It's happened. Your pet end of society collapse scenario has come and gone, and you survived. For the first time ever in industrial history, society is unable to regain what it had before the disaster of choice, and is actually regressing, much like post collapse of Rome. Technology is lost, civilization only survives within the reach of your rifle, and you and your merry band of militiamen/preppers/co-religionists define what exactly is civilization and you are probably (depending on how you favor your scenario to go) waging war on other bands of people/surviving government/bandits/the wrong religion/culture.
Anyway, while your children and maybe grandchildren are growing up to be paragons of this new regressed era, you still hold fond memories of the the Before Time and really would like to charge up your iPod and listen to Sir Mix-A-Lot sing about his love for big butts one more time before you die. Sadly, the last solar panel was destroyed in a raid, and your alternator windmill generator didn't work out as well as you planned, and besides, you had to eat the copper coil for food after two successive years of crop failure, because you chose to stash another thousand rounds of .22 instead of garden seeds, thinking the other guy had the seeds.
So, how are you going to drag your group kicking and screaming back into the glory days of the mid 19th century? Why through Science(TM) of course! Presuming you don't adhere to some weird religion or political party that fears science you have choices. Of course since we are living on the scrap heap of the superior civilizations that preceded us, you might try rigging wind and water power or even crude steam power to get some salvaged and cobbled together generators working, but let's face it, we kinda gloss over that sort of thing in our imaginary planning, and jump right to a Jules Verne Mysterious Islandesque scenario, where our noble survivors are full of advanced knowledge, but in a land without any machines.
So we are going to make batteries, and hope that whatever charging devices you have left can deal with dirty and sometimes fluctuating current, because you don't know the advanced electronics to make the circuits and voltage regulators to safely charge your precious pre Hillary 4th term MP3 player, and neither do I.
So what are we gonna do? Well for one, the quickest answer if you can scavenge some wire and metal rods is an earth battery.
https://en.wikipedia.org/wiki/Earth_battery it was good enough for the early telegraph systems, it's good enough to get one more sweet playback extolling the truth of liking rotund backsides.
However, if you are a little more advanced or have access to a convenient supply of basic chemicals (don't worry, the heroic Last True American always has this deus ex machina) then we can make any number of wet cell batteries. In it's most primitive form, soak some cloth discs in vinegar (you have figured out how to make that by this point in your pet collapse setting, right? If not, you've got more important things to worry about than getting your radio back on the air.) and alternate a stack of pennies, or other coins to make a crude cell. It works, because science and maybe Jesus and shit.
Moving along though, since you probably glossed over canning too, and are now back to crude drying, smoking and brining techniques (see vinegar again) we can put some of these old timey early 21st century mason jars to good use.
The gravity or crowsfoot battery (so named because of the shape of it's electrodes)
http://w1tp.com/mbatt.htm was a long time workhorse of the telegraph industry. All you need is scavenged copper, zinc and umm copper sulfate as an electrolyte. No copper sulfate? No worries, just cook some up with copper, and some sulfuric acid. No sulfuric acid? Ouch. Hope after you got enough tactical operators in your group you found somebody with basic chemistry skills. Sulfuric acid can be made in pre industrial conditions, or even better still scavenged from car batteries if there are still any left.
http://chemistry.about.com/od/makechemi ... t-Home.htm meanwhile, consult your group, and when in doubt, kick one of the 20 self proclaimed DM/Snipers out and find somebody who knows more about chemistry than how to cook meth.
Of course if you've got magnets and some copper wire, you can make a generator
http://www.wikihow.com/Make-a-Simple-Electric-GeneratorEither way, let's pretend you've got some sweet, sweet volts flowing, and your noble band of survivors is standing around oohing and ahhing like cavemen when seeing fire for the first time.
Well shit. You gotta regulate that voltage now don't you? Well, you're on your own here, there are plenty of homemade voltage regulators out there, but they all rely on modern chips and components. Long story short, if your doomsday scenario includes a future where mankind has regressed technically, either stock up hella good for yourself, and enjoy the batteries while they last and while your charging technology lasts, or sit around the campfire someday holding up a dead cellphone with an extended radio antenna and try to convince the future generations that you really could text through the air with this tiny thing.
The reality is, for most of us, once you lose the ability to charge batteries, you are fucked three ways from sundown. Fortunately, the reality is this probably won't be the case. Just keep a good solar charger, and maybe a hand crank generator, and for that equally unlikely time you find yourself lost in the woods playing noble savage with modern technology, you can keep the phone charged.