LiIon or NiMH?

[DMA]

Suppose you have a BLE controller which could work from two AA (or even AAA) NiMH batteries.
It could also work from Li-ion - although power management for the li-ion will about double the BOM cost-wise.

What would you chose and why?

I mean yes, NiMH would require some external charger and the battery pack will need to be on the outside of the keyboard, probably bolted to the back side or even on a double-sided tape. But is it that big of a hassle, really? Battery life is expected to be a week+ anyway.

[Muirium]

Is this for your BT/FSSK? There's likely plenty of room for neatly tucking AAs away inside, so I’m guessing this is something different.

Assuming this is for a compact project keyboard project, and you're asking about people's attitude towards general use, I'd say go with the Lion, as an internal battery (recharged over USB) is a lot more elegant than popping out AA/AAAs and rummaging around for a charger. Consumer electronics is heading that internal battery / USB powered direction, it's what people expect now.

There are arguments for AA carbuncles, however. I just don't value them. Even a dead Lion keyboard is still a USB keyboard.

[Findecanor]

I used to prefer NiMH, but I'd vote li-ion and internal charging over USB ... For a keyboard.

BTW, I'd like to see a wireless mouse that supports both, or rather either - in the same compartment but physically with different contacts. To insert a AA battery you'd have to unplug the Li-ion battery.
That way you wouldn't get stuck without a working mouse if the Li-ion battery runs out and you don't have the USB lead with you. But that scenario is not as likely with a keyboard, I'd think.

[DMA]

BTW, I'd like to see a wireless mouse that supports both, or rather either - in the same compartment but physically with different contacts. To insert a AA battery you'd have to unplug the Li-ion battery.

Definitely not both - making something work equally well on 2.4 and 3.7V is unnecessary hard and will never be needed, really. In my particular case, PSoC6 is a 3.6V device, AMR is 4.0V - which means it can't be directly powered from li-ion and will need at least an LDO, but ideally a buck.

That way you wouldn't get stuck without a working mouse if the Li-ion battery runs out and you don't have the USB lead with you. But that scenario is not as likely with a keyboard, I'd think.

no USB cable with you? What are you - some sort of prehistoric caveman?

[Findecanor]

Definitely not both - making something work equally well on 2.4 and 3.7V is unnecessary hard ...

I don't know much about Li-ion battery charging, but I do know that the voltage from batteries usually changes as they discharge. So you'll probably need voltage regulation of some kind or another anyway.
And you'll need voltage regulation from 5V to whatever the SoC uses when connected to USB.

no USB cable with you? What are you - some sort of prehistoric caveman?

Well, I did come up with the idea years ago back before USB-C had become ubiquitous.
I was travelling to a convention and had a laptop/tablet with me at all times, but left my USB leads (...) with different chargers at the hotel room.

And BTW, the tablet had only a single USB port. And while I might want to run the mouse on the tablet's tiny battery, I won't always want it to charge it as well.

[DMA]

Is this for your BT/FSSK? There's likely plenty of room for neatly tucking AAs away inside, so I’m guessing this is something different.

Well, _my_ instance of BT FSSK will not have any "inside" to speak of - I don't have FSSK case. But the problem is that those AAs will need to be changed - not super-frequently, but somewhat frequently, and taking your SSK apart weekly will grow old real soon.

Assuming this is for a compact project keyboard project, and you're asking about people's attitude towards general use, I'd say go with the Lion, as an internal battery (recharged over USB) is a lot more elegant than popping out AA/AAAs and rummaging around for a charger. Consumer electronics is heading that internal battery / USB powered direction, it's what people expect now.

There are arguments for AA carbuncles, however. I just don't value them. Even a dead Lion keyboard is still a USB keyboard.

Main problem with lithium battery isn't even the cost - it's shipping restrictions..

[DMA]

I don't know much about Li-ion battery charging, but I do know that the voltage from batteries usually changes as they discharge. So you'll probably need voltage regulation of some kind or another anyway.
And you'll need voltage regulation from 5V to whatever the SoC uses when connected to USB.

well, it's 10% vs 160% difference. Also obviously NiMH-powered BLE device won't have any USB on it - and li-ion will have something like LTC3554 on it, which will provide both battery management and stable power supply. (and cost relatively arm and leg)

no USB cable with you? What are you - some sort of prehistoric caveman?

Well, I did come up with the idea years ago back before USB-C had become ubiquitous.
I was travelling to a convention and had a laptop/tablet with me at all times, but left my USB leads (...) with different chargers at the hotel room.

And BTW, the tablet had only a single USB port. And while I might want to run the mouse on the tablet's tiny battery, I won't always want it to charge it as well.

Ah, _that_. I never separate charger from my laptop - too unorganized to keep them reliably charged overnight, and also battery life on my T420s is like 2 hours, so I know I'll need to charge if I ever pull it out of the bag