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Old 04-10-2006, 06:37 PM   #16
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Quote:
Originally Posted by v0rtex
My opinion is that it seems you're slightly confused. When cells discharge under race conditions, they dont all discharge at the same rate and have different voltages and capacities after the race.

Throw a pack on a Novak Smart Tray some time after a race. You'll see that not all cells get to the cutoff voltage at the same time, hence different capacities remaining.

can you prove they discharge at different rates? i tested the discharge a single cell for 120seconds at 20amps on different cells and they both gave me the same total mah amp draw.

and yes put it on a smart tray the lights will flick off at different times, that goes to show they have different voltages and run time.
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Old 04-10-2006, 06:40 PM   #17
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Originally Posted by Drewdc90
The only problem with charging each individual cell is that the first cell charged will end up having less charge than the rest because it will sit there the longest. If you charge each individual cell at the same time it will work better than a discharge tray but not if you don't charge them at the same time.
Drew.
i am not saying charge at them indivudually then use the pack. i you read it, i say charge them individually. this will ensure that the packs are 100%full. then let them rest for a few hours, maybe even overnite. this will ensure that your packs rest to its natural levels. that is the equilisation process.

then on race day you discharge them say 3300's, discharge about 2500mah. let it rest, then charge it again in series.
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Old 04-10-2006, 06:45 PM   #18
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Originally Posted by InYourFace
can you prove they discharge at different rates? i tested the discharge a single cell for 120seconds at 20amps on different cells and they both gave me the same total mah amp draw.
Yes, your next point is exactly how to prove that under race conditions they discharge down to different capacities. If you did the test you just suggested above to a pack of 6 cells at a time, they would all have different capacities remaining.

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Originally Posted by InYourFace
and yes put it on a smart tray the lights will flick off at different times, that goes to show they have different voltages and run time.
That is your answer
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Old 04-10-2006, 06:47 PM   #19
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Originally Posted by Turtlemaster
lets assume your water tank theory is correct. your not over filing the first tank in order to reach the full level of the largest tank, BECAUSE, if your delta peak set correctly, then when the first tank reaches capacity the water shuts off. the tanks that didn't get topped off don't effect voltage, only run time, and with most of our packs getting over 7 minutes of normal run time. well you get the point. just a different opinion

the way delta peak works is if the PACK's voltage reaches a peak, then drops then it will delta peak.

but what in fact happens is this....
take this scenario and yes it does happen even for mathced packs.

we are near the end of the cahrgeing process, the typical voltages will read something like this.

cell 1 1.55v (voltage still going up)
cell 2 1.59v (voltage going down has "peaked")
cell 3 1.55v (voltage still going up)
cell 4 1.54v (voltage still going up)
Cell 5 1.57v (voltage going down has "peaked")
cell 6 1.55v (voltage is still going up)

as you can see cells 2 and 5 have received their full charge and the voltages for the cell is dropiing. but for cells 1,2,4 and 6 the voltages are still increasing.

the charger can only detect the voltage drop across the whole pack, and if the voltage increase of cells 1,3,4,6 is GREATER than the voltage drop of cells 2 and 5, then cells 2 and 5 will infact be overcharged.
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Old 04-10-2006, 06:51 PM   #20
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Quote:
Originally Posted by v0rtex
Yes, your next point is exactly how to prove that under race conditions they discharge down to different capacities. If you did the test you just suggested above to a pack of 6 cells at a time, they would all have different capacities remaining.



That is your answer
they flick off at different time NOT because the discharge at different rate. but because the cells are different to begin with.
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Old 04-10-2006, 06:54 PM   #21
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Originally Posted by InYourFace
they flick off at different time NOT because the discharge at different rate. but because the cells are different to begin with.
So you're saying that each cell in a 6 cell pack will discharge at the exact same rate during a race?
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Old 04-10-2006, 06:59 PM   #22
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here guys i give you another simple analogy.

take a 2 cell pack. one cell is a GP3300 and the other cell is a IB3800.

on the widely accepted process, what we do now is discharge to 0.9v for each cell and then charge it up.

cell 1 will get 3300 and cell 2 will also get 3300. as we can see cell 2 has been undercharged by about 500mah. not good for max performance.

but now if we did this charge/equlizing method.

step 1. equilize down to 0.9v/cell
step 2. charge the pack in series to around 3000mah
step 3. charge cell 1 till it peaks (another 300mah)
step 4. charge cell 2 till it peaks (another 800mah)

now both cells are full charged.

step 5. let them rest for a day or so.
step 6. discharge them in series about 2500mah.
step 7. let them rest.
step 8. charge them up for racing, they should both now accept the same amount of charge and peak at the same time.


yes this is a extreme example to highlight how to charge packs that are not matched perfectly, and lets face it no packs are matched perfectly anyway.
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Old 04-10-2006, 07:00 PM   #23
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Believe me, I'm the first to jump into a good argument on theoretical electronics. The only problem is that the theory really starts to fall apart when you take a rechargable cell and pull 10x its capacity out of it.

The 1st and 2nd order effects (the water analogies and their extensions) dominate when you are discharging at fairly low rates (ie 1C and below). When you start to yank 5x, 10x, or even 20x(!!!) out of a pack, the little parasitic elements of the chemical reaction start to dominate. BATT_MAN from Axxis racing can supply the sordid details, but the upshot is that, the things that "make sense" to you aren't necessarily correct anymore when you are this far outside the normal performance envelope.

Now that said, the way we treat our batteries - high charge currents, higher discharge currents, dead-shorting, etc. IS bad for them. If you ask any of the manufacturers, they'll tell you that the way we use our batteries absolutely kills them. That's why they lose capacity so consistently over their lifetime.

But hey, we're RACING here!!! It's about going fast, and I can tell you that even my year-old packs (I don't currently have any new ones) are absolutely ballistic after they've been charged, trayed hard down to nothing, and then recharged hard. The first run, they just don't have the snap like they do the second one.

And it is for information like that which I trust the matchers and the racers here. I can't afford to buy 10 packs, try 10 different methods, and come up with 1 pack of the 10 that performs well. I have yet to try the IBs, but I'm glad everyone figured out that you don't want to dead-short them (or even tray them below 0.9) before I bought any - I'd have been screwed!!

So basically, I trust the common wisdom. Is there a potential advatage to be gained by taking the road less traveled? Sure, but I don't have the time to risk being REALLY slow (verses the pretty slow I am now) for a small upside poptential.

But, as they say, Your Mileage May Vary...
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Old 04-10-2006, 07:12 PM   #24
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Quote:
Originally Posted by v0rtex
So you're saying that each cell in a 6 cell pack will discharge at the exact same rate during a race?
Whoops, this one was posted while I was having my novel printed!

All of the cells have to discharge at the same rate during a race. The bulk of the current flows through all cells. It has to, to reach the motor and do work. There's a little that gets chewed up in each cell's leakage, but it's minimal compared with the total current flowing.

What isn't the same is the capacity of the cell. You pull the same amount of energy out of each cell (more or less), but since each cell's capacity is slightly different, you'll see slightly different behaviour when it hits the discharge tray.

But if you're sticking with the Novak tray, at 2 amps, each second is only 0.5mAh of difference in capacity between cells. A full minute is only 30mAh. That's 1%. If all of the cells are within 1 minute of each other, you've got 1% matching, which is pretty good in my book, given how these things get hammered on!

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Old 04-10-2006, 07:17 PM   #25
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Thanks for the explination, I always assumed the cells were close in thier capacity when they peaked - now I have a better understanding of how & why they're different when they come off the track.

Thanks.
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Old 04-10-2006, 07:48 PM   #26
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the best process I have come up with which seems to work very very well is the following:

lets say you rock up to a race meet with discharged pack's (to 5.4v) and they've been in your pit box for a week.

1) equalise cells on your tray (one that has per-cell cut off like novak smart tray or much more CTX-D) before charging. then let them cool for 30-60min if required.

2) charge at your desired amperage on your charger (lets say 6A)

3) race, then remove pack, then discharge your pack at the highest amperage you can, this might be 10A on your ICE charger, or whatever, in my case it's 30A pulse on my CTX-d. Down to 0.9/C or 5.4V.. then pack them away in your batt box for your next race meet.

I do not see any reason to equalise cells before a pack-away. I'd rather equalise them on the day, right before i charge them (and they are cool)

This process works very very well for me.

Previously i was discharging my pack's @ 20A linear (in series) off my supernova competition charger. then packing them away. but since I picked up a Much more CTX-D (yesterday), I'll now be 30A pulse discharging them after each race... and 5A linear 'equalising' them, before a charge up.

My battery matching guy says this technique is ideal and should see my cell's happy and well performing.

I've had my fair share of poor performing cells due to experimenting with different ways to maintain my cells.. my current technique is working out really nicely

everyone has their own way.

I think equalising tray's are excellent (but only if they have an electronic controlled per-cell cut off.
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Old 04-10-2006, 07:53 PM   #27
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I think equalising tray's are excellent (but only if they have an electronic controlled per-cell cut off.
I love my integy 030, its great for abusing my 18month old 3300's before deadshorting
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Old 04-10-2006, 08:01 PM   #28
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Originally Posted by v0rtex
I love my integy 030, its great for abusing my 18month old 3300's before deadshorting
Well I run IB3800's, which i dont trust under 0.7V....

if i was running 3300's I'd probably dead short them too
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