Sky Energy LiFePO4 Bulk Buy this Week

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Post by antiscab » Thu, 16 Jul 2009, 07:38

the cell degredation data does seem to correlate with my experience.
my scooter cells are at 80% of original capacity at around 550 cycles (equivalent to 80% dod).

however i have over discharged a few (8) times and discharged at 1.4C continuous, 5-6C peak.

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Post by coulomb » Thu, 16 Jul 2009, 14:24

Our 212 SE 40 Ah cells are due to be delivered this morning.
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Post by coulomb » Thu, 16 Jul 2009, 20:32

And here are some of them:

Image   

Image
Each box came with a hand filled in form with various numbers and the cell's internal resistance (as they measure it).

The largest box actually had only 2 cells in it, but also the threaded rods, end clamps, bolts, etc:
Image    Image

First order of the day was to charge one of the 2 "runts" (with significantly higher internal resistance than the others).
Image
We have a very preliminary sag figure: 131 mV/C. This is about 3/4 of the sag of the Thunder Sky at room temperature, and compares with an average of 24 mV/C indicated on the packing lists (as measured with their crazy AC meter).
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Post by acmotor » Thu, 16 Jul 2009, 20:40

Now if only you could just mount that shipping box as is ! Image
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Post by Electrocycle » Fri, 17 Jul 2009, 00:39

yeah it's a pity they don't pack them with every second cell reversed so you can just link them up in series :)
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Post by acmotor » Fri, 17 Jul 2009, 02:14

Not hard to turn half around. I did with mine and looked at them for a while ! Image
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Post by coulomb » Fri, 17 Jul 2009, 02:18

These are the straps that come with the Sky Energy cells:

Image     Image
Note how the straps are laminated (for lower eddie currents Image ), are kinked (so the cells can expand a little without undue terminal stress) and are insulated in the middle. I'm quite impressed.

These Sky Energy cells are not quite as tall as Thunder Sky cells:

Image     Image
(Edit: note the different spacing of the horizontal ribs, yet the same spacing of the vertical ribs.)

Note the brass and aliminium (or steel?) posts on the Sky Energy cells, and the anodised aluminium lock nuts. Not really visible in this photo is that the top of the cells have slightly different details, such as thickness of plastic ribs. Under the Thunder Sky logo, the vent detail is extremely similar to that of the Sky Energy.
Last edited by coulomb on Thu, 16 Jul 2009, 16:21, edited 1 time in total.
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Post by coulomb » Sat, 18 Jul 2009, 02:18

I forgot to mention that these are a bit heavier than the Thunder Sky 40Ah cells. On my kitchen scales, the Thunder Sky weighed in at 1.4 kg, but the Sky Energy cell I measured at 1.55 kg. Both are nominally 1.5 kg, according to the information I can find.
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Post by coulomb » Sun, 19 Jul 2009, 04:34

I feel that my lithium cell information is getting splinched (parents of children of Harry Potter age would understand this term.) Maybe I'll post a summary and links post one day when all the data is ready.

Here is the first of hopefully several graphs of charge voltage for currents of 40 A (1C), 80 A (2C), ... 240 A (6C). This is for the Sky Energy SE40AHA cell marked "best" (it had equal best internal resistance, according to the manufacturer's information.) The time at the bottom is in tenths of a second. The data doesn't end at exactly the same time because the current was programmed off manually.

Image

Rather pretty, if I do say so myself Image . Thanks once again to Tritium James for letting us use the fancy power supply, and driving the PC software that programmed the currents and logged the data.

See also Weber's calculation of lift per C (cf sag per C, but these are for charge, not discharge) in this Li cell discharge tests post.

Edit: realigned 3C data.
Note that the data don't start "in order" because we waited a variable amount of time between tests for the voltage to settle. I think it's largely luck that they are "in order" at the end of the graph.

Edit 2: The data are arranged so that the effect of the current is seen starting at sample 3 (3 on the x axis).

Edit 3: these results are quite temperature sensitive, and unfortunately temperature measurement wasn't done. All cells were at 17.3°C at the start. Ambient temperature was some 21°C. About 10-15 minutes after the test, this cell was at 20.2°C.

The stepped voltage at the start is probably due to some artifact of the way the power supply samples voltage. It seems to always get the same result two samples in a row. I'll attempt Weber's suggestion of taking out every second sample; presumably that would smooth the graph.
Last edited by coulomb on Sun, 19 Jul 2009, 05:25, edited 1 time in total.
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Post by coulomb » Sun, 19 Jul 2009, 16:21

my good self: wrote: The stepped voltage at the start is probably due to some artifact of the way the power supply samples voltage. It seems to always get the same result two samples in a row. I'll attempt Weber's suggestion of taking out every second sample; presumably that would smooth the graph.

Well, it's not actually every second sample that is the same. Sometimes it's 3 in a row, sometimes 4. It possibly depends on how fast the data is changing.

This is what every second sample looks like for the 4C (worst looking, to me) voltage curve:

Image

As you can see, it's still quite stepped. So I think I'll just report the data as sampled.
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Post by Tritium_James » Sun, 19 Jul 2009, 17:23

I'll have another look at our windows software, but I suspect the problem is that you can ask for the latest voltage and current values from the power supply faster than it's actually sampling them.

Apparently the actual V and I control in the guts of the supply is analog, so while the user interface might be digital the micro's not actually running the control loop itself. Sucks a bit, but the word on the street is the guy behind this family of designs is an analog guru, so that's what you get...

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Post by coulomb » Sun, 19 Jul 2009, 18:29

Here is the original Thunder Sky cell (we only have the one) under the same conditions as the Sky Energy SE40AHA cell above. Initial temperature 17.3°C, ambient temperature ~21°C.

Image

As you can see, the lift (antisag? rise?) is greater. In fact, the cell could not take 5C for more than a second without hitting the 4.25 V limit. ALthough we performed a "6C charge", because of the voltage limit, about the same current went in as with the 5C test apart from a few percent more in the first half second.

To be fair, the Sky Energy SE40AHA cell didn't quite take the full 6C charge for 5 seconds; the last 4 samples (0.4 sec) recorded 184 A (to keep within the 4.0 V limit we set for that cell). If anyone has any definitive voltage limits for the Sky Energy or China HiPower cells, I'd love to hear it.

The stepping at the start seems worse here for whatever reason. 10-15 minutes after the test, this cell was at 21.8°C.
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Post by coulomb » Sun, 19 Jul 2009, 19:18

Here is another cell to compare the Sky Energy cells with: High Rate China HiPower cells. This is the test cell that has about 32 Ah of capacity. Here, it is being treated as a 40 Ah cell; if you treat it as a 32 Ah cell, then the charge marked "6C" is really 7.5C. I've tried to scale the graph to be similar to the Sky Energy graph.

Image

The lines are closer together because this high rate cell has a lower internal resistance, and hence lower sag under load, and lower rise under charge/regen. Note: not all China HiPower cells are high rate; these are quite new and few production versions have been made. These high rate cells cost about 7% more per Ah than the standard cells, and are less energy dense. They would appear to be a good compromise in EV applications, fitting between Thunder Sky and Sky Energy SE (lower cost, higher internal resistance), and A123 cells (I believe these have even lower internal resistance, and considerably higher cost).
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Post by coulomb » Sun, 19 Jul 2009, 21:27

For what it's worth, here is the fourth cell we tested. This is another Sky Energy SE40AHA, initially 17.3°C, ambient ~21°C. This cell had the second worst internal resistance (of the set of 212), according to the manufacturer.

Image

We didn't seem to record the initial voltage for the 1C test. Also, we didn't seem to do a 6C test either, possibly thinking that the voltage at 5C was getting too high.

Edit: we didn't record this cell's temperature after the test.
Last edited by coulomb on Sun, 19 Jul 2009, 12:06, edited 1 time in total.
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Post by Powered By DC » Sun, 19 Jul 2009, 21:52

coulomb wrote: If anyone has any definitive voltage limits for the Sky Energy or China HiPower cells, I'd love to hear it.


Per the Sky Energy Specs 3C max Charge current and 3.6v high voltage cutoff


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Post by coulomb » Sun, 19 Jul 2009, 22:02

For what it's worth (not much), here is the data for the final Sky Energy cell charged. This one must have been fairly full as it came from the factory. (Edit: but see next post.) This must have been the first one we tested, before we decided to keep the charges to about 5 seconds. We also had the voltage limit at 3.6 V, so it didn't even take a 2C charge:

Image

The voltage limit was obviously lifted to 4.2 V after the 2C charge.

Some 10 minutes after the test, his cell ended up at 20.8°C, and like the rest, started at 17.3°C with the ambient temperature around 21°C.
Last edited by coulomb on Sun, 19 Jul 2009, 18:45, edited 1 time in total.
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Post by coulomb » Mon, 20 Jul 2009, 04:44

me wrote: This one must have been fairly full as it came from the factory.

Actually, we pieced together the sequence of events, and it's likely that we charged it while doing a discharge test on another cell. So it's probably not inconsistency on the part of the factory, just us not recording in enough detail what was happening. It gets hard remembering the SOC of 5 cells being tested.
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Post by Kearon » Mon, 20 Jul 2009, 23:56

Guess what?
I was all ready to rant about these great cells, but Sky Energy seem to have lost the funds I transferred. No, I'm not kidding....it's been 14 days since the funds left my account. AND SE have confirmed the account details I used were correct.

AND, to top it all off, I've just been told they have since run out of stock of one of the battery types I ordered.

My advice - be VERY cautious. Sure, I'm trusting - and I may have gotten stung this time. Hmm.

EDIT: I've just been reassured by Sky Energy that they're looking into it. Again. A trip to the bank tomorrow to track the $ down may help...
Last edited by Kearon on Mon, 20 Jul 2009, 14:49, edited 1 time in total.
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Post by juk » Tue, 21 Jul 2009, 00:43

If it was an electronic funds transfer, your bank should be able to recall the funds.

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Post by acmotor » Tue, 21 Jul 2009, 03:37

I can't say I have yet had a paypal or EFT go wrong and I purchase many goods both privately and business. In fact the one eBay purchase (nothing to do with the funds transfer) that the seller renigged on was refunded to me by PayPal.

I would guess that the funds may have gone into the SE account without / misleading description and they need to track it down.

Otherwise the payment bounces back if there details don't agree.
It is your bank's responsible if they transfered it to the wrong account.

EFTs are typically not reversable by definition, unless the bank made the mistake, they can do what they like.

Don't delay in following it up though.
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Post by Kearon » Tue, 21 Jul 2009, 09:16

Latest from Sky Energy:
"Sorry for all the inconvenience and bad feelings coming to you. I understand you felt bad about that. So do i. Last week we've consulted the bank, someone told us abbreviation is ok. But actually, they kept the money. I communicated directly with them this afternoon. Finally, figure it out. The accouting department is making application to admit that the funds is ours. The bank said that the abbreviation of INTERNATIONAL is INTL. They won't release the funds to us for the wrong abbreviation. I thought it is caused by different culture and understanding, since Chinese character is very different from western language."

OK - so basically I'm being told the problem is that the Chinese bank did not accept 'INT' as an abbreviation of 'International' - as the Chinese character is quite different for each. (There was only enough room for 'INT' on the transfer form online....)

Since when is it MY responsibility to know how an English abbreviation might translate into Chinese? Not cool.

However, SE have now agreed that it is a communication issue between themselves and their bank. At least the funds haven't gone missing...
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Post by 7circle » Fri, 04 Jun 2010, 09:09

Has anyone considered this battery manufacturer.
International Battery

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Post by antiscab » Fri, 04 Jun 2010, 11:36

they manufacture cells under licence from thundersky.

their cells are expensive (~US$3/ah) and are aimed more at US military applications, where hardware isn't allowed to be sourced from overseas.

not really useful to us.

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Post by 7circle » Sat, 05 Jun 2010, 02:00

Thanks Matt
Made new topic Lithium Mono-Block Manufactures
With reply as it's seperate to this bulk buy.

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