Bad battery?
 

I think my battery is shot. My son was playing with the light and must've run the battery pack all teh way down. I recharged it overnight and the light flashed when I mated the plugs and the illuminated ring glowed green. So far, so good. When I went to change modes, the light just died- no more glowing ring, won't turn back on- won't even flash when I unplug and then make the connection again. Place it back on charger for a bit and it is more of the same- flash, glow, on, then dies when I touch the switch.

Here is the light in question-http://dx.com/p/hi-max-lz-u3-3-x-cre...x-18650-219001

I would put the battery on a voltmeter and check the voltage after letting it charge. It should read 7.4v. It could be the switch on the light. If your son was just clicking away he may have done something to it. If you don't have a DVM, just try the battery on something else that takes that voltage. Do you have another light?

The only other light I have is a Trustfire in 1x18650 format.

My wife was an Electronics Tech in the Navy, so there probably is a DVM stashed away somewhere around here. I foresee a frozen 'rita on our next date night if she agrees to fiddle with it.

The fully charged voltage should be near 8.2v.

If the battery were completely discharged, you may have activated the protection circuit. Overdischarge can lead to cell damage which renders the cells unstable and unsafe to continue using. The protection circuit basically locks the batteries so that they can be used in this unsafe state.

It should read between 6.0 (as low as they should ever go) and 8.4 volts (fully charged.)

But more to the point, the plugs these use are not conducive to the use of a voltmeter unless you get a male plug to plug into it and then measure the voltage on that. If you stick the voltmeter probes into the plug, you're quite likely to short it out, even if you know exactly what you're doing and have very steady hands. And shorting it out, even just for a second, can easily damage the protective circuits.

If you can't get a matching plug, you're better off cutting off the outer layer of insulation on the wires and putting your sharp probes into the inner wire's insulation and measuring the voltage that way.

In any event, cyccommute's comment is probably right on -- it's probably gone below the voltage that the protective circuit will allow you to charge it. If so, the battery could probably be revived by cracking it open and bypassing the circuit and charging the individual cells back up past that level, but there's a risk to that too -- the circuit prevents charging an overly discharged cell because the cell is likely damaged by that. If it's only overly discharged by a little, it's probably OK, but the circuit is playing it safe.

That said, you can buy replacement batteries fairly cheaply. The $16 Magicshine clone on Amazon includes a battery that's probably pretty much identical to what you had, and it comes with a charger and light head to boot at that price. (And it's also possible that your charger is what's bad.) The only possible gotcha is that the plugs may not quite match, and you might have to cut some wires and move plugs around or replace them. And there are other replacement batteries out there as well.

Those 4 cell battery packs that are sold along with the china clone lights are not made with the best quality cells. The Fenix BT20 allows a user to drop in two quality 18650's that have been charged on a quality charger. The case they drop into is waterproof. Fenix doesn't make outrageous lumen claims for their light. It's rated at 750 lumens, and it puts out just a tad better than that. I'm surprised more of those battery packs sold by fastech and dx haven't exploded and burned peoples houses down.