4AA == USB!
Whenever I have to replace four AA batteries, in my head I’m thinking, “This could be a USB rechargeable battery”. This is because a AA battery is 1.5v, give or take. The same voltage that runs all the things that you plug into USB!*
*well, OK these days there’s higher voltages on USB-C, but we’re talking classic USB here — the lowest common denominator — 5V.
So at some point there’s that nagging voice inside you that actually wants to try to minimize waste and knows that all the batteries that we use in this country could probably fill Mt. Trashmore once a month. Why not replace those four AA batteries with a little 5V rechargeable USB battery pack that you can plug into any USB outlet to recharge!
To do this, you of course need a USB battery, but these aren’t hard to find. In fact you can find hundreds of form factors, colors, and prices out there. My favorite ones cost $1 at a yard sale, or even better have been tossed in the recycle bin. The key thing is to pick a USB battery that will fit into the space currently occupied by the AA batteries (or another space if one exist…preferably where you can make a little hole for the charging cable to go.)
So let’s look at what’s inside one of these things. Answer: a single battery cell and a small circuit board usually with an “in” (charge — usually a micro USB) and “out” (5V supply USB-A). Compact enough to fit in the battery compartment of whatever battery powered thing you want to upgrade.
In general all you need to do is take the 5v pins of the “out” and solder them to the wires that come from the battery compartment. OK, OK, for some electronics it might get more complicated than this, but for radios, flashlights, speakers, toys, etc. most of the time there’s just a +/- coming from the battery compartment supplying 5v.
For charging, I usually just use a knife or soldering iron to melt a 1x2cm hole then superglue or JBWeld the USB battery to the wall of your battery powered thing. Here are some examples:
If you’re intimidated, frustrated by or otherwise against trying to put the USB battery inside, you can always use USB cables to connect to the battery. In every case, I’ve always found that the bigger wires in the USB cable are red and black, and these carry the voltage (red=positive | black=negative). So you can clip a USB cable and connect the same colored wire to the ones where the AA’s went and you should be able to power the device.
Oh, one helpful hint — and probably the reason I find so many USB batteries in the recycle bin. If you short out (i.e. pull too much power) from the USB output of these things, they generally go into a “protection” mode and stop providing any power at all. People assume they’ve broken the thing and toss it — but nay this is not the case. All you have to do is plug it back into charge again and it will reset the protection!
