Why are most NiMH batteries 1.2v not 1.5?

soccerballtux · 10-23-2009, 08:37 PM

Do they not make higher voltage NiMH AAA batteries? I would prefer to get a higher voltage rechargeable for my laser pointer. That way it would stay brighter for longer (voltage tapers off as it loses juice).

bobsmith1492 · 10-23-2009, 08:58 PM

The electric potential between the chemicals is fixed. It's not something that can be changed for a given type of cell. I.E. NiMH/NiCd are about 1.2V, alkaline are about 1.4V, and Li-poly/Li-ion are about 3.7V.

Also, NiMH cells don't lose voltage much; their discharge curve is pretty flat after a few minutes:
http://www.shdesigns.org/batts/battcyc.html

Alkalines aren't 1.7, though. They may start about there but under load will quickly settle to a lower voltage:
http://www.powerstream.com/z/AA-100mA.png

soccerballtux · 10-23-2009, 09:02 PM

Wow, /thread, thank you sir. I love ATF.
I simply recall taking a voltmeter to some duracells and thinking, jeez, 1.7v, that's 3.4 for 2 of them.

bobsmith1492 · 10-23-2009, 09:17 PM

You could put 3 NiMH in series for about 3.6 and probably get away with it... probably need some tape to hold it together though!

Rubycon · 10-23-2009, 09:17 PM

What kind of laser pointer - i.e. is it red or green?

In any case a driver designed for slightly lower terminal voltage will give you more runtime/stability over the length of the charge cycle.

soccerballtux · 10-23-2009, 11:25 PM

Green. Straight from China, $10 shipped from Hong Kong.

Rubycon · 10-24-2009, 04:01 AM

Yeah those have no regulation. Increasing the input voltage using three cells as Bob mentioned will definitely increase brightness however the MCA (crystal set) may not handle the additional output of the pump diode or the pump diode may over heat if the button is held in for more than 10 seconds at a time. (more likely)

Best bet is to use a good quality 800+ mAh nimh cell and keep it charged IF you use it a lot. (or just buy a lot of Alkalines!)

The more expensive Leadlights have APC and will keep the power right at 5mW throughout the battery life whether you're running nimh/nicd/alkaline or Lithium.

soccerballtux · 10-24-2009, 11:41 PM

Quote:

Originally posted by: Rubycon
Yeah those have no regulation. Increasing the input voltage using three cells as Bob mentioned will definitely increase brightness however the MCA (crystal set) may not handle the additional output of the pump diode or the pump diode may over heat if the button is held in for more than 10 seconds at a time. (more likely)

Best bet is to use a good quality 800+ mAh nimh cell and keep it charged IF you use it a lot. (or just buy a lot of Alkalines!)

The more expensive Leadlights have APC and will keep the power right at 5mW throughout the battery life whether you're running nimh/nicd/alkaline or Lithium.

Why would I want one stuck at 5mW...

As I'm sure you know, heat is definitely not an issue for semiconductors...even the 150mW solid state greens don't have any dissipation mechanism. The diode is not getting anywhere close to 125C (where I would start worrying about the lattice).

Rubycon · 10-25-2009, 12:52 AM

Quote:

Originally posted by: soccerballtux

Why would I want one stuck at 5mW...

As I'm sure you know, heat is definitely not an issue for semiconductors...even the 150mW solid state greens don't have any dissipation mechanism. The diode is not getting anywhere close to 125C (where I would start worrying about the lattice).

Anything above 5mW is not classified as a pointer any more - it's a module and must have additional safety features.

Additionally, these cheap pointers output nowhere near claimed levels of 532nm. Most omit the IR filter so if you measure total output you will indeed see an artificially inflated reading which most will be invisible and dangerous IR.

How DPSS lasers work

Pump diodes MUST be kept cool (below 35 deg centigrade!) for best lifetime AND to keep their outputs close to 808 nanometers which the vanadate needs in order to lase at 1064 before getting halved to 532 by your other half of the MCA (KTP crystal).

Heat definitely WILL kill the pump diode and MCA. These cheap lasers don't use discrete optical elements (Vanadate/KTP) instead use a MCA (Monolithic Crystal Assembly) in which the two are bonded together. If these are over pumped dark spots in the bonding agents will appear which will rapidly heat up and your MCA is done at this point. Luckily mode hopping occurs long before this point so if you're trying to shoot for higher outputs you can back off once you see mode hopping taking place.

A true 150mW 532nm laser will not only have discrete optical components including adjustable output coupling mirrors but also thermoelectric cooling to keep vanadate/KTP stable throughout the operating range.

herm0016 · 10-25-2009, 04:24 PM

if you want to measure the voltage under load use few hundred ohms in parallel with your volt meter, or get a real battery tester with a load built in.

deputc26 · 11-03-2009, 03:37 PM

For highest acceptable voltage in two standard cells go lithium, here's the link to the forum where this testing was done. http://www.candlepowerforums.com/vb/...ad.php?t=64660 They are fairly rabid battery enthusiasts.
http://img.photobucket.com/albums/v4...parison05A.jpg

Modelworks · 11-03-2009, 04:47 PM

yeah lithium is the way to go if voltage is a priority. I have a camera that can only be run off lithium batteries. AA alkaline will fit but they don't provide the minimum 3.7V the camera requires. So now I use lithium ion rechargables.

RedSquirrel · 11-03-2009, 05:47 PM

I've always wondered this too. It sucks as it limits the uses of rechargeables, and I try to stay green and use them for everything I can. But lot of stuff wont work due to less voltage. For example my programmable thermostat, my phone etc. I suppose another option would be to mod stuff so it can handle an extra battery but then it's giving it a bit too much juice.