generally speaking ... my own two bits is that if you are not willing to put in the additional time to quantify the important variables ( Capacity Wh or Ah, Resistance Ohms , Self-Discharge Rate ) for each individual cell ... if you aren't going that far than it isn't worth going past the 6 cell subpack stick as a unit level... but that's just my personal preference.
Individual cells out of a 6 cell subpack stick can get hotter than other cells in the same stick because of more than one reason.... such as:
- Differences in internal resistance ( this would be useful information to have ... if you can quantify it and rule out the influence of other factors. )... otherwise identical cells with more resistance will generate more heat... and thus get hotter over the same period of time.
- Differences in SoC ... even with everything else identical including internal resistance ... if cell A is at 80% SoC and cell C is at 60% SoC ... cell A reaches 100% SoC first ... it will be converting all the charging current into heat while the cell C catches up charging up it's 20% SoC difference.
- Differences in Capacity even with everthing else identical ... first cell to 100% SoC ... converts any additional charging current into heat , while the other cells catch up.
- Differences in heat dissipation ... even if everything else is identical ... the two cells at the ends of the subpack stick have more exposed surface area with which to dissipate their heat to the ambient ... meaning even if everything else were identical , due to the increase exposed surface area of the end two cells , we would expect them to be slightly cooler in temperature.
Unfortunately everything else will not be identical ... you can have any one of or all of those 4 influences happening simultaneously ... SoC difference effects influencing Capacity differences effects influencing resistance differences effects influencing differences in heat loss area effects... all you will see is a temperature ... without additional data you don't have a way of knowing what is the cause... thus without being able to narrow down the cause of the difference in heat it is useless information by itself.
Quote:
Originally Posted by MetroMPG
Question (pardon my noob-ness):
|
Nothing to pardon ... It is just a hobby / interest for me ... I see no good reason not to share some of what I've learned with others... So feel free to ask anything you like.
Quote:
Originally Posted by MetroMPG
Good idea. But since voltage begins falling as soon as the discharge begins, at what point do I record V to get a best estimate? 1 second after starting to discharge? 10?
|
The easiest is as soon as the rate of current stops rampping up and has stabilized at whatever number of amps you are using... this is usually a very short period of time.
Technically there will be some other influences ... due to a non-ideal battery ... but I wouldn't worry about it.
The purpose of the testing it not to specifically determine the resistance to be accurate within +/- 0.000001 mOhms or whatever ... It is just getting a reasonable estimate ... and as long as your testing methods are consistent than you can compare your results to the results from any other test you have done the same way ... and the errors rates should be nearly identical and for all practical purposes cancel out.
Different test methods will give different results for the same item ... so keep it as consistent as you can , within reason.
- - - - - -
Glad to hear things are going well with the testing.
Looks like more than a 2.8 Ah SoC difference had built up for whatever reason ... from the BCM point of view that is effectively about a ~40% loss of usable battery capacity.
As you get more testing data in , it will be interesting to see how much actual discharge capacity Ah difference there is between the 20 subpack sticks ... from the strongest to the weakest ... I strongly suspect they will be much much small of a difference in actual usable discharge capacity than the 40% caused by the ~2.8Ah SoC difference.
Good stuff
