Radio Lithium batteries for old radios
I've recently been looking at an old Bush TR122 radio with a corroded battery compartment, powering from a PSU I found it still worked well without any component replacements but could not fit batteries to it. Now 18650 lithium batteries are readily available from scrap laptop battery packs, poundland usb power banks etc. and I've been using these for various things, light strings, torches, battery power tools etc, and I found that MT3608 DC-DC converter pcbs are available very cheaply from ebay or ali, so I could get up to 16V DC from an 18650 so I though I'd try it with a transistor radio, I was doubtful it would work but using a page 1 cheap Hong Kong radio from the seventies the results were good (MW only), I then tried the TR122 and this also worked well (MW and LW), this is good since I could fit an 18650 with a DC-DC converter and safely leave the set for occasional use without fear or leaking batteries. I've not got an FM radio to try this with at the moment, but it would be interesting to see if anyone has had success in using these modules, I'm very pleased with the results.
@mfd70 mobile phone batteries can be used too. The bonus is they have built-in protection and are generally better quality than dodgy Ebay 18650 batteries in my experience. Official manufacturer batteries are often sold really cheaply when they're for an older model of phone.
All you need to do is solder directly to the + and - terminals, they're well isolated from the actual battery so there's no danger of the heat getting to it. Obviously exercise caution.
@wayned It could be worth having a sift through Big Clive's channel on YouTube, or possibly even 'Diode Gone Wild'. They're often dismantling and reverse engineering such things, and often give schematic diagrams. From there you can build up your own protection circuitry for lithium batteries.
Mind you, there is a lot, and I mean, a lot, of videos to go through!