NIMH Question

davidra

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OK...I'm still trying to figure out the relative value of running a Mag85. I'm currently running mine with Litho's double stack; now I've got some battery holders and the time has arisen to get them or not. This thing will be used rarely; maybe on a boat for a while, or for a wow factor fun kind of thing. I know they will discharge without being used, so: If I charge up 9 AA's, use it for 5 minutes, and set it aside for say 2 weeks, will all batteries need to be recharged? What is the "shelf life" of charged AA NIMH's?
 

SilverFox

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Hello David,

NiMh cells self discharge somewhere between 1-2% of the remaining capacity per day.

If we figure 1.5% self discharge rate and start at 100% full charge, they would be down to about 80% after 16 days.

Tom
 

VidPro

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is that compounded daily :)
they are basically usable for a month from my experience, there might be 10-20% in them, enough to light an led before the month ends.

the LI-ION though just keep going , and will still light after 4-6 MONTHS , much bettern
 

SilverFox

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Hello VidPro,

You must have some very bad NiMh cells... If that is all the better they are doing, I would replace them.

My cells and packs are showing less than 1% per day self discharge. It takes over 5 months to get to the 20% level.

I do agree that the Li-Ion rechargeable cells hold up better. They come in at about 1% per month. However storing them at 100% causes them to loose capacity. You can loose (and never recover) up to 20% of the capacity in one year. If you are storing them for longer than a couple of weeks, you get better life by keeping them at 40%. This is about the same level that NiMh cells end up at after sitting for 3 months.

Tom
 

newo

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[ QUOTE ]
SilverFox said:
However storing them at 100% causes them to loose capacity. You can loose (and never recover) up to 20% of the capacity in one year. If you are storing them for longer than a couple of weeks, you get better life by keeping them at 40%.

[/ QUOTE ]

Tom

I have a head cold, and I believe that all that sinus drainage has short-circuited my brain. Is there a more or less no-brainer way for the non-gifted amongst us to get there from here?
 

SilverFox

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Hello Newo,

/ubbthreads/images/graemlins/evilgrin07.gif No. /ubbthreads/images/graemlins/evilgrin07.gif

Brains required... /ubbthreads/images/graemlins/grin.gif

Tom
 

newo

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Okay. Fair enough. I did ask for that.

So, is the easiest approach to run the cell down, and do a timed charge at a known rate, or is there some technique using a DMM that is not occurring to me in my weakened state?
 

VidPro

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your right silver, i thought you were saying they were shot in 16 days, that is what some naysayers of nimhy were saying , and it is not that bad at all.
but i know when i reach for, or leave a set in the digicam ready, or the wireless mics, or the led lights in the stuff ready to go, they are not ready to go :-(
we have taken to leaving a rack of them on a trickle charger.
running out in the middle of the show, is never enjoyable.

as far as the Li-ION we do leave them charged, but i noticed that the manufacturers say to discharge them if your going to store them for a long time.
so we started doing that a bit more, what value is a battery that is discharged when you get a call :)

1/2 of the li-ion camera batteries we purchaced when they went li-ion years ago DIED, they died of the poor quality circuit stuff in them, that did not protect them from venting, they are completly detached, no ohmes, no resistance no nothing. sometimes its a few cells that died, sometimes its more.
but the ones that have lived forever, and have been kept mostly charged , are just about as good as the new ones.
according to the "info lithium" readout, and my direct testing of that with a timer.
a dead pack will read "info lithium" data incorrect if one cell set is dead, it doesnt know it (or right away)
so i have tested them by timing the 250 or 350minutes to make sure they are making it.

the big camera has 3 batteries, that cost $800ea for the pack, and they have ALWAYS been charged, and it has been 3 years for them and they are at least 95% still, they never fail and i never even need the backup i bring for them
they never even fully discharge, but i do cycle them around on the charger, one is ready to go, 2 parked on the charger.

which brings up a great question.
is SELF discharge (once it has done that through time) concidered leaving the pack down ?

are they saying 40% down so they dont get into reverse discharge issues of a pack when doing a maintanace discharge?
 

Phaserburn

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I thought leaving Nicad pack lights on the charger all the time was ok. Isn't that the point of nicad vs nimh? Like a stock Magcharger, SL-20X or Big D Nicad. The stock charger will cook the cells if left on? The documentation says that leaving the light on the charger is ok.

Confused!
 

SilverFox

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Hello VidPro,

The self discharge of NiMh cells is a royal pain. Several solutions are available...

A few primary cells my get you started, but if you need to go non stop through the "whole show" that is not a good solution.

You can leave cells on a charger, but that tends to cook them over the long haul.

The best solution is to leave cells on a charger that is plugged into a timer. I have mine plugged into an Intermatic TN811. They charge for about a half hour a day and are always ready to go with no concern about overcharging.

I have lithium packs for my camera as well and keep them charged at 100% also. I have noticed that the voltage is about 4.16 per cell which is technically about 95% of full charge. I have also noticed that the low voltage cut off is at about 3 volts per cell. This duty cycle gives great performance. The charger also seems to not abuse the cells and charges at a moderate rate keeping the cells from heating up.

If I were supplying an $800 battery pack and did not want to have returns, I would do something very similar. If you limit the high and low voltage, you give up a bit of the capacity, but get very reliable service.

Li-Ion's last longer when they are frequently "topped up," rather than going through a full discharge. You might give that a try. There is no memory effect and no crystals in the chemistry, so discharging has no benifit.

Li-Ion chemistry ages. 3 years is a good life from them, but understand that you are approaching the tail of the bell curve. I hope you get many more years of life from your packs, but I would start a savings account now just in case.

I keep my "non daily use" cells at about 80% (4.0 volts) thinking that they are still ready to go and not aging as bad.

Tom
 

SilverFox

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Hello Phaserburn,

I would suggest not leaving NiCd or NiMh (or Li-Ion) cells on the charger for extended periods of time.

If you leave cells on the charger, NiCd cells are able to take more abuse and will last longer.

I believe Marketing (and not Engineering) was responsible for the documentation comments... /ubbthreads/images/graemlins/smile.gif

Tom
 

Phaserburn

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Thanks, Silverfox. Well, doesn't that stink! The reason I'm using the stock nicads was because they could be left on the charger! Guess not!

Battery Univ says trickle charge of Nicads should be .05 to .1C. My charger says it's 1A, charging a pack of 6 D 5000ma cells. Is this ok?
 

SilverFox

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Hello Phaserburn,

Let's see... 0.05 X 5.0 = 0.250 = safe maintenance trickle charge rate.

Your charger is putting out 1.0 amp.

No. Not a good idea for an extended time. However, if you use the light every day and run it down a bit, it should work out fine.

Tom
 

VidPro

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my NI-MHYs (the AAs) that i leave on the charger, have some sort of really low trickle rate.

it was a cheap charger that shuts off and turns to trickle, at about 10ma

the cool thing about it, is it will do each unit singularly, and it will do a dischager if i want it to.

The bad thing about it is sometimes it thinks incorrectally, doesnt know if its fully charged or turns to trikle to early. i think i fixed that, by putting in SETS all over again :-( back where i started.

i thought it would have been cool to have each battery charged and discharged on the rack of 10, independentally, but aparentally it is not good enough to think totally independantally.
 
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