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Old 10-12-2006, 06:39 PM   #1
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Default Voltage goes up during discharge?

I'm running some year old IB3800's and now that I'm between seasons I've been cycling them with the intent to keep them from going bad. I notice that when I discharge them (@20amps), during the discharge the voltage starts out low, and slowly but steadily rises as they discharge. What's going on here? Is this because as they discharge they warm up and make more voltage?

They do seem to take an ok charge, between 3600 and 4000 depending on how they are discharged.
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Old 10-12-2006, 06:49 PM   #2
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I noticed this once before a few years ago cycling some panasonic 3000's.

I came to the conclusion that, as you stated, the cells warmed up and produced more voltage for awhile.

If you discharge them right off the charger and they are warm, it should be normal.

If you are discharging right off the charger, they are warm, and they start low and get better, I would think that something is wrong with the packs.
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Old 10-12-2006, 09:30 PM   #3
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Yes, this is with cold batteries that are not right off the charger. Makes me wonder what I'm missing out on by not charging them at the track right before the races. Same thing must be happening on the track, cold cells producing an inferior voltage.
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Old 10-13-2006, 02:03 PM   #4
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i had the same problem with 2 two year old 3300, they were heavily used and abused. the cause was a very high internal resistance within the pack as a whole. what happens is as the charger is discharging, a part of the pack is discharged while a little "head" of crystalization builds up and continually builds up behind the "head". this would cause the voltage seen on the screen to be higher and rise. eventually the chargers pulls enough amps through that "head" to make it pop like a pimple. heat helps along with this process, as with heat, the gases in the cells, expand, move faster, and help to pop the chemical pimple. after the pimple os popped, voltage returns to normal.

what would help to eliminate this is to charge the packs as per normal, but to discharge the pack at 30 amps, dynapulse style, meaning it pulses very rapidly, and this helps to break down the crystalization within the pack.

-Zac
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Old 10-13-2006, 05:23 PM   #5
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Be prepared to wait quite a bit form them to discharge on a dyna.. A fully charger 3800 will take several hours to discharge. Not quite feasible on race day..
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Old 10-13-2006, 06:59 PM   #6
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Quote:
Originally Posted by Z.Hallett
i had the same problem with 2 two year old 3300, they were heavily used and abused. the cause was a very high internal resistance within the pack as a whole. what happens is as the charger is discharging, a part of the pack is discharged while a little "head" of crystalization builds up and continually builds up behind the "head". this would cause the voltage seen on the screen to be higher and rise. eventually the chargers pulls enough amps through that "head" to make it pop like a pimple. heat helps along with this process, as with heat, the gases in the cells, expand, move faster, and help to pop the chemical pimple. after the pimple os popped, voltage returns to normal.

what would help to eliminate this is to charge the packs as per normal, but to discharge the pack at 30 amps, dynapulse style, meaning it pulses very rapidly, and this helps to break down the crystalization within the pack.

-Zac
That makes no sense.
Dont mean to be an ass, but, what he is saying as that while Discharging (nothing to do with charging) the voltage starts out low and increases a bit as the cells warm up.

This crystal pimple as you call it, uh, wouldn't that be a benefit if the voltage goes up during discharge?

It is clear that what he is seeing is lower voltage due to cold cells that have been sitting around for awhile.

It is best to charge your packs right before you use them for maximum performance. Its a known fact with NiMh cells. They put out more power when they are warm, also a known fact. (Which is why you charge them right before you use them cause they come off the charger warm.)
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Old 10-14-2006, 06:29 AM   #7
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what would be nice to find out is iff it is the source voltage going up wile heating up OR that it is just internal resistance going down a bit wile heating up ...

this can be simply measured by measure voltege without load before anny discharge so cold .... then start discharge to the point where you notice voltage goinig up ... release the pack from its load and while the pack is still warm measure voltage again if its the same or lower but it stil reads a higher voltage when discharging then its just internal resistance going low

wich seems to me a lot more logic !!!!
chemical voltage normally doesn`t change under temp as long as temp rise / fall is the same on positive and negative side .. but resistance is always changing under temp. difference`s ...

please measure ands let us know !!!!!
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Old 10-14-2006, 07:06 AM   #8
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Nimh batteries are VERY sensitive to temperature. If they are room temp when the discharge begins, then the process of discharging will heat up the cell, the voltage will go up.

Nimh's need to be quite warm for best performance.
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Old 12-17-2006, 09:24 PM   #9
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I also experience this problem. In addition, all of my battery packs that increase in voltage during discharge, decrease in voltage during charging for a time being (around the first 5 to 7 minutes) before the voltage increase.
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