ErgoMechKeyboards

Some boards that use the nice!nano have room to tuck a small battery (I think 110mAh) underneath the controller. If they use the 3 middle pins on the nano though, that's not an option. Rechargeable coin cells are an option, but for boards with the usual LiPO pouch cells, you'll probably want some kind of case. Some folks just use double-sided tape to hold the battery to the PCB but I'd rather have some protection.

If you build using nice!nano controllers, you can get batteries that fit neatly under the (socketed) controller, you can just tuck it in there loose or tape it in place. If you use those batteries, which are very small, it's best to build the board without any power-drawing features, such as LED illumination or OLED display. Most PCBs that are designed for batteries will put contacts in that vicinity, or you can wire the batteries directly the nice!nanos.

If you build this way, there's no impact on the external size or portability. If you want to use larger batteries, you will need to make room in the case for them, if you're using a case. [I have some chunky batteries taped to the bottom of my caseless Kyria, which is obviously not doing anything good for portable use or aesthetics, but it does work.)

If you haven't used ZMK before, that will be a bit of a transition. ZMK is great, and in some ways it's a much more elegant solution than QMK, but it can be a little tricky to get used to at first and if anything goes wrong, troubleshooting is usually more annoying.

For portable use, a PCB that supports a power switch to disconnect the battery from the controller is kind of essential IMO, since without a hardware switch, there's no way to turn the board off or put it to sleep. So if it's in your bag, it'll constantly be waking up and connecting to your phone if paired.

Batteries require management with control circuitry normally on the MCU. This means you need to pick MCUs that support batteries.

You will also want a on off switch for the battery somewhere.

I always use LIR2450 button cell batteries where I can. The steel body makes them more resistant to damage where as pouch cells can't take much of a scuff.

They're very portable! Just make sure the battery can't be smushed if the board itself gets some pressure, and that it can't get poked by sharp / conductive things.

ZMK for the nice nano and XIAO nrf52840 should even report battery life! Your primary half (generally left) drains faster than the accessory half.

RGB / screens that aren't designed for low power are utterly destructive to battery life. I get ~1-2 weeks on 120mah splits. With RGB I'm lucky to get an hour.

You don't change these batteries as they're rechargeable. Most boards will function while recharging. I offer QI wireless charging on my boards if you request it.