Don't do that, it's very dangerous even if it seems to work ok, and stays dangerous even after you take the batteries out of the light. Batteries from different manufacturers don't always have the same chemical composition and the differing chemicals can create an electropositive feedback loop which can make permanent zeta resonance in the flashlight body. Then later if you put in a fresh set of batteries whose chemicals match the resonance, the flashlight body can be charged with up to 20,000 volts. You need to desresonate the flashlight by N-ray discharge on a good, inverse suspended seismic stability platform (you don't need a really big one, 20-25 tons is enough) or else it can go up in a large explosion completely unexpectedly at any moment! If you don't have proper deresonation equipment, since you're such a nice guy I guess I can take care of it for you. Just send me any flashlights you might have ever put mixed battery brands into and I'll make sure they're disposed of properly, especially Arcs, McLuxes, and Surefire E2e's in HA finish. Please send them immediately, the safety of your neighborhood depends on it. [img]/ubbthreads/images/graemlins/wink.gif[/img]
I've mixed a few in my time. Usually as you said, when I'm in a jam and really need some power and quick. But I've always kept them together with similar chemistries, ie alkalines in one pile, carbon-zinc in another. Never mixed carbon-zinc with alkalines before, and never mixed rechargeables with non-rechargeables.
If the chemistries are similar, you should not see too much problems with the mix. If, however, you mixed different chemistries, there could be quite a severe difference in internal resistance, power output, power curve, etc. This will result in reduced runtime, reversed charging and other nasty behaviour. That nasty behaviour includes the batteries leaking or blowing up, though with fingers crossed, there should not be enough available current for something quite that dramatic.
But yeah, alkalines from different brands all use pretty much the same chemistry and internal construction, so there usually isn't a big enough difference in performance parameters to cause serious problems to your flashlight. Just be sure to check regularly for leaks.
Within the same chemistry, I would think brands matter far less then useful life remaining.
Few if any of us know the remaining life in each battery in the 'junk box' anyway, so in an emergency it seems safe enough. Battery life is sure to suffer as the first cell to die brings on dark.
However, I've recently been made aware of a possibly serious (but hopefully fairly remote) threat, blowing up the flashlight.
It seems that alkalines when seriously abused (like a discharged cell in a high current battery string being very reverse recharged) outgassing hydrogen. This is a small amount of gas, not enough to rupture the flashlight case, but if you've got good seals (like in a dive or other waterproof light) it forms an explosive atmosphere (at about 7% as I recall) with the oxygen already there and waits for the spark of turning the light on or off to explode.
I'm told for this very reason 'all dive light makers have been including platinum catalysts to recombine the hydrogen for the last 25 years or so'. About the time alkalines have been in common use.
For sure they're not increasing the cost of lights that sell for less than $10 without reason.....
How hard you have to abuse them (if it's even possible in a low power flashlight) I don't know. For sure it's a bigger problem with long battery strings as well.
So I guess it's safe enough with true dive lights (the ones that rattle from PT and Streamlight for instance), but perhaps not so with others? And I think it's more OK to mix brands than same brand cells of differing remaining capacities. I'm not sure mixing chemistries is a problem, either.
I wouldn't worry about explosions as much as I would worry about the batterys leaking. If you use batteries with different levels of charge and one gets discharged before the others, it can reverse the cell and it can leak all over the inside of your light. I lost a mag that way. I left it for too long and one of the batteries leaked and started corroding the inside of the light.