Alkaline to lithium

[arowana]

Hi, I just got myself an unbranded 9led light that comes with 3 AAA Alkaline batteries. I'm just wondering whether it will be risky for me to try putting in lithium batteries.. does anyone think I will wreck the leds or circuit if I try that? Also, would it likely be brighter than using alkalines? Thanks in advance for any replys.. /ubbthreads/images/graemlins/smile.gif
(Don't worry just post what you think even if you're not 100% sure cos I didn't spend much on it so if it breaks so be it /ubbthreads/images/graemlins/smile.gif)

 

[rwolff]

Safe route: get another flashlight of similar brightness. Load up a set of 3xAA alkaline in the new light, and turn it on, pointed at a wall. Briefly turn on the other light, pointed at the same wall. Compare brightness. Leave the "test" light on for an hour, briefly turn on the other light, and compare brightness. Repeat until the "test" light goes dead (uses up a set of batteries, but it's better than finding out the hard way how long a set will last - of course, that's why you've got an ARC AAA for EDC /ubbthreads/images/graemlins/grin.gif ).

If the "test" light dims throughout its run, it's probably got a "brain dead" drive circuit, and Lithiums may blow it. If it runs at fairly constant brightness, it's probably got a decent regulator, so you've got a better chance of it working with Lithiums (not familiar with AA Lithium - are they 3V like the button cells?).

 

[led_replacement]

I'm not sure about that led flashlight but I do know Lithium batteries will burn out the Tektite LT-1 led replacement for AA and AAA Mini-Mags. Lithium AA and AAA batteries measure 1.7 Volt and the alkaline measure 1.6 volt when new.
led_replacement

 

[arowana]

[ QUOTE ]
rwolff said:
Safe route: get another flashlight of similar brightness. Load up a set of 3xAA alkaline in the new light, and turn it on, pointed at a wall. Briefly turn on the other light, pointed at the same wall. Compare brightness. Leave the "test" light on for an hour, briefly turn on the other light, and compare brightness. Repeat until the "test" light goes dead (uses up a set of batteries, but it's better than finding out the hard way how long a set will last - of course, that's why you've got an ARC AAA for EDC /ubbthreads/images/graemlins/grin.gif ).

If the "test" light dims throughout its run, it's probably got a "brain dead" drive circuit, and Lithiums may blow it. If it runs at fairly constant brightness, it's probably got a decent regulator, so you've got a better chance of it working with Lithiums (not familiar with AA Lithium - are they 3V like the button cells?).

[/ QUOTE ]

Hi rwolff, thanks for your suggestion. I'm not sure what you meant by the first part of your reply (do you mean get another light to test it?) but I will remember the second part. I haven't use the light enough to see if it goes dimmer or it blacks out suddenly so I guess I have to be patient and use the light more to get the answer. /ubbthreads/images/graemlins/smile.gif

 

[arowana]

[ QUOTE ]
led_replacement said:
I'm not sure about that led flashlight but I do know Lithium batteries will burn out the Tektite LT-1 led replacement for AA and AAA Mini-Mags. Lithium AA and AAA batteries measure 1.7 Volt and the alkaline measure 1.6 volt when new.
led_replacement

[/ QUOTE ]

oh no.... /ubbthreads/images/graemlins/icon23.gif

 

[watt4]

yeah. what about the 2 led, 3xAA tektite/trek lights?

 

[markdi]

3 aa probably just has a resistor to limit current

or is direct drive

 

[eebowler]

arowana, many of the 3AAA lights I know of are direct drive, ie, between the battery and LEDs, there is no added resistance. The LEDs therefore gets 4.5V (1.5Vx3) from three fresh cells. This voltage overdrives the LEDs but isn't much of a concern since the relatively high internal resistance of AAA cells prevents too much current from flowing and frying the LEDs.

Lithium cells when fresh have a pd of arround 1.7V. Three of them in series will provide 5.1V to the LEDs.Worse, they have a lower internal resistance and therefore are capable of providing higher currents. That combination will very likely start killing off the poor LEDs within minutes...

Oh, and to answer your question, Lithium cells will be noticably brighter than alkaline...

 

[arowana]

Thanks eebowler, guess I will give up the thoughts of using lithium cells in my led light.

Thanks everyone for your inputs. /ubbthreads/images/graemlins/smile.gif

 

[LightObsession]

eebowler, I measured some fresh alkaline and lithium AAs last night and some of the alkalines were over 1.6v, some were about 1.5v depending on the brand. The lithium was 1.7 as previously stated. The batteries WERE NOT under load when measured with my Fluke 73 DMM.

My understanding from reading other posts is that the alkaline battery peak voltage will sag much more under load than the lithium peak voaltage and therefore the 1.6v initial votage isn't much concern.

I'm still learning about these things and haven't acutally tried the lithium AAs I purchased last night in anything. The lithium battery package states that the Energizer will repair or replace any device damaged by the batteries if you want to go through the inconvenience and expense of shipping both items to them.

David

 

[mrme]

All is not lost. If you want to change out the resistors, you could put in ones that match what you need to run on lithiums.

The main problem is that the lithiums have a lower internal resistance. The light designers were counting on the batteries to have a certain internal resistance, which you won't have with the lithiums. So, take a 50 ohm or so potentiometer (variable resistor) and a DMM with current measuring ablilities. Figure out what resistance you need to kepp the current on EACH LED below a safe level, then install resistor(s) of that value. Make sure your batteris are fresh, otherwise fresh batteries with less internal resistance will toast the LEDs.

But why bother?

I don't know why you would feel the need to put lithiums in a cheap LED light anyways. Lithiums outperform alakalines in high drain devices, but their benifits are diminshed in lower drain devices (like yours). They may have lower self discharge rates(10 years instead of 5) and be more cold tolerant, but these rarely justify the increased cost, especially in cheap lights.

Just put some good old fassioned alkalines in it and enjoy cheap light. If you use lithiums, you will the economy the LEDs would give you.

Save your money for a light capable of benifiting from the expensive batteries.

 

[rwolff]

[ QUOTE ]

Hi rwolff, thanks for your suggestion. I'm not sure what you meant by the first part of your reply (do you mean get another light to test it?)


[/ QUOTE ]

What I meant was a second light so you could do "head to head" comparisons of brightness between the LED flashlight whose brightness/time you were wanting to check and a light of known brightness. That's why I said to only turn the other one on briefly - so that you wouldn't drain its batteries.

You need to go "head to head" because trying to determine changes in light level of a single light source using the "Mk. 1 Rev. 0" eyeball as an instrument are doomed to failure. Better than a "head to head" test judged by eye (if you have the equipment available) is to use a camera with light meter (either "match needle" metering, or auto exposure that tells you what it's planning to do). Choose an ASA rating and shutter speed to put the illuminated surface at the smallest aperture available at the beginning of the run (you're not taking pictures, you're just using the camera as a light meter). If your camera is aperture-priority, choose an aperture that will give the fastest shutter speed available at the beginning of the run. Every hour, see what aperture (or shutter speed) is recommended. Remember that one "F" stop difference represents a doubling (or halving) of the light, and that doubling/halving the number represents two "F" stops (i.e. F2 lets through 4 times as much light as F4).

This test must be done where the light source under test is the only source of illumination, and you need a focal length/distance to subject combination where the entire viewfinder area is filled with the area illuminated by the light under test. It doesn't matter if you need to get too close to focus properly - again, you aren't taking a picture, merely using the light meter. If, a couple hours into the test, the camera recommends an "F" stop or shutter speed that lets through twice as much light as at the start of the test, it means that the light under test has dropped to half brightness.

 

[Lynx_Arc]

If you can get an ammeter inline with it and have a 1k variable resistor you can test how much current is going to the LEDs. Just take the variable resistor, turn it to one side and figure out how to connect to get maximum resisrance. Then put both the resistor and ammeter inline with the light and LEDs and slowly crank the resistor the other way noting the current. Do this with alkalines and note the final current (0 ohms) then you can reset the resistor to max and use other batteries and when you hit the same current draw break the circuit and test the variable resistor and write down the resistance. You can overdrive LEDs but the amount depends on the LED. Probably 40-50 ma overdrive is a good limit to stay under or the current that the alkalines drive it at.

 

[bjn70]

I gave 2 Gerber Trios for Christmas, and loaded them with Lithium AA's beforehand. I told the recipients to save them for emergency lights- one in a car glovebox and one in the house. I wanted the Lithiums to be able to sit there for years without going dead or leaking, and I don't have faith in Alkaline batteries to do that.