NiMh's to Alkalines?

[logicnerd411]

NiMh\'s to Alkalines?

WHat is the difference between the rechargeable NiMh's and plain alkalines? Less voltage, more amps? What does this mean? Please compare it to a water hose with water flowing(the best analogy for electricity, IMO).

 

[Brock]

Re: NiMh\'s to Alkalines?

Hummm, well that is a hard one. Alkaline’s actually have a bit more power in them (total water) but it comes out at a slower rate. Sort of the problem is as you increase the load on an alkaline cell the voltage begins to sag rather quickly. Alkalines start at about 1.5v, while NiMH start at about 1.25v Under a really small load, like a single LED, the alkaline will be brighter. Now if are trying to run a high power incandescent lamp like say a Streamlight 4AA the alkalines will drop below 1.2v rather quickly, then the NiMH's will actually be brighter longer after the initial charge comes off the alkalines.

Another thing to consider is the output curve. An alkaline cell has a slow slope down the entire time as the battery is used. This means most of the time after about 1/2 the batteries life you will replace it because the voltage has fallen to far (under 1.0v). NiMH's on the other hand hold their voltage much better; they start at 1.25 and run most of their life about 1.2 pretty flat.

Also NiMH's self discharge at about 1% a day or 30% a month, while alkalines self discharge much slower, I think it was like 2% a month?

This probably didn't help and hopefully someone else will come along and help clear it up.

Basically if you go through batteries a lot (every other week or less) switch to NiMH, if you replace them every other month your at the questionable point, and more then that stick with alkaline. Unless your like me and put NiMH's in just about everything and charge them once a month. Except the lights that use lithium AA's, but that’s a whole different story.

 

[logicnerd411]

Re: NiMh\'s to Alkalines?

Basically, my question is:

Why can't I use NiMh's in some flashlights?

 

[ikendu]

Re: NiMh\'s to Alkalines?

The Streamlight 4AA 7 LED seems to have a problem with NiMHs (and Lithiums) because it seems to have been engineered to REALLY OVERDRIVE the LEDs for brightness. New alkalines sag in their voltage fairly quickly and apparently the SL 7 LED can survive this brief period sufficiently. However, if you run this light on NiMH, the NiMH don't sag like that and continue to overpump the LEDs greatly overheating them and leading to their destruction.

SL apparently is recognizing this problem and has added a resistor to help limit the current load to the LEDs.

Other than this sort of circumstance, many people use NiMHs in their lights quite successfully. I use 4 AAs in a UK 4AA AS2 incandescent and use 4 AAAs in a PT Attitude 3 LED light. Both have been just fine.

The Opalec Newbeam 3 LED module for a 2 AA Minimag is a favorite for NiMH 'cause its regulation circuit keeps the light at contast brightness regardless of battery type until the batteries are basically exhausted.

 

[PeterM]

Re: NiMh\'s to Alkalines?

Originally posted by logicnerd411:
Basically, my question is:

Why can't I use NiMh's in some flashlights?

<font size="2" face="Verdana, Arial">From what I've gathered on other posts here, NiMh's have less internal resistance than alk's. so in some, mostly direct drive, lights they will overpower the LED, whereas the alk's internal resistance will inhibit the voltage despite the higher published rating. There is a fairly extensive discussion of this in the latest LGI-2 thread where the consensus agrees with my experience that if you don't use NiMh's hot out of the charger, you probably won't get any magic smoke. LED life is perhaps another story.