Is an external batter charger powered by AA batteries a viable option? Most of the external chargers I have seen are rechargeable units. Just thinking a long distance ride, with short stops and questionable availability of electrical outlet charging options, would make this type of charger a good option. I've never used such and don't know if they provide sufficient charge to be worth it. Or would they be better as a "backup to the backup"? ***Edit-- Looking at this option just due to convenience and possible lower weight penalty. This would be in a long distance, randonneuring situation, where were have a limited time to cover 400, 600, 1000 or 1200 kms.***

What are you trying to charge? They make a large assortment of solar chargers, one of them might fit your need.

The typical (USB type) Lithium Ion battery charger outputs 5.0 volts. The batteries themselves are likely 3.6-3.7v, so in theory you might be able to charge with 3 alkaline batteries, (4.5v), though this may be a bit skimpy achieve a maximum charge.

HOWEVER lithium batteries are prone to overheating during charging, and you may run a high risk if charging without some kind of resistor to slow the process. I'm sure you can get better and more specific information if you search the net asking about charging lithium batteries.

The voltage depends upon the design of the battery pack. I have battery packs from 3.7V (single 18650 Li-ion battery) to 12V (lithium polymer battery intended for CCTV backup). A lot of the newer lights are designed to run on 5V so you can use the USB phone charger but there is no combination of Li-ion batteries that achieves that voltage so they have to incorporate electronics to get 5V. I have bike headlights that run on 8.4 volts and use an external battery pack with 4 18650 batteries. I'd never go back to throw-away batteries as the Li-ion batteries pack a lot more energy for the same battery weight. It's also so much cheaper to use a battery that can be recharged 500 times versus the good quality but expensive ones like Energizer or Duracell. It's a good idea to charge your Li-ion batteries on a non-flammable surface and always use the appropriate charger that comes with the batteries. If you do that, they are safe. It's easy and cheap enough to carry a spare 8.4 volt battery pack or buy one designed to switch individual 18650 batteries. A single 18650 battery only weighs 36 grams so it's not a lot to carry.

Iphone, Garmin, Jaybirds,possibly lights. I edited my original post. Convenience and such during Randonneuring events is the main reason I'm looking at this option.

I edited my initial post. Randonneuring events are the real reason I'm looking at this option.

Right now I'm designing a portable solar charger for my phone which I can also plug in a 9V battery for a quick charge. Just need the time to put it together.

Lithium-ion and other Lithium type batteries carry a lot more energy per gram than the other types. If you bring a battery made of AA or AAA cells, you might carry less weight than a Li-Ion battery, but it will have less charging capacity. That doesn't matter, though, because either will probably be acceptably light, even if it has enough capacity.

One advantage of using AA or AAA cells is that you can get them at any corner store. This is not a great strategy if you plan to use the battery a lot, because of environmental and financial cost, but it is a real winner for unexpected needs.

Then again, if you bring enough capacity -- or even more than you expect to need -- in Li-Ion batteries, life will be simpler.

As Francis warns, don't let the Li-Ion battery overheat. It can happen from charging, and it can happen from exposure to the sun, so insulate it in the summer.

I'm not sure a DIY phone charger makes a lot of sense because the components would cost you way more than buying a readily available one. Searched a site that I used to buy my bike lights for "solar charger" and came up with 41 results. Price depends upon the capacity of the lithium polymer battery built into the charger. Starts at $10 including shipping. solar phone chargers - Buy Cheap solar phone chargers - From Banggood

Well it's a good thing I happen to have all the components I need on hand then. I already had the solar panels from a battery charger project I made a number of years ago but fell into disuse when I got a larger, higher current solar panel. The PC board, voltage regulator, capacitors, & diodes are all out of my junk box, and some I bought in bulk pretty cheap at some point in the past. And the USB jack I salvaged out of an old printer/scanner/copier which no longer worked.

I have seen a video online where someone took a cigarette lighter USB charger plug and basically soldered a 9V battery connector to it so that you can charge your phone from a 9V battery. This I will also incorporate into my charger as the 5V voltage regulator will allow the 9V battery to charge the phone, as well as whatever voltage I'll be getting from the two 3V solar panels.

I might work on this project tonight, and whenever I get it completed I'll post a thread about it here.

Anything a AA cell external battery pack can do, a D cell external battery pack can do longer. We used those years ago for extended running of devices like the original laptop, the AA powered Tandy Model 100 and 102, or field DXing with AA powered shortwave radios. That was pretty trick 20something years ago. You could even buy or make D cell battery packs with bean bags to support the powered device, stuck together with Velcro.

But nowadays I'd stick with Li-ion batteries. More bang for the buck, lighter weight, more versatile.

A single larger external Li-ion battery can handle two or more USB ports to simultaneously run and charge compatible devices (other than my Light & Motion headlight, which can only charge while plugged into a USB external battery). Spare Li-ion batteries with a solar charger on the rear rack or on top of the panniers.

I can recommend the Jackery USB batteries. I've used one for about six months and abused the poor thing. I rarely drop anything but for some reason I keep dropping that Jackery battery -- possibly because the darned things are shaped like a bar of soap and about as slick. Keeps on ticking. I usually keep it in a small padded handlebar bag Velcro'd across my handlebar (and retained with a backup safety strap -- which saved my bacon this weekend twice when rough roads and a crash popped the main Velcro strap loose). Run the USB cable through the bag to the device that needs charging or extended running.