I noticed that my batteries to my digital camera are AA NiMH batteries. On the batteries it states the voltage as being 1.2v compared with the 1.5v that are stated on the AA Alkaline (unless my eyes deceive me cause the print is ever so tiny). Anyway the AA NiMH batteries will drive the halogen bulb that comes with the MiniMag but it is brighter to my surprise than the Alkalines. How could this be. Both set's of batteries are fresh, fresh charged an the Alkalines did not have any use.
[ QUOTE ] Blikbok said:
NiMH has less internal resistance than Alkaline. More power is getting to the bulb, because of the increased amperage, even though the voltage is lower. So the bulb is brighter.
Some unregulated LED's can be seriously damaged by NiMH cells because the greater current flow is...unregulated.
Another way to explain it is alkalines start at about 1.5v and NiMH start at about 1.25v If you could measure the voltage with the Mag's lamp on you would see the voltage drop to about 1.1v to 1.15v per cell, while the NiMH under load only drops to about 1.2v. Thus you actually have more voltage even with the lower initial voltage battery. The reason is as stated above. In many high drain applications, such as digital cameras and flashlights, NiMH's will be a better fit. In a low drain device, like a single LED alkaline will win out.
Now if you want to beat either in any case, go to lithium AA's. A lot more $, not rechargeable, but they maintain about 1.5v per cell, even under large loads and cold.
To expand slightly on Brocks post. It is really a power rating or power calculation. Though you have less voltage, you have vastly higher current and volts X amps = power. So the extra current you have picked up with the use of the NiMH cell way offsets the slightly lower voltage.
In this vein, I ask a question, or rather present a situatiion. My Blaster II+ pulls 600+mA's with fresh alkaline D's, and 350mA's with 9Amp NiMH D's, testing 1.3 volts each. Why the difference? I do know that my Blaster II+ has built in resistance. Is this the problem? Too much resistance for NiMH's? No such problem exists for my EL Tri-Star, comparing NiMH's to alkalines.
It has to do with the alkaline D cells. They have a lower internal resistance then their tiny AA brothers so they don't drop voltage under load as much. So instead of dropping to 1.1v per cell they probably only drop to about 1.35 or so, which is higher then the NiMH cells to start with. Think of a D cell as a bunch of AA’s in parallel.
The resistor does play in to it also. It basically lowers the voltage even further from the start. The lower the voltage, the lower the mA draw.
[ QUOTE ] Brock said:
It has to do with the alkaline D cells. They have a lower internal resistance then their tiny AA brothers so they don't drop voltage under load as much.
[/ QUOTE ]
That's actually not true, although it's a widely held misconception. Check it out.
Eveready, for instance lists the internal resistance of the E91 AA cell at 146 miliohms. The E95 D cell is 173 miliohms. This is becuase the current path is much shorter in the AA. Capacity (a function of chemicals) is another matter, and is in play in the case in hand.
It is far from 'a bunch of AA cells in parallel', at least at this level.
I just checked out http://data.energizer.com/
According to them, internal resistance for D Nimh ranges from 11-21 milliohms.
For nimh AA's, its 30-40 milliohms. [img]/ubbthreads/images/graemlins/smile.gif[/img]
It has to do with the level of current draw. Alkalines are usually rated at higher voltage, and under light current demands will maintain the higher voltage.
As power demands increase alkalines suffer more voltage drop than NiCad or NiMh rechargables, so at higher power drain levels rechargable batteries can actually end up delivering higher voltage and current.
Alkalines start out at 1.5v, but under heavy load can drop to 1.1v or less. NiCad and NiMh batteries start out at around 1.3v, and will typically maintain 1.2v even under very heavy load.
This is why alkalines work better in low current demand devices like radios, clocks, remotes, etc., and rechargables work better in high drain devices like digital cameras. (And why you can take a set of "dead" alkalines from a digital camera and put them in a radio or remote and they will work fine - they've still got plenty of power, just not enough current capacity.)
As far as flashlights are concerned, it all depends on how much current at a given voltage the light demands. For example, if your light is running 4 cells and a 5 to 6v lamp, then rechargables won't deliver maximum brightness because the voltage required is more than the typical load voltage of NiMh batteries (1.2x4=4.8v). On the other hand, if it was say a 4.5v lamp that was hungry for more than an amp of current, then NiMh would be the better choice, since under that kind of load they will still deliver enough voltage, while alkalines will usually choke.
The "break even" point is not an exact number, as different battery brands and variations in chemistry will affect performance.
All of the above is based on unregulated lights. Electronic regulation circuits found in many LED and all HID lights will compensate for different battery types and you will find little if any performance difference in regulated lights (at least as far a brightness is concerned).