true fast charge capable NiMHs

[**DONOTDELETE**]

Hi,

I have used a range of NiMH AA and AAA cells for quite some years (I guess I was among the first using NIMHs which became available to the ordinary consumer in the early 90s, from GP, and have since used Varta 1.1Ah, Friwo 1.1Ah, and Conrad Electronic 1.7Ah AAs, plus a range of 550 and 700mAh AAAs) but so far I haven't found any which I felt are capable of being truely fast charged, at around 1C, or at least 0.7C. They all get very hot at this charge rate.

Now I need to buy more and wonder which brand I should take.

Any recommendations?

Apprecciate your help

wus

 

[Klaus]

WUS,

get the Sanyo Twicells - I do charge them at 1C - and yes - they get hot

Klaus

 

[Ron Schroeder]

Over-heating is also a sign of improper end of charge detection and late shut-off. How are you charging them?

 

[Willmore]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Ron Schroeder:
Over-heating is also a sign of improper end of charge detection and late shut-off. How are you charging them?<HR></BLOCKQUOTE>

Ummm, I'm not quite sure I agree with this. For NiCd, the charging process in endothermic, that is, the reaction that occurs during charge takes up heat. For NiMH, that's not true, it's exothermic. You'll heat up *no matter waht you do* when charging NiMH cells. Sure, you'll heat up more with improper end of charge detection, but *some* heat is unavoidable.

I use a Ray-O-Vac PS4 to charge my NiCd and NiMH AA and AAA cells and it works great. It would say that it's the single best/most reliable charger I've ever used. Better than the Maha CF-777.

 

[Ron Schroeder]

Hi Willmore,

Endothermic???? I have never seen that characteristic with any battery. Durring the bulk of charge, electrical energy is converted into chemical energy mostly but at the end of charge and durring overcharge, the electrical energy is turned into thermal energy. Also "I" squared times the internal resistance of the cells is turned into heat. The NiMH internal resistance is somewhat higher than NiCad so they heat up more thruout the charge than Nicads.

I just finished data-logging the temperature rise of a nicad pack durring charge here at work this afternoon and it had a slow steady rise in temperature just as expected, not an endothermic drop.

Ron Schroeder
Brookhaven National Lab
WD8CDH
Day 631 344-4561
Nite 631 286-5677

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Willmore:
Ummm, I'm not quite sure I agree with this. For NiCd, the charging process in endothermic, that is, the reaction that occurs during charge takes up heat. For NiMH, that's not true, it's exothermic.<snip><HR></BLOCKQUOTE>

 

[Willmore]

Hey, Ron!

Do a web search for the terms and NiCd an NiMH and you'll get a lot of hits. One good one is: http://data.energizer.com/batteryinfo/application_manuals/nickel_metal_hydride.htm

Endothermic means that a reaction takes heat to perform. Exothermic means that it releases heat. Think of one of those 'squeez here to activate' ice packs as a good endothermic example and as, uhh, fire as a good exothermic example.

Sure, you're right, their internal resistance will give you some heat, but the internal resistance of both NiCd and NiMH are *tiny* and don't lead to much I^2R heating--unless you really give it to 'em.