Interesting lipo question for the masses...

[kulangflow]

Howdy all. I made an observation and I'm interested in your analysis.

I have two semi-equivalent lipos.

1) TRC 2s 15C 8k (10 cycles old)
2) MA 2s 20C 8k (5 cycles old)

I received both packs within a week of each other and tested them individually for voltage drop over a period of time in a specific vehicle on a specific course. They tested so similar that I couldn't tell one pack from the other. Having heard that the cells come from the same source, that's pretty easy to believe.

Anyway, today I charged them both up, threw them in my E-Revo with an MM/CCBEC/9XL geared 24/65, and ran them down to a custom LVC of 12.8V. Both packs were a tad warm, but nowhere near hot. I lost my temp guage, but they were cool enough that I could've put them on a sleeping person without waking them up.

When I brought them in, this is what I found for the individual cell voltages:

TRC - 3.70, 3.66
MAX - 3.26, 3.44

If these truly were identical cells, then that would be really badly out of balance. Is the TRC pack just not delivering as much capacity or is it holding its voltage much better? The TRC pack does have 5 more cycles on it, could that be why it's performing better?

After charging, this is how much mAh was put back into each pack and the resistance shown by my 0610iDUO:

TRC - 7664mAh - 16mOhms
MAX - 7325mAh - 17mOhms

Anyway, I'm interested in your thoughts and analysis.

Thanks,

Brian

[Sammus]

Originally Posted by kulangflow

they were cool enough that I could've put them on a sleeping person without waking them up.

Lol thats an interesting way of putting it. Did you test that hypothesis? :P

On topic though, im not sure if theyre the same cells or not, but it sounds like the max amps ones are more worn (less capacity that is)

Lipo's have a very short cycle life compared to other chemistries, I've heard they drop to less than 70% of their original capacity even after 50 or so cycles at the kind of discharge rates we use them at - compare that to 1000+ cycles for lifepo4 or NiXx cells. Maybe those extra 5 cycles made the difference?

[Just go Play]

The TRC pack looks like it is right about where I would expect it to be (depending on discharge rate). The MA pack appears to have been discharged below 3vpc while giving up less capacity.

Since I know how determined you have been in performing as fair and balanced testing on these packs as reasonably possible I'm assuming that the average discharge rate during both runs was close enough not to have been the cause of the difference you found. This would make me think that the MA packs dropped more voltage under load...

I think you need to run them both again with the eagletree and let us know what you find.

Assuming near equal amp demand in running both pacs, I doubt they are made from the same sells. The TRC's appear to just be better quality cells since they hold their voltage better under load. ET data could add another level of certainty to an evaluation though.

[kulangflow]

Here are the charging logs of each pack. Cell 1 and 2 are the voltages of each cell at the beginning of the charge. Side shows which side of the DUO the pack was charged on. Res is the resistance as indicated by the charger. Peak T is the peak temperature. If it's higher than 75F, it's likely because it was warm when the charge started.

Anyway, you can see with the MA pack that I never even used half the capacity before last night's run.



Here's the EagleTree data of the two packs, along with the 25C MA pack. This is when all three were new. I ran them in my Kyosho 1/8 scale buggy (MM/UBEC/8XL) around a "track" in my front yard for a timed 5 minutes each. I then averaged the volts recorded at each Amps unit. I did the best I could to make the runs as equal as possible and the count at each Amps unit was pretty even, so I think it's a fair comparison.

You can see why I said they are very close in performance when brand new. If anything the MA pack was a hair better, but I'm pretty sure it's not anymore. I'll have to test them individually again and see what happens.

[BrianG]

lol, the "sleeping person test" should become the gold standard by which "cool" is determined! Make it so!

Interesting test. The fact that the TRC packs have a higher voltage at the end of the same run (and assuming the same use) means to me that they are better packs. Especially since the TRC packs charged up more.

If both MA and TRC may be based off the same cell, I wonder if the variations noted are just because of the batch the cells came from?

Although, both packs exhibited an imbalance, the MA packs more so (>4x more).

[kulangflow]

Brian, I threw this over on NK to get their opinions on it too. Just a heads up, as I know you'll see it over there.

[Sammus]

lol you mean your not saying that maxamps are the best packs and cells available worldwide from anyone for any price? wow you'll probably be banned in the next few hours

[lutach]

TRC does not over rate their packs. The 0.04V is not really out of balance for the TRC pack.

[kulangflow]

I went ahead and threw it over on TRX-Revo too just to get more opinions.

[VintageMA]

Sorry if I missed it above, but just wanted to be clear as I get the impression some people are thinking you ran the MaxAmps pack and then the TRC pack.

It sounded to me at the top like you ran the MA and the TRC in series with each other to make a 4S pack - is that correct?

I've stopped buying MA packs since last winter and have been much happier with the Enerland based TP packs - and there is no surprise based on what you are showing and what I experienced that MA overrates their packs by at least 5C continuous, maybe even 8C.

The straw that broke the camel's back for me with MA was when I ran a 3S 5Ah pack with a 72A peak that lasted about 9 minutes - 33Amp average - and when I pulled the battery out it was over 140* and was puffing.

[kulangflow]

I ran them individually for the initial testing. Yesterday I ran them in series in my E-Revo as a 4s pack. Fortunately I didn't pay for this MA pack. They gave it to me for testing purposes along with the 25C pack.

[wade7575]

You should put the same amount of cycle's on the MA pack as the TRC and run them with a series harness and then see what your testing show's.I would like to see if the cells need a few runs to run about the same as a TRC pack with the same amount of cycles pack for pack.I don't think it will but who know's,I also heard on the Traxxas board the Lipo's take 3 to 5 charge's to get them woke up so to say and get the performing at there best.I olny heard this and can not say if it's true.

[kulangflow]

I wondered about that as well. I know it's true that NiMH packs need a few cycles to wake up. It will be interesting to see if the MA pack will wake up as well after a few packs.

[Serum]

assuming both packs have got the same capacity, I would expect the pack with the lower internal resistance to discharge faster than the other. In that case the true-rc pack would be on a lower voltage sooner than the maxamps pack. (there is a 6 percent difference in resistance.)

Bottom line is that the truerc packs deliver better results. i wouldn't use them together again.