I build a few wireless keyboards by now, and those are the 3 ways I mounted my batteries, all with their own upsides and downsides.
Most wireless builds out there socket the controller. Those sockets add some height to the controller.
This gained space underneath the MCU is perfect for small 100 - 150mAh batteries.
anything with roughly 12x30x3mm110mAh & Mill Max socket
Small battery capacity
A lot of keyboards cover the controller with acrylic covers, sometimes for an OLED, sometimes just for some protection.
It's possible to replace the default standoffs with larger ones, which means we can mount rather thick batteries there.
Use some double-sided tape to secure the battery to the acrylic cover, that way it'll stay in play during normal use.
They'll do the job at home, but I wouldn't dare throwing such a keyboard in a backpack.
Those acrylic covers snap very easily at the screw holes, raising them for battery clearance isn't helping that.
That being said, having batteries last multiple months instead of a few weeks certainly is appealing.
The 1100mAh batteries on my Lily58 were mounted that way, and it worked just fine. Only recharged those once after around 3 months, soon after that I moved on to a Microdox.
anything with the footprint of an MCU1100mAh batteries
Not very sturdy if you travel with your keyboard
A lot of split keyboard cases are just two plates, the switch plate and the bottom plate, screwed together with some standoffs.
With MX switches, there is usually very little space left between the PCB and bottom plate.
But Chocs don't have the 2mm between the top plate and PCB, thus gaining those 2mm between the PCB and bottom plate.
This doesn't mean we can't use this mounting style with MX switches, we just need to use longer standoffs.
The batteries can't be very thick, but there are very little restrictions otherwise.
All the unused space between the PCB and bottom plate is battery space.
Spicy pillows are dangerous
I've been using such a setup on my Microdox for about a year now.
The battery is 4.1mm thick and I measured 4.4mm between the PCB and bottom plate. It still slides around a bit, but if the battery was on the thicker side (product page said 4mm +-.5mm) it could get dicey.
It may be advisable to get some slightly longer standoffs and use double-sided tape to secure the battery, instead of squishing the battery between the PCB and bottom plate.
anything with longer standoffs830mAh batteries
|Low Profile MX
|only adds to the top
|raises the whole keyboard
|Low Profile Choc
|only adds to the top
|pretty much hidden
Battery life are based on my usage, with the mentioned battery sizes and no additional features.
Wireless keyboards with RGB and/or OLED will draw significantly more power.