Surefire E2DL going dead/battery question

[tonywalker23]

Before I start I'll say that the extent of my knowledge about batteries and electricty is don't touch the metal when plugging something in… So,

I've had my E2DL for maybe a month or so. Couple of nights ago I was shining it around and noticed it stopped coming on high. It isn't like the high slowly got weaker (at least my eyes didn't think so). It went from high to low to staying on low. The next night at work I got someone who uses a voltmeter? to measure the batteries. The two batteries together were measuring 5.8 volts. Well I kept the same batteries in it and last night I used it for about 2-3 hours on low until it started flickering.Tonight the batteries are measuring 5.5 volts.

I guess my question shows how little I know about the subject but my brain says batteries are dead when there's no volts left but it seems like my surefire says the batteries are dying when they go from 6 volts to 5.5. Am I understanding this correctly/can someone maybe enlighten me a little bit?

 

[rookiedaddy]

Hello tony,

I believe the readings from your batteries are off-load. To reliably get the state of the battery reading, you should do a load-reading, e.g. using a 10 Ohm resistor. There is an excellent guide by HKJ here on how to do that:
Simple guide to using a DMM for measurements

However, at 5.5V (divided by 2, that's ~2.75V per cell) off-load reading, it's an indication the batteries are almost fully drained... from my experience.

 

[65535]

The batteries are dead.

When loaded their voltage will drop, when sitting around and attached to the meter they have what's called resting voltage, which is higher than the voltage under load.

Batteries rebound after being discharge they will read at a higher voltage than you might expect but when loaded they just don't provide much power anymore.

Surefire flashlights use regulation on their LED models (most if not all) which rather than dimming out will stay bright until the battery no longer provides enough power, at that point in order to prevent failure of the battery the light switches to the lowest mode in order to keep the light usable and prevent a total battery failure.

A new set of batteries and all will be well again.