I liked my Lithium Ions until the batteries died. They cannot be rebuilt. The other issue I have with them is that they give 100% of their charge up until the last. This is OK for most applications, but I refuse to use the flashlight. I don't want to go under a house or into an attic & have the light just up & die without a little warning.
Actually, with Lithium Ion batteries you really want to keep the batteries 100% topped up between uses any how. Lithium Ion batteries thrive on shallow charge-discharge cycles (just the opposite of NiCd) but have a short service life if repeatedly deep-discharged (again, the opposite of NiCd). So, with Lithium Ion batteries you will want to regularly top off the batteries at the end of day, preferably well before they are anywhere near dead. Since they do not self-discharge anywhere near as aggressively as NiCd batteries, this means that they will still be at nearly full charge the next time you need them -- even if it is several *months* later.
Since the batteries will always be at, or very near, full charge, you really don't need to be too fearful of the flashlight going dark anytime soon. In the event you run the flashlight so long that you do run the battery dead then the solution is to swap the dead flashlight battery for your drill/saw/etc. battery and make a strategic exit. In the event that your flashlight and drill batteries are not compatible, a small pocket-sized penlight (esp. if a bright LED) will provide plenty of backup light to make your exit and swap your discharged battery(s) for a fully charged battery(s).
Even so, if you pay attention you will usually get 1-2 minutes of warning before the Lithium Ion batteries cut out (it is actually a protective circuit that cuts the power to prevent irreversible damage). The light (or tool) will begin to dim (or lose power) by about 10-20% in the minute or two before the protection circuit cuts the battery out.
I dare say, I'm much more sensitive to your concern than most as my sense of balance is primarily visually based (so much so that I look like a drunkard trying to walk in a tall building on a windy day -- since the swaying means that vertical and horizontal references no longer correspond to plumb and level -- I can not even stand for more than a moment without light). Because of this I keep a very small pocket flashlight on me at all times (about 3/8" diameter and 2-1/2" long) on my keychain -- just enough light to get out of most any bind safely.
By extension of the above, however, users who regularly and repeatably run their batteries until dead will typically suffer premature battery failure (i.e. short service life) with Lithium Ion batteries and are actually better served with traditinoal NiCd or NiMH batteries. Those who, like myself, tend to have frequent shallow charge-discharge cycles will most greatly benefit from Lithium Ion technology because this is where Lithium Ion batteries truly shine. The shallow charge-discharge cycles greatly extend the expected service life of a Lithium Ion battery.
HTH
