15 minute charger bad for my NiMH AAs?

[4EN[sic]]

I have a Sony BCG-34HUE 15 minute charger that I bought awhile back. I have some Eneloop AAs and some 2700 mah Sanyos as well as the 2500 mah Sony batteries that came with the charger. I read that it could be bad for the batteries to charge them so fast, can anyone elaborate on this for me.

 

[lowatts]

15 minute charging can be bad for cells not designed for it. Even for cells that can handle that charging rate, you will compromise capacity and charge cycles with fast charging. I think someone mentioned in another thread in CPF that he rapid-charges his Eneloops without obvious damage.

 

[Black Rose]

Here is the last page of a thread where SilverFox put some Eneloops through a torture test using a 15 minute charger.

http://www.candlepowerforums.com/vb/showthread.php?t=114943&page=5

 

[Mr Happy]

Feel how hot the batteries get as they reach the end of charge. If they get "hot hot hot" it's probably not doing them any good. If they only get slightly hot it's probably OK. Eneloops do seem to handle fast charging better than some other cells.

 

[Zenster]

Even for cells that are designed to be fast-charged, it's still harder on the cell to do that than to perform a typical C10 charge.

It's all about heat, and fast-charged cells get hot while C10 charged cells generally only get warm. Heat is one of the major causes of batteries not reaching their full life span, so if you don't have to heat them up, then you might as well not.

Being what the price of AA batteries are these days (i.e., they're pretty cheap for the use you can get out of them), I think the best strategy is to simply have enough spare, fully charged batteries on hand so that you never have to fast-charge any of the batteries.

I use the Maha-C9000 charger exclusively which is only a 4-cell (AAA/AA) charger, but with my stash of NimH batteries at around 30, I always have freshly charged LSD batteries when I need them, and I simply charge the ones I need to whenever I accumulate more than 4 that need it.
No rush. No fast charge. Batteries last a long time.

My .02

 

[4EN[sic]]

The Sony charger has a fan built in that blows air across the batteries while charging so the batteries don't seem to get hot, just warm. I am looking at getting that c9000 charger mentioned, so I will just start using that to be safe.

 

[Jackson]

The Rayovac I-C3 charger I had also had a fan and the cells still came off of the charger so hot that I accidentally pulled back their covers when trying to get them off of the charger without waiting for them to cool down.

 

[jhMadCow]

A fan can help keep the heat down. But anything more can lead to a false peak (like a heatsink on the cells), or at least that is what I have learned in the RC world. Don't use a 15min charger, those will get the cells charged up quick, but not to their full capacity and it takes some life out of them, as mentioned above.

 

[Turbo DV8]

A fan can help keep the heat down. But anything more can lead to a false peak (like a heatsink on the cells), or at least that is what I have learned in the RC world.

I notice when I use a fan to keep the cells cool on my BC-900, it very frequently causes the charger to miss termination. The voltage will hover at around 1.52v indefinitely, but when I switch the fan off, the cells all terminate charge within five minutes, like clockwork. Now, if I want to keep the cells cool with a fan, I make sure I monitor the voltage level, and when I see it is about time for the charger to terminate charge, I turn the fan off, at say 1.48 volts.

 

[JWP_EE]

I am not so sure that cooling the cell with a fan is really helping. The internal temperature is whats really matters. I don't know how well the cell conducts the internal heat to the outside. I measured the case temperature of some charging cell. When the charge terminated the case temperature continued to rise for about 5 minutes. The fan may help some I just question how much.

 

[Turbo DV8]

I am not so sure that cooling the cell with a fan is really helping. The internal temperature is whats really matters. I don't know how well the cell conducts the internal heat to the outside. I measured the case temperature of some charging cell. When the charge terminated the case temperature continued to rise for about 5 minutes. The fan may help some I just question how much.

Very good thoughts. Visualize it this way. There won't be just an "internal" temperature and an "external" temperature, but an infinite range of temps from the deepest guts to the outer wrapper, as the heat is conducted away from the core. One thing that effects the rate of conduction is temperature difference. The rate of conduction is directly proportional to the temperature gradient. Anything you can do to lower the external temperature of the cell's skin will increase the rate at which the core is able to cool. You can't "draw" heat from the cell, only encourage it to be "given up" through a temperature difference. By cooling the cell's outer surface, through convection or forced fan, you increase the rate at which heat can be lost from the cell. That can only be good in terms of the cell's inner core temperature. But, not always so good in terms of the cell being able to provide a reliable end-of-charge voltage drop when being force-cooled.