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Old May 25, 2012, 04:30 PM
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The importance of characterizing performance of new lipolys

Based upon published test results in the Lipoly performance database, I recently purchased 4 new 3S 2600mAh lipolys. Upon receipt, I performed my normal characterization tests consisting of individual cell voltage check, individual cell internal resistance (IR) measurement, and baseline capacity measurement. These are very simple tests that only require my iCharger and a simple discharger consisting of an automotive taillight bulb.

I then performed a few break-in cycles and placed these lipolys into service. I was so thrilled with the performance these four packs that I ordered four more 8 days after making my initial order. I then proceeded to do my normal characterization tests on the second four as outlined above.

I was initially surprised when the 'as received' individual cell IR on the second batch of four was higher than my first batch but very consistent, indicating cells of a different pedigree. I then performed baseline capacity measurement with my iCharger and all four packs had significantly reduced capacity when compared to the first four packs that were purchased 8 days previous, clearly confirming that these packs were very different. The below CBA discharge curves further demonstrate that the second set of 4 packs were clearly from a different lot (or manufacturer altogether) than my first 4 packs. (Note how both the first and second sets of 4 are so consistent with one another).

The lessons here are that it is very wise to perform basic measurements with available equipment prior to placing new lipolys into service, and that past success with a certain pack or manufacturer is not necessarily a reliable predictor of future performance.

Also, when considering the purchase of a new charger, strongly consider one that is capable of performing basic battery diagnostic functions consisting of capacity verification and individual cell internal resistance measurement.

Mark
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Old May 25, 2012, 05:38 PM
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Interesting confirmation on something we have always said in the past.
Come'on Mark, give it up... What brand are these?

Rick
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Old May 25, 2012, 05:47 PM
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Come on Mark, Give.
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Old May 25, 2012, 06:41 PM
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Gents, can I suggest Mark not tell us what these are, at least in this thread.

I think the theme of his post is clear. The manufacturer and label are no guarantee of consistency. Other such well documented examples as this would be useful data about that; a brand comparison is not. The LiPo performance database thread is a good place to put that, and I'm sure he will if he establishes the results are systematic and not just a bad batch of cells.

A soon as he tells us they are Brand X we will just have another "Brand X is crap, I always buy Brand Y" bunch of postings or "I have flown Brand X for 300 years in my Super Whizz Bang and they always perform well." (Not from you I must emphasise - but I have seen it happen so often it is now almost a fact of life.)

IMO Mark is tenaciously trying to bring some sensible simple techniques to the average flier to make it easier for us to understand our LiPo performance. Unfortunately battery brands attract the same emotional response as computers and cars. Almost invariably, threads on performance attract the more-heat-than-light crowd.

Maybe this one could be an exception?

John
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Old May 25, 2012, 07:50 PM
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Right you are John.

I deliberately omitted the specific make and model of the subject packs as this is not salient to the conversation at hand and in fact would detract from the point of this thread. This point being - there is lot to lot variation among ALL lipoly manufacturers and with a few simple measurements when new packs are received, the consumer does not need to be a victim of this variation. Case in point - I am no longer the owner of the second four packs. Armed with the above data, I contacted the vendor and explained my findings and was cheerfully provided a full refund.

Consumer wins! Be a winner, check your packs!

Mark
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Old May 26, 2012, 05:43 AM
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And you do not intend to publish results of the second batch in "Lipoly performance database"?
It's not good for an average modeller this time?
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Old May 26, 2012, 06:29 AM
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How about the physical dimensions and weight? Is the last batch identical to the first batch?
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Old May 26, 2012, 12:15 PM
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I was very curious as to whether there were any physically measurable characteristics that would indicate a cell switch as well so I broke out my postal scale and caliper and measured to the nearest gram and .001". Interestingly, the measurements between the two batches were essentially identical, pointing to the same manufacturer but different lot or chemistry.

Mark
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Old May 26, 2012, 12:24 PM
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I notice your graphs Mark.

I have seen the small whoop-de-doo at the start of the constant current graphs many times. This is apparent on the first batch (the better ones) I have not seen the whoop-de-doo toward the end of the graphs before like on the bottom 4 tests (the worse latest batch)

In my experience niether graphs not the IR tests lie , but can anyone explain the two whoops and why they are and/or why they are where they are?
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Old May 26, 2012, 12:28 PM
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Side question: What current was used for graphs?
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Old May 26, 2012, 12:40 PM
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I have seen similar bumps on graphs in the past and am at a complete loss to explain exactly what is going on. It's a characteristic of the electrochemical conversion process during discharge but exactly what is happening is beyond my level of comprehension. I merely accept it as 'that's the way it is'.

Graphs were made at 2C (5.2A) discharge current.

Mark
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Old May 26, 2012, 12:43 PM
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Looking forward to seeing how these two different "batches" compare to each other in the future. Nice job.

Oops, I see you may not be able to keep them.
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Old May 26, 2012, 12:44 PM
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I have received several PM’s regarding the specifics of my method of characterizing new lipolys. In addition to answering the PM’s, I decided that I would post it here for discussion as well.

Here’s what I do. It’s really quite simple:
  1. I will first measure voltage of each cell within the pack with either a multimeter or one of my iChargers to ensure that each cell is between ~3.75-3.9 volts.

  2. I then perform individual cell internal resistance measurement with my iCharger to make sure that the pack will perform as expected. This is covered in the iCharger manual but the easiest way is to: 1) Connect pack, 2) Press and hold ‘Stop’, 3) Press and hold ‘Start’. I then compare this number to that obtained by the Lipo performance calculator: http://www.jj604.com/LiPoTool/

  3. Lastly, I perform a balance charge to 4.18V/cell, discharge to ~3.73 V/cell resting voltage and then charge again at 1C to 4.18V/cell and note capacity. I then divide this by .8 to get a reasonably accurate representation of total pack capacity.

I perform these simple tests prior to swapping connectors and have made various conversion adapters to allow such so that if the packs need to be returned, there is no objection that they have been ‘modified’.

If all parameters are within expectation, I will then place the pack into service. If something is awry, I contact the vendor.

Mark
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Old May 26, 2012, 04:35 PM
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Well done.
Thanks for the recent data.
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Old May 26, 2012, 04:41 PM
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And thank you for confirming my reason for delaying the publishing of my data as expressed in posts #4 and 5 above.

I was hopeful that the desire to keep this thread solely about the importance of performing simple tests on packs upon receipt would be honored. Alas, to little avail.

Cheers,
Mark
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