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How do you store and manage your electricity?
Nevilleh
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Post by Nevilleh »

coulomb wrote: Neville,

per this post, you're supposed to be getting SE cells with some 0.6 mR internal resistance, or 24 mV/C. That's radically different to what has been measured above.

Our cells are at Brisbane port, so yours may not be far off.

I don't suppose you've actually got yours and had a chance to make an IR measurement, have you?


I have an arrival notice that says they should be available for collection next Tuesday.

The QC report they sent quotes internal resistances of around .55 - .6 milliohms and capacities of 47 - 48 AH measured at .1C discharge.

I'll be very interested indeed to see if those figures stack up!

They also quote an open circuit voltage of 3.3v at the state of charge the measurements were done at (and 20 deg C ambient).
That means that if I suck 200A out of one, the voltage drop should only be .12v ie the terminal voltage will be 3.18! Believe it when I see it....
Nevilleh
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Post by Nevilleh »

weber wrote:
I'm starting to think their test figures are not internal resistance at all (as stated), but possibly internal impedance (at say 1 kHz).
[Edit: spelling]


Yes, that could be the case.
The test report says:
"internal resistance by AT520SE Alternating Low Resistance test" whatever that is.
Edit:
I just Googled AT520SE and found it to be an ac milliohm meter - amongst other things. So it IS measured using ac.
Last edited by Nevilleh on Sat, 11 Jul 2009, 08:12, edited 1 time in total.
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weber
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Post by weber »

Well spotted. And yes it is at 1 kHz. 10 mA max.
http://www.applent.com/products/detail. ... el=AT520SE
Completely irrelevant to EV applications where the impulse loads are applied for seconds, not milliseconds.

It seems it can display only the resistance component (not reactance or impedance) as it says it can be used to measure supercap ESR.
[Edit: Spelling]
Last edited by weber on Sat, 11 Jul 2009, 08:51, edited 1 time in total.
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Nevilleh
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Post by Nevilleh »

Hmm
If it's measuring capacitive reactance of .5 milliohms at 1 KHz, that works out to be .32 Farad! Rather surprising.
I suppose the only value that measurement has is as a means of matching cells.
Can't wait to get my hands on them and try some real measurements.
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weber
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Post by weber »

Here's another low-cost high-current test-rig idea.

Once you have the magic number of 3 cells to test (they don't have to be the same brand, only approximately the same capacity) you can use 2 of them as the charging source for the other one. And looking at it the other way, you can use one of them as the load for simultaneous discharge tests on the other two. All at very high currents. Then you only need a slow charger.

Of course you still need something like the previously-mentioned home-made water-cooled copper-wire resistors in between, but these don't have to dissipate as much power as they do as sole loads. And of course you still need short-circuit protection, isolation and a current-measuring shunt.
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Post by Nevilleh »

That's a good idea! I have been using a big coil of resistance wire with a computer fan blowing on it, but it is about .05 ohms so the current is only 60 or 70 A.
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Post by jstan »

I have a Camira station wagon with 45 thundersky 90AH LI-ions and am considering buying another 45 40AH set to run in paralel under the bonnet, let them charge from the main pack's charger and then rig the BMS to cut them off when a cell reaches 4.2, any comments.
Some figures from my dash voltmeter, fully charged 150 V, after driving 10 KM =148.9V, 20 KM =148.2V, 30 KM =148.0V, 40 KM =147.4V, 50KM =146.3V, 60 KM =144.3V these readings are after letting the car rest for one min.
I normaly pull less than 100A and try to never exceed 200A.When pulling 150 to 200 Amps the voltmeter reads 135 to 140V at start of trip and gets down to 125 to 130 after 40 to 50 KM. Is all this normal.
I am a complete novice and at my age I don't think I can absorb much detail such as internal resistance etc.
Thanks     John
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Post by antiscab »

Hi John,

125v @ 150A from 45 90AH cells is fine.
What you describe is normal.

Matt
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Post by zeva »

jstan wrote: I have a Camira station wagon with 45 thundersky 90AH LI-ions and am considering buying another 45 40AH set to run in paralel under the bonnet, let them charge from the main pack's charger and then rig the BMS to cut them off when a cell reaches 4.2, any comments.

So long as you keep the same number of cells in your parallel string, it'll work fine - they'll share the load proportionately.

The downside is, since they'll be in a separate part of the car you can't directly parallel each cell individually, so would just be paralleling the string end-points and would need a second set of BMS modules to monitor each 40Ah cell too - bit of an extra expense. They can all work with the one BMS master unit though (if you're using the EV Power one? I forget)

The other option which comes to mind would be adding say 20x 90Ah cells in series, giving a 208V 90Ah pack. There are advantages to this in terms of performance, though you'd have to make sure all your HV components were OK with the higher voltage. I guess the main issue (OK more like showstopper) is that there are no 208V+ DC controllers on the market anymore! Image (Maybe I'd better build one..)
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Post by acmotor »

IMHO

The TS cells seem to fail to zero V (s/c) so you would be safer to parallel only at the full pack level, not the cell level. Otherwise a good cell dumps into a faulty one, kills both, or causes physical damage in the way as well.
If one cell goes s/c in a string, the other share the voltage change i.e. 1/45th of 3.2V in your case. This they will do happily. The BMS will let you know if there is a problem but generally nothing catastrophic will occur.

So yes you need two sets of BMS. Treat them as two separate strings of cells. I would also isolate when not charging or operating vehicle.
Possibly even diode isolate during charge otherwise the good cells can pick on the faulty ones and without a spanner handy you'll have losses !

The thing with batteries is it is all fine while the cells are in good condition, however you must allow for the day when they start to die !

Hoop's 208V idea is good, if the system is up to the extra voltage. Image
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