Upon further investigation there no "internal charger" to speak of. The 817 dumps approx. 250mA into the battery connector for a set period of time. (regardless if the battery is present or not, 70mA is used for the display and LED in off state). NiCd and NiMH can 'cope' with it, as when fully charged, they dissipate the extra energy in heat, and coincidentally, there's plenty of heat conductors in the battery area.
Of course this process damages the cells slightly (esp. NiCd), as it's not IEC specified 0.1C for 16hours.
The only measure that's keeping 817 from charging Alkaline cells is that green wire.
Cell balancing is not necessary provided the cells are capacity, voltage and IR matched from the start (100mAh tolerance is generally accepted). Laptop batteries rarely implement balancing and do not usually exhibit explosive tendencies.
No reason not to opt in for li-ion cells IMO.
Gone are the days where I'd willingly pack 40Wh/kg lead-acid for QRP backpacking. AAs don't fare much better.
Li-Ion is a way to go, NCR specifically both due to lower temperature tolerances, higher currents and higher densities.
There are a few aftermarket solutions:
3D printed holder
https://www.thingiverse.com/thing:2527434
and
3000mAh battery with back cover and adapter:
https://www.aliexpress.com/item/NEW...-Hatch-Cover-Charger-Bracket/32798891704.html
Neither of those solutions I'd consider optimal.
The 3D holder seems good solution at start, but requires protected cells and springed contacts increase resistance.
3Ah battery pack looks great, but it's expensive ($60) and replacement would mean buying proprietary battery, likely another $40 value.
Best solution IMO would be to 3D model the battery door to allow accommodating 18650 with their 18mm height + some extra room for heat-shrinked taping, 5.5x2.1mm jack and 5A+ switch.
This would enable swapping 3S 18650 packs on the go, low resistance across contacts and retain all the benefits of above battery pack, also by being non-proprietary (3S packs are readily DIY-able for $5-10 using NCR18650B or INR18650-35E).
With that, a question follows - has anyone managed to 3D model the battery door?