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Old 11-10-07, 12:18 AM   #1
nomo4me
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Why and when to discharge Nimh AA's??

Just received my high zoot charger from Tiger Direct and see it has a one touch discharge feature to use when batteries "have a small amount of electricity left in them".

Do I need to drain my Duracell 2650's before recharging? I suspect the answer is "no", but a high degree of ignorance in all things electrical means I must ask the question.

Thx

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Old 11-10-07, 07:41 AM   #2
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Supposedly, cycling a rechargeable occasionally (completely discharging it before charging) will prevent the development of "memory", which is a condition in which a battery may be fully charged, but will not discharge beyond a certain level, the result being that it doesn't last as long as you would expect.

This was an issue with NiCads, but I'm not too sure about NiMhs- some makers claim "no memory problems" for their NiMhs, but as I say, I'm not sure. Here's a link to a page that discusses battery memory, and if you google, you can find plenty of info:

http://www.dansdata.com/gz011.htm

My charger has the capability of cycling the batteries, but I usually forget how many times I've charged a set (it's suggested to cycle them every 10 charges), and just don't do it. I guess you could keep a log, but that seems like a lot of trouble.

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Old 11-10-07, 09:06 AM   #3
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Originally Posted by nomo4me View Post

Do I need to drain my Duracell 2650's before recharging? I suspect the answer is "no", but a high degree of ignorance in all things electrical means I must ask the question.
You are correct; Ni-MH batteries do not develop a memory so you never need to completely discharge them before recharging. Your Ni-MH battery life will be extended by never allowing them to become completely discharged.
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Old 11-10-07, 09:11 AM   #4
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My quick research on the subject shows there's no need to discharge NiMH batteries.
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Old 11-10-07, 10:28 AM   #5
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You are correct; Ni-MH batteries do not develop a memory so you never need to completely discharge them before recharging. Your Ni-MH battery life will be extended by never allowing them to become completely discharged.
All Ni chemistry batteries can develop crystallites that are the cause of the 'memory' problem for NiCd and, to a lesser extent, NiMH. It just takes longer for the NiMH to do so. According to Buchmann, regular full discharge is still required but less frequently. I have chargers that have a discharge function and I use it about once a month, especially during summer storage (or I run them in an RC car which is a lot more fun).
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Old 11-10-07, 05:37 PM   #6
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My quick research on the subject shows there's no need to discharge NiMH batteries.
My additional research says that completely discharging the NIMH is very bad.
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Old 11-10-07, 07:07 PM   #7
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My additional research says that completely discharging the NIMH is very bad.
You don't want to discharge the battery completely but discharging to a controlled voltage...I use a Maha C777 plus II...doesn't harm the battery and does help keep crystallite growth to a minimum. Like I said before, I do it about once a month.
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Old 11-11-07, 08:28 PM   #8
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If I am reading the Battery University website correctly, nicads should be discharged once per month based upon a GTE Government Systems study commissioned by the Navy and though no study has been done on nimh's an educated guess is every 3 months.
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Old 11-11-07, 10:18 PM   #9
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If I am reading the Battery University website correctly, nicads should be discharged once per month based upon a GTE Government Systems study commissioned by the Navy and though no study has been done on nimh's an educated guess is every 3 months.
I find that a monthly discharge is just easier to remember. I do it on the first Saturday of each month.
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Old 11-12-07, 07:25 AM   #10
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Whenever you think the performance of your NIMHs have degraded. I'd go with cyclocommutes advice. Once a month, whenever you remember or whichever comes first.
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Old 11-12-07, 10:38 AM   #11
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Originally Posted by n4zou View Post
completely discharge them before recharging
I'll stay away from the to discharge or not discharge debate, but want to clarify "completely discharging".

Nimh, Lead-acid, LiIon and LiPoly cells can get damaged quite easily by discharging them beyond a certain voltage. For Nimh, that voltage is 1 volt/cell.

The only time I've correctly used the discharge feature of a charger is when I was storing a battery as most types of batteries do not like to be stored fully charged. <----- The key word in that statement is correctly as I've destroyed many batteries over the years
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