For those of you who have a ZX300...Yes, you can still get OEM Sony batteries from Sony parts directly in the US. They're out of stock at the moment, as I've bought the last ones. But, they say that it'll be restocked at the end of this month. Sony parts (for the consumer) comes via Encompass and the part number is X-2595-744-3. Here's the link to the page:
Encompass X-2595-744-3. If you do buy one, please don't hoard them so that everybody can get one...
You'll get the OEM part in the standard Sony parts box:
Simply pull the rubber sheet off the back of the player, starting at the BOTTOM, near the WM-PORT. There's a plastic cover there underneath that makes it easy to stuff a spudger underneath. Don't do it from the side, as there's one side that has the NFC flex board and you don't want to accidentally rip that. Then, unscrew the metal cover and plastic plate. There's a T-8 for the cover and a handful of JIS phillips screws. Use a 0 or 00 JIS driver so you don't strip them. Pull the little black plastic cover over the NFC plate, pop the latch on the ribbon socket, and carefully pull the NFC ribbon away. Then, slowly lift the socket off the cover. Use a toothpick or nylon/teflon flat. Once the cover is off, you'll see the following. The new battery will have a protective wrap that keeps the adhesive foil from sticking until you're ready.
There's also a plastic shield held by one screw at the top covering the contacts. Remove it too and set the cover pieces aside.
Separate the battery from the plastic tray. Do NOT pry up on it as there's a lot of adhesive foil. You can bend the battery, puncture it, or otherwise start a fire. Use dental floss, like Reach (a Teflon floss), and stuff it around the battery. Work the floss until it it starts to "cut" the adhesive. There's a spot near the microSD slot that has no adhesive on the battery, so it's a good place to start. Work from the bottom of the battery towards the wires. You can also add a bit of very high grade isopropanol to the edge you are cutting to soften the adhesive. I'm using a 100ul pipette and dropping reagent grade 99.999% isopropanol in the second picture. You want something that is plastic safe, evaporates quickly, and does not have water in it!
Eventually, you'll cut through enough of the glue to get the battery loose. You can then use a wood or nylon spudger underneath to push the glue and release the battery. Once loose, cover the adhesive side with something to keep it from sticking back down. A silicone release sheet or plastic film works good if you want to save the battery. Otherwise, use a piece of scrap paper. I'm using a bit of pink static bubble wrap...
Then, unsolder the old pack - starting with the ground shield foil, and the black (negative) wire. Protect each wire as you remove it from touching something else. Use a real soldering iron, not some hobbyist "woodburning" toy. Since this solder is the lead-free crap, you'll need some heat. Start around 350C with a micro-tip in your handpiece. Once completely unsoldered, simply install the new pack, and solder it all back. Be sure that your solder connections are clean, not cold, and not shorting.
And no, you don't need some special "dehydrated Unicorn urine encapsulated solder". It's solder. Actually, in my case, I'm removing the lead-free crap and replacing with a 63/37 Kester so that the joint will hold up better over time.
Reverse your disassembly, and you're good to go. The battery will have little charge (for storage). You'll need to do a full, uninterrupted charge once this is done for the battery to relearn. Remember, this is a new battery, so it'll take a few full charge cycles to reach full capacity. Be patient. For those with a lot of patience, you could also just run this down until self power off and then charge uninterrupted to full.
The batteries have a date code at the top of the pack, on the back side. The old one of 2018/04/07 and the new one of 2019/12/09. Sony also usually uses letters for revised components, so the "AA" of the original pack vs the "CA" of the new usually denotes 2nd revision:
Since my old battery still seems good, I'm going to safe it and keep it in case I want/need it in the future. If you do that, cover the exposed conductors with something proper before stowing. I'm using black Kapton so that it won't come back loose. You could always use the tube that came with the new battery or some heat shrink. I don't recommend "electrical tape" as it'll leave a sticky residue and will eventually come off by itself.
I hope that helps you ZX owners out there who have the tools, time, and want to change your ZX's battery. I'm an electronics engineer, so with a full lab and proper gear, I did this all in 20 mins and that's taking my time.