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Old 03-11-2005, 10:58 AM   #76
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shouldn't a batt put out more power when cold
Maybe it does, at least this report from a Freie Universitšt Berlin shows a cell has higher voltage at lower temp.

Check it out, excellent write up.
http://robocup.mi.fu-berlin.de/buch/...up-battery.pdf

Maybe this was a source of the practice I've heard some chilling batteries, although I'd question that too.

Note their results on NiMH memory.
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Old 03-11-2005, 11:39 AM   #77
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I think what you are refering to is where the statement was made that at 45 deg celsius, the battery reached 1.4 volts, and at 0 degrees celsius, the batt reached 1.5. This was while charging......

I don't think that means that on discharge the voltage is going to be higher. I have tried this before, I charged a battery while in a refridgerator. The results were not very good to say the least.

I can say that the packs that I have that are older and of lower performance, peak at a higher voltage. Say upwards of 9.4 volts for the pack. My good packs peak at 9.0. So it seems to me that a lower peak voltage may indiciate a better cell. Of course if the cell peaks at 1.0 you can't expect 1.2 out of it.

I still think the cell produces more current when warm because the chemicals inside the cell are more excited.

They also state the Nimh doens't really have memory effect like NiCds did. This I don't think was ever really an issue with RC anyway. I think the main benefit of using a discharge tray is not to prevent memory (although it should) but to force all cells in a pack to an even state prior to charge, to help reduce the effect of some cells peaking before others. This would be increased in packs not equalized.
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Old 03-11-2005, 05:13 PM   #78
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Yep, the data did not report on discharge results, I did say "maybe". Without unbiased scientific test data, I reserve judgement on the performance effects of hot or cold batteries. In any case, I do believe it is unquestioned that heat does reduce battery life.

Regarding equalization, if two cells charge and discharge at different rates,
why would making them equal in a discharge state improve their equalization in the charged state? I argue they would not, in fact, the opposite, you are amplifying their differences with respect to their charged state.

In my view, it isn't equalization but a method of fully discharging each cell,
which is important for NiCad batteries, but is not for NiMH. Furthermore, I argue that performing a full discharge before charging for NiMH batteries is simply reducing the battery life and making it take longer to charge them.

I realize the desire to find some undiscovered, unorthodox treatment of batteries to find some marginal performance gain, no matter what the cost in battery life.

I will say that if you take a warm-up lap before a race, you have just reduced your battery performance more than any trick might improve it, but that warm-up lap is worth more in your driver confidence than any possible increase in milliamps.

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Old 03-11-2005, 05:40 PM   #79
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I can tell you my car seems faster when I tray them before charging.

I have not verified with a radar gun.... However, I highly doubt that its psychological, as when I go up on the drivers stand, the though that "oh gosh I didn't tray this pack" does NOT even enter my mind.

What does enter my mind after a race is "Wow, that pack felt strong, as I could clear the tripple without having to go wide on the 180 turn right before it" whereas with a softer pack I need to get a bit of a run.

I can kinda see what your saying about the pack being more unequalized at charged state after traying, but I highly doubt that traying harms the cells.

So what you should try is to run a pack, and never discharge after its run in your car.. Let it cool or save it for next week, and just charge it without discharging. Do this a few weeks, then compare on the bench to a pack that you cycle normally (discharge and tray) on a regular basis. Thats really the only way to tell.
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Old 03-12-2005, 12:16 AM   #80
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Quote:
So what you should try is to run a pack, and never discharge after its run in your car.. Let it cool or save it for next week, and just charge it without discharging. Do this a few weeks, then compare on the bench to a pack that you cycle normally (discharge and tray) on a regular basis. Thats really the only way to tell.



Right ! A year ago, when I got my first NiMh packs, that is just what I was doing. Race, charge, race, charge. Ocasionally I would dump the packs before charging for around 2 min. with the bulbs, but that's it. Having learned more since, I feel a big difference in power after a full dump and tray before charging.
Even on those old packs from a year ago that are now for practice.
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Old 03-12-2005, 11:19 AM   #81
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I don't know why people use their worst battery pack for practice it just doesn't make sense!

the good battery will feel so much different and probably the car will handle weird if you adapt to the way it feels when driving at it's full potential you will performe much better as a driver!

sorry if I sounded rude not my intention
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Old 03-12-2005, 08:31 PM   #82
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Quote:
Originally posted by Quiet
I don't know why people use their worst battery pack for practice it just doesn't make sense!

the good battery will feel so much different and probably the car will handle weird if you adapt to the way it feels when driving at it's full potential you will performe much better as a driver!

sorry if I sounded rude not my intention
thats because we need out best batteries to stay at the best performance the more we use it the more performance it may lose!
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