Good way to ruin your batteries, though any decent charger will know to stop charging.
With modern low self discharge batteries this isn't necessary.

That's so simple that I never thought of this. Does anybody know if that would also work with a single AA flashlight? I'm pretty sure I know the answer, but I don't want to find out the hard way...

Whether it works with a single AA flashlight depends on many factors, but the sad answer is likely no.

With an incandescent bulb, polarity is rarely an issue, and the light should still work, unless the battery connections are such that they will not make positive contact with a reversed battery (which is unlikely).

With an LED, there's a chance that there's some minor circuit protection, but in likelihood there's not much, if any. Reversing the battery would simply forward-bias the LED (as with a normal diode; most LEDs must be reverse biased to light up IIRC), and the battery would simply drain.

Both cases also require the switch to be on, of course.

For $20 gear just head to Target, lots of nice options

Apparently the earlier posters learned this is not good enough in the real world where people let batteries run down and do not replace them.

Reversing one battery is a multi-battery flashlight option.

Homeless Depot has got new Duracells that have 7 yrs. guaranteed Fresh on All the battery types,I just Refreshed my supply last week!I've been Reversing batteries in ALL of my lights & equipment for many,many years & I've had NO Ill effects regarding power loss or drain,I do however reverse all the batteries,whether 1 or more,& for the CR2032's Lithiums& such,I place a piece of (yellow) gas line teflon tape over the contact point,eliminating any power drain, short of water intrusion.The Main thing here though,Is to Check your equipment,Often,Regardless!

Wow - second only to candlepowerforums.com folks, I would have expected everybody here to have a fully charged ready-to-go flashlight on their bodies at all times.

Dead batteries when you need them? I'm shocked ETS'ers!

I've always got about a dozen Eneloop NiNH's in various sizes charged and ready to go and a small handful of Li-Ion 14500's ready to go as well (for the devices that can take these batteries). And the fallback is several dozen alkalines in every size imaginable. And just in case my flashlight itself fails, I've got about two dozen (or more) spares.

Yes. "My name is Dave, and I'm a flashaholic."

In a perfect world all flashlights should be checked rehularly, perhaps monthly.
In the real world however this gets neglected.
A compromise might be a number of cheap basic flashlights fitted with new fresh alkaline or lithium cells.

Premiun alkaline cells are warrented good for 7 years from production, so if fresh when purchased should be good for at least 5 years storage.
Checking for leakage or other failure would be prudent just in case, but they should be fine if neglected for 5 years.

Lithium cells are warented to be good for about 15 years from production. If fresh when purchased they should be good for at least 10 years storage.
Again it would be best to check them, but if this is neglected they should be fine after 10 years.

At work I keep about 20 cheap basic flashlights as loaners.
Most are 2D rubber ones with a 0.5 amp incandescent bulb.
This gives enough light for most purposes and gives a run time of 25 hours from alkaline cells.

I do of course have better lights for my own use, but for lending , the cheaper and simpler the better.

Flashlight technology is improving rapidly and I would urge that those who chose to upgrade to the best/latest types should store the old ones.
You, or somone else might be glad of these in an emergency.

In a perfect world, batteries would live up to their warranties...I use either lithium batts or Eneloops ro avoid corrosion and leaking problems. I have no use for alkalines. What works best for me is a single cell AAA on the keyring and another close by in the pack, belt, or briefcase.