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Old Jan 07, 2009, 12:12 PM
bambino is offline
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Is it necessary to discharge NiMh batteries before charging them to avoid overcharge?


Hi, I just got Piranha CX15 charger which is a timed charger which I guess it has no peak sensitivity to stop charging process.

Now, I know it is recommended for NiCd batts to fully discharge them prior to charge, because of the memory effect and so on.

But what a bout NiMh batteries? Wouldn't it be also convenient to discharge them prior to charge them to avoid overcharging them with a charger that has no peak detection?

Thanks all
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Old Jan 07, 2009, 12:24 PM
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Nimh battery still have the same effect that is know as memory but less then Nicad. If you are using a TIMED charger and you have a VOLTMETER handy, you can use it to check to voltage and when the voltage starts to DROP then stop charging, youre pack as reach it max storage capacity. If you dont have a volt or multimeter, the other way is to monitor you pack for heat, some heat is normal but when it starts to get hot you should stop charging. You can get a good multicharger that are not that expensive, if you are serious about the hobby, it would be a good/wise invest to buy a decent multi-type charger (NiCd, NiMh, LiPo-LiOn, Life-A123 & even Lead-Acid) with or without Lipo balancing capability.
Old Jan 07, 2009, 12:26 PM
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I normally just charge prior to needing the batteries, regardless of the state of charge. Every 10-20 cycles I fully discharge then recharge at 1/10C for 12 -14 hrs.
Old Jan 07, 2009, 01:06 PM
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Hi guys, thanks for your advice,

Mr freechip, may you recommend me a decent multi charger? (Im not very interested in charging/balancing LiPo batteries but wouldn't hurt either)

I bought this Piranha CX15 charger because it is cheap (under $35 in Tower Hobbies).

Is there any decent multicharger in that range of price?

I know the triton jr, but it is about $70 and it requires a fine, decent 12v power source.

Is there something more economical than taht which includes its own power source?

Thanks!!
Old Jan 07, 2009, 02:42 PM
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I Love.


FMA Cell Pro 4. For charging up to 4 cells of LIFEPO4 Or even LIPO.
It has proven to do everything perfectly. I have delibertly given it a " BAD " pack. 1 cell way lower. It went in the safe mode to try and revive the cell. It did bring it up to the rest of them. THEN it went to a normal charging.

Someone has stuffed a person inside of that charger. He really cares about my batteries. It runs off of a charged up car battery.

Buy one.
Old Jan 07, 2009, 04:41 PM
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There is no need to discharge either NiCad nor NiMh prior to charging if you are slow charging (0.1C or very near that value). If you are fast charging (which is always harder on your batteries and will lessen their lifetime in terms of charge/discharge cycles) still no need to discharge first for either as they will still respond to the peak when charged. One of these days, the myth that NiCad have more memory problems than NiMh will hopefully be forever lost as it just does not happen in the way these batteries are used in RC. Now both NiCad and NiMh can suffer from voltage depression--which is often erroneously diagnosed as memory--and this can often be at least partially if not completely remedied by a slow charge of 0.1C for 16 hours (and you do not have to discharge before doing this forming/reforming charge).
Old Jan 07, 2009, 04:56 PM
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Hi

Cyclops, do you know a general price for that charger? (will look into towerhobbies)

Rodnye, the problem is that Piranha CX15 charger does not have peak detection feature, so I think there is a risk of overcharging if I charge them partially charged.

This charger I have can only fast-charge. I wonder how many cycles less this practice (fast-charging) will cost me.

Thanks
Old Jan 07, 2009, 05:13 PM
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Quote:
Originally Posted by Rodney
Now both NiCad and NiMh can suffer from voltage depression--which is often erroneously diagnosed as memory--and this can often be at least partially if not completely remedied by a slow charge of 0.1C for 16 hours (and you do not have to discharge before doing this forming/reforming charge).
I completely agree with you until this point. The slow charging will help to balance the pack and make sure all the cells are fully charged but only a full discharge to approx. 0.9V/cell, and the subsequent recharge, will break up the larger crystal structures that have reduced the cell's active surface area (increasing its internal resistance) and allow the formation of many more smaller structures that lower the internal resistance, preventing voltage depression.

Bambino, I have a bit of info regarding NiCd/NiMH cell care on my site at: http://www.camlight.com. See the Why Discharge? and Tech Tips sections.

John
Old Jan 07, 2009, 05:54 PM
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The concept that NiMh have no memory is a myth, is a myth. The cells do lose ability to hold full capacity, particularly when idle for a long time, whatever you want to call it. This was proven most recently when I tested several dozen AAs for capacity last week and noticed a few were quite low. Usually ones that had been sitting at 0v for a while. Trickle charging didn't help. 3-4 discharge/charge cycles brought them right back.

Another clue is that all NiMh chargers worth anything have discharge feature. I highly recommend Tenergys 8 and 10 cell AA/AAA units. Picked couple up day after thanksgiving for $10. I don't like those that do many types of batteries but prefer single purpose units myself.
Old Jan 07, 2009, 06:05 PM
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I think it's getting down to semantics now.
The classic "memory" problem only occurred for satellites a long time ago. I agree that the term is commonly used now to refer to voltage depression in a cell (which Rodney and I referred to), but it's quite confusing for a lot of people when used that way.

John
Old Jan 07, 2009, 06:19 PM
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Quote:
Originally Posted by JohnMuchow
The classic "memory" problem only occurred for satellites a long time ago. John
At least that's what NASA tells us. Remember, they also tried to convince us that a man was actually landed on the moon.
Old Jan 07, 2009, 06:32 PM
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Bambino.


Click on the.....Members......word in the blue band above. Then type in everydayflyer. Then select send a ...PM... He is a great wealth of anything battery related. He Knows ALL about what you need to know.

Rich
Old Jan 07, 2009, 06:43 PM
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NiMH discharged by uncertain mode of simple charger, or by attempt to run them flat empty, will die very soon.
NiMH packs charged at quite high current, using extremely simple charger, timed, with no peak detection, will overheat and die instantly.

I think that first good pack or two, costs more than such charger, but new charger will save the packs, otherwise they may last for no more than few charges. Even good pack, if used this way, may perform well only once or twice..
Low voltage under load, impotency to provide high enough power, and also reduced capacity, is far higher problem than hardly measurable, temporary(!) voltage decrease by few percent, at the point to which was the pack discharged few hundred times (that is the "memory effect")..
Old Jan 07, 2009, 06:49 PM
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Quote:
Originally Posted by rich smith
At least that's what NASA tells us. Remember, they also tried to convince us that a man was actually landed on the moon.
?! Well at least NASA pulled off the 911 job successfully.
Old Jan 07, 2009, 06:54 PM
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Quote:
Originally Posted by rich smith
At least that's what NASA tells us. Remember, they also tried to convince us that a man was actually landed on the moon.
Neil Armstrong was a WOMAN?


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