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Old 01-29-2003, 05:26 PM   #1
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Default discharging batteries (NiMH)

I have some promatch gp3300. Because they are NiMH do I have to discharge them at 30amps. Do i have to discharge them at all. According to the chemistry of NiMH batteries they do not suffer from memory effect therefore to discharge them at 30amps to create a memory is not necessary. I've read all over the internet about people discharging their NiMH, but i do not understand why. Because they are NiMH they never suffer from memory effect and maintain constant voltage. Please help if i am misunderstanding something
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Old 01-29-2003, 05:54 PM   #2
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Here's what I do with my Gp3300 packs, and as a bonus I'll tell you why I do it:

After a heat or main, I discharge the pack at 25 amps to .9 volts per cell. I don't do this because of memory, I do this because my 1/12 car averages around 25 amps throughout the run, and I want to see how much run time was left in the pack.

After the discharge I put the pack away until the next race day.

The next race day, I charge the pack (5 amps for mod, 6.5 amps for stock) and race it.

Approximately every fourth or fifth time I race, I'll tray the pack down just before I charge it on an equalizing tray. I do this just to get the cells working from an equal standpoint, I don't think it's necessary every time, however)

I've got almost a full season on my 3300 packs, racing every weekend, with no loss of voltage or runtime so far.

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Old 01-29-2003, 09:43 PM   #3
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Check out this link from Trinty:

http://www.teamtrinity.com/tips/batts.asp

I've pretty much used Trinity's recommend tips on my batteries and haven't had any problems.

I hope this helps...
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Old 01-31-2003, 03:34 PM   #4
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Trips,
I did a search on this topic and found your advice. I just got some Promatch 3300 packs. I have an Indi Reactor 20 discharge device that brings packs down to an auto cutt-off. What's the difference between 20- and 30-amp discharge as far as battery life and performance is concerned? Is one better than the other for certain applications? Should I buy a 30-amp discharge device for NiMH?

Thanks for your help.
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Old 01-31-2003, 11:50 PM   #5
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coolrcdad,

Back in the NiCad days, we liked to discharge at a high rate because NiCads seemed to perform better when discharged that way (less tendency to feel "flat" near the end of a run.)

NIMH cells apparently don't exhibit this memory effect, thus I don't think that it matters whether you go for a 20 or 30 amp discharge after a run. I try to choose a discharge rate that will be approximately the same as what the car sees on the track, so I can time the discharge after the run to get an estimate of how much runtime was left in the pack. For 1/12 scale, we run 8 minutes, which would average approximately 24 amps during the race if we dump jsut after the heat is over. By discharging at 24 amps right after the race I get an idea if I had enough runtime left to go to a hotter motor or gearing setup.

For 5 minute touring car racing, I'd discharge at 38 amps if I could, again just to get an approximation of true remaining runtime. My Victor won't go that high, so I settle for 35 amps.

If you don't care to get as compulsive as I do, I think either a 20 or 30 amp Reactor will do the job just fine. If I were racing touring cars, I'd go with the 30, but again, I'd go that route to get a better idea of remaining runtime, not because I feel that 30 amp discharges benefit NIMH cells more than 20 amp.

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Old 02-01-2003, 09:45 AM   #6
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Thanks for replying. Good info and it applies well to us because we do both 12th stock and 10th stock and 19t mod.
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Old 02-01-2003, 10:52 AM   #7
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Hi guys. Greetings from Malta.

INTERESTING!!!!!!!!

We don't run 3300s yet. We still have a top limit of 3000 so I can only speak about them.

In Mod we run Panasonic UMH on my son's TC3. In stock I run Sanyo HV on my MR4. We treat them all the same way.

I charge (mod at 4 amps, stock at 6 amps) so that the cells are ready about 15 minutes before our race. I then peak once at 8 amps just before the race.

After the race I leave the cells as they have finished the race, and put them away that way.

At the next meeting, before charging I discharge down to 5 volts.

After about 6 cycles I tray them, just to equalise. I have just done that with cells that are just 5 months old and they were pretty unequal, so it was a necessary exercise. Incidentally we run the top quality Corally Pro Team cells only.

What I have found is that with every cycle the cellsloose a bit of time. The first time I tested them, when they were new, I was getting 510 seconds on my Protrak. They are now down to 450, but still give good power.

Regards

Joe from sunny Malta.
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