nimh charge rate Q

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blackdragonx1186

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fresno, cali
i have some titanium D cells, their 11k mah size ones. my question is whats the best rate to charge them at? i use my rc car chargers on them, so i can charge from 100ma to 10 amps. im currently going at 3A, so im hoping thats ok. i dont mind them taking awhile, but not overnight, as my charger beaps when its done, and doesnt stop until i press one of the buttons.
 
Hello Blackdragon,

The normal advice for charging NiMh cells is to charge at a rate that completes the charge in 1 to 2 hours. Some D cells have problems with 1 hour charging, so you will want to monitor the cell temperature while charging. If the cell heats up early during the charge, reduce the charge rate.

The charge rate is dependent on your chargers method of termination. The Maha 808 charges D cells at 2 amps and terminates properly and keeps the cells cool.

This gives you the range of 2 - 10 amps, depending on the cells and your chargers termination method...

Tom
 
SilverFox,

I commute to work on a 2 wheeler. I need to learn more about batteries and charging. Last year I bought a LaCross H-900 charger and have been using Energizer 2500 NiMH cells. I also have some LaCrosse cells that came with the charger.

After a full refresh the LaCrosse lasted for quite a number of trips to and from work. Fearing they were about to die, I replaced them with what I thought was a set of fully charged Energizers. The charger reported ~2500 mAh after the charging cycle was complete. The Energizers lasted only about two or trips (90 minutes) in a headhight advertized to last 5 hours on fresh cells.

I've always been uncertain on what charging rate to use for these AA batteries. Can you help? Any idea why the Energizers died so quickly? Someone in another forum said the Energizer 2500 was the worst battery produced. You've done a lot of testing. Any commets? The recommendation was to use the Maha Powerex 2700. I didn't see that cell tested in a test report of your, which I believe was a bit dated. How do you view the Powerex 2700?

Several questions so I'd appreciate any insight or knowledge you can share. Thanks.
 
Hello Dbsoccer,

Welcome to CPF.

Most of the 2500 mAh cells (all brands) have developed high rates of self discharge. They go from fully charged to fully discharged in as little as 24 - 30 hours. As a result of this, most people have recycled those cells and moved on to others.

The cells still work well and deliver good capacity, but you have to charge them up then use them immediately. I am not sure what happened to these cells, but speculate that there were some "issues" with the separator material.

You will get the best performance from your NiMh cells if you charge them at a rate that completes the charge in about an hour.

I have not tested the Powerex 2700 cells. If you would like to send me some, I would be happy to check them out and return them to you afterward...

The last update to the NiMh Shoot Out thread was 3/11/08. I have had a busy summer and things should settle down in the near future so I can get back to running some more tests. November marks the end of summer for me... :)

Tom
 
I can confirm that many Energizer 2500 cells blow. I charge mine very carefullly and so far they have lasted as well as any other Nimhs I have (Duracell, LaCross, etc)...

BUT my girlfriend sticks hers in a crap wall wart charger and leaves them there for days. And 3 out of 4 of hers are dead. I think that if you're not SUPER careful with them you can kill them easily.

Of course they might just be ultra crap cells.
 
As has been suggested, it's most likely that you are experiencing the typical behavior of Energizer 2500s, unless you charged the cells up immediately before using them, then much of their capacity was probably lost to self-discharge...

if you contact energizer via email and complain about their crappy cells they will send you a $10 coupon towards any energizer or everready product. I did not feel the least bit bad about complaining about these cells as I feel they are crap cells in every respect and should not be sold to unsuspecting consumers as all they tend to do is generate a negative attitude towards the concept of rechargeable cells in the general public.

The other possibility is that the headlamps runtime is falsely advertised.. this would not surprise me or anyone on this forum.

Another possibility is that the headlamps runtime is based on the diminishing output provided by Alkaline cells, which will drag out a long but diminishing output in many designs, but with NIMH cells, the headlamp *might* probably provide a more "regulated" output through the run as a result of the discharge characteristics of NIMH, and cause the light to not run as long, but maintain more steady output levels through the run. This is all hypothetical and depends on the specific headlamp. Good regulated flashlights will almost always run longer on a good set of NIMH than they will on alkaline cells.

Eric

I suspect a combination of the 2.

dbsoccer said:
SilverFox,

I commute to work on a 2 wheeler. I need to learn more about batteries and charging. Last year I bought a LaCross H-900 charger and have been using Energizer 2500 NiMH cells. I also have some LaCrosse cells that came with the charger.

After a full refresh the LaCrosse lasted for quite a number of trips to and from work. Fearing they were about to die, I replaced them with what I thought was a set of fully charged Energizers. The charger reported ~2500 mAh after the charging cycle was complete. The Energizers lasted only about two or trips (90 minutes) in a headhight advertized to last 5 hours on fresh cells.

I've always been uncertain on what charging rate to use for these AA batteries. Can you help? Any idea why the Energizers died so quickly? Someone in another forum said the Energizer 2500 was the worst battery produced. You've done a lot of testing. Any commets? The recommendation was to use the Maha Powerex 2700. I didn't see that cell tested in a test report of your, which I believe was a bit dated. How do you view the Powerex 2700?

Several questions so I'd appreciate any insight or knowledge you can share. Thanks.
Click to expand...
 
Thank you all for your replies.

Is it just the 2500 mAh size that is the biggest issue? Are there others that last longer? My LaCross cells seemed to last and were still going strong before I change them out thinking they should be about ready to die.

If a rechargable cell will self discharge in 24-48 hours it would seem that are not much good for mos applications. The headlight on my bike is used in the mornings but sits off most of the time. The flashlight in my truck is hardly ever used but needs to work when needed. I have small battery powered devices (trimmers, remotes, etc) that sit idle most of the time.

Over the weekend I purchased 8 Maha 2700 mAh Powerex. I put 4 of them in my Lacross charger on Refresh with a 200 mA charge rate. 3 of the 4 were FULL this AM (48 hours of refresh) sitting at just under 2700 mAh. I left them on until the forth is FULL as well. I read somewhere where for first charge should be a refresh to get the best performance. I don't know if 48 hours is expected.
 
SilverFox said:
Hello Blackdragon,

The normal advice for charging NiMh cells is to charge at a rate that completes the charge in 1 to 2 hours. Some D cells have problems with 1 hour charging, so you will want to monitor the cell temperature while charging. If the cell heats up early during the charge, reduce the charge rate.

The charge rate is dependent on your chargers method of termination. The Maha 808 charges D cells at 2 amps and terminates properly and keeps the cells cool.

This gives you the range of 2 - 10 amps, depending on the cells and your chargers termination method...

Tom
Click to expand...

humm, sounds good. towards the end of my charge cycle, the cells were very hot. im thinking 3A might be a tad high for them. i didnt monitor them while charging, other than looking at the charger to see how many mah had be put into the cell.

my chargers that i use use a - v delta to detect a full cell, same as other individual cell chargers. im charging my sanyo 2700's at 700mah. sound a little low? how fast should their voltage climb while charging? ive had them going for 20 mins, and they are at 2.90 volts. (a pair in series. they started at 2.36) i noticed that if i raise it much higher, the voltage goes up quicker, and it doesnt completely charge the cell. thanks for your info!
 
2.9V for a pair is fine, as you have noticed, the voltage will climb higher when charged at a fast rate. At a standard charge rate the termination will happen somewhere ~1.4-1.5V per cell on most NIMH chargers. 2.9V for a pair means a perfect 1.45V per cell, that's fine.
 
whoops my bad, misread that number. I thought he said it was going to 1.9v each, which my crappy BTY batteries did before they started spitting and whistling.

That'll teach me to skim the threads :o
 
mdocod said:
2.9V for a pair is fine, as you have noticed, the voltage will climb higher when charged at a fast rate. At a standard charge rate the termination will happen somewhere ~1.4-1.5V per cell on most NIMH chargers. 2.9V for a pair means a perfect 1.45V per cell, that's fine.
Click to expand...

yea i figured as much. they terminated at 3.1 ish i believe. were pretty warm, but not hot.
 
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