LED flashlight- can I drop in NiMH batteries?

[Comdrpopnfresh]

I got a B&D LED snakelight flashlight for the holidays. Since it is running LEDs, would going from 3.0V w/ two alkaline C's in it to 2.4V w/ two NiMH C's in it still drive the LEDs?
I use AA and AAA NiMH everywhere, but getting the rechargeable cells and a charger to fit 2 C's just to find out if they do work or not is lame.

 

[DayLaPaul]

Most LED's can accept a range of input voltages. My guess is that it should work, but the only way to know for sure is to either look at the spec sheet for that particular LED emitter or just try it out and see.

 

[Rubycon]

It depends on whether or not the light has a driver or is direct drive. Also some drivers switch to direct drive when input voltage falls below a threshold value. The difference between chemistries (alk vs. nimh) certainly would pose a real change in performance unless the circuit (driver) is specifically designed for a wider range of terminal voltages. If it's specific then expect a very short period of run time before switching to direct drive. Nimh has a relatively flat curve on discharge so expect decent output, albeit lower than alkalines.

Of course it really depends on the driver and type of emitter. Best way is try it out!

 

[edro]

Yes, it will work fine.

 

[the DRIZZLE]

The Fenix brand lights are actually designed for NiMh batteries. Apparently the low light modes don't work right when using alkaline batteries.

 

[sygyzy]

Put it this way, it won't hurt it. You might get decreased output though but you probably won't care.

 

[Comdrpopnfresh]

maybe I'll put two NiMH AA's I use in my LED maglite (rayovac platinum rechargeable that are 2100mAh) in a 3V 2xAA serial holder and put some 'gators to the contact. Don't need to run the lamp, just the flashlight end and don't need to run it long, just get the 2.4v from my best AA cells. It's a chemistry issue, not a mAh condition.

 

[Comdrpopnfresh]

the flashlight didn't get very bright. the 17-LED lamp got bright, but seemed to be flashing. When run on the alkaline cells, the lamp hums like a transformer is in it.

 

[Jeff7]

It depends on whether or not the light has a driver or is direct drive. Also some drivers switch to direct drive when input voltage falls below a threshold value. The difference between chemistries (alk vs. nimh) certainly would pose a real change in performance unless the circuit (driver) is specifically designed for a wider range of terminal voltages. If it's specific then expect a very short period of run time before switching to direct drive. Nimh has a relatively flat curve on discharge so expect decent output, albeit lower than alkalines.

Of course it really depends on the driver and type of emitter. Best way is try it out!

Or else they just do away with the drive circuitry completely and run the LED right off of the battery.

With a white LED, the forward voltage should be high enough that the current won't be a problem.
Uneven and suboptimal light output, but hey, it's cheap, so ship it!

 

[sygyzy]

Or else they just do away with the drive circuitry completely and run the LED right off of the battery.

With a white LED, the forward voltage should be high enough that the current won't be a problem.
Uneven and suboptimal light output, but hey, it's cheap, so ship it!

The OP is clearing asking about a plug and play solution. He's asking if he could drop in some NiMH rechargeable batteries in place of his alkalines and if his flashlight would still work. And you are suggesting he rewire it to by pass the driver?

 

[Rubycon]

the flashlight didn't get very bright. the 17-LED lamp got bright, but seemed to be flashing. When run on the alkaline cells, the lamp hums like a transformer is in it.

Yep there's a boost circuit, the hum is inductor whine/sing.
The flickering is caused by the terminal voltage near the switching point.
My Surefire L4 (designed for two 3.0V CR123) does this when powered by a single Pila at 4.2V. When the cell is nearing its end of charge it will strobe like crazy often many minutes before its protection (the Pilas are protected) kicks in.

 

[Jeff7]

The OP is clearing asking about a plug and play solution. He's asking if he could drop in some NiMH rechargeable batteries in place of his alkalines and if his flashlight would still work. And you are suggesting he rewire it to by pass the driver?

No, I was just saying that an alternate way that some manufacturers do simple LED lights is to run the LED(s) directly off the battery, without any kind of driver circuit.
I could stick a white LED on 2 AAs and it should light up without issue. Most white LEDs that I've seen have a forward voltage above 3V, so they'd be underdriven at 3.0V or less, assuming nothing wacky happens that would severely reduce the forward voltage.

So my post may not necessarily apply to this specific device, it's just general information. And Rubycon's already mentioned the fact that it sounds like there is some kind of driver circuit in there. Good to know that B&D went the extra distance to put some kind of real electronics in there. (Whether it's a genuine constant-current driver, or just a constant-voltage-output booster...don't know.)


Edit: Duuuurrrr...and now I see that Ruby also said "It depends on whether or not the light has a driver or is direct drive." in post #3.

 

[Comdrpopnfresh]

charged up my 2 rayovac AA's in an eight hour overnighter. Put them in a 2-AA holder that serializes them. the LEDs were flashing before because the AA cells were not so fully charged, and the contacts on the snake light were in need to pressure, not just contact. It ran just the same, just as bright.

So Now...
Whats the best NiMH C-cells for a 17-array LED lamp with a 3-LED flashlight mode? Runtime on regular alkaline cells was claimed to be 5hours on the working lamp, and 64 continuous hours on the flashlight. I am looking for low-self discharging ones. I saw a variety that had 85% of charge after being on the shelf for a year, and 70% at the the two year mark.
How many mAh are necessary to equivocate a pair of NiMH cells with 2 of the Eveready Super Duty it came with?

 

[Rubycon]

Get Enloops and don't look back.

 

[shortylickens]

http://www.candlepowerforums.com/vb/forum.php

 

[Comdrpopnfresh]

what about those other 1.6V NiZn cells I've seen- Are they better suited to replacing the alkaline than the 1.2V NiMH?

 

[0roo0roo]

Yea candlepower has some threads with runtimes/brightness graphs alkaline vs nimh... for most nimh is perfectly fine, if not better
Alkaline only good for low drain devices.